'Bad weather could raise your blood pressure and even kill you,' is the unnecessarily alarmist headline in the Daily Mail. It reports on a large, complex study that looked for any association between changes in weather and blood pressure rates.

The research focused on patients at a blood pressure clinic in Glasgow and looked at two consecutive visits the patients made within a 12-month period. The researchers combined these findings with Met Office weather data from the time of these visits to assess whether changes in patients' blood pressure were related to changes in the weather.

They found that decreases in temperature and sunshine, or increases in rainfall and frost, were associated with a slight increase in blood pressure.

In the longer term, individuals whose blood pressure seemed sensitive to decreases in temperature and sunshine had slight increases in blood pressure. They also seemed to have overall shorter survival than people insensitive to weather changes.

We know that our bodies respond to temperature changes, so it is plausible that temperature could influence blood pressure. But factors other than the weather may have had a role to play in the blood pressure results seen.

It is also important to point out that the minor increases in blood pressure detected by the study could in many cases be compensated for by taking more exercise or improving your diet.

Where did the story come from?

The study was carried out by researchers from the University of Glasgow. One of the study authors was supported by a Wellcome Trust Capacity Strengthening Strategic Award to the Public Health Foundation of India and a consortium of UK universities.

It was published in the peer-reviewed Journal of the American Heart Association.

The quality of the Daily Mail's reporting of this study is mixed. On the negative side, it presents an over-simplistic conclusion that cannot be drawn from the complex analysis used in this study. The claim made in the headline that 'bad weather...can kill you' is needlessly sensationalised.

On the plus side, its story does contain useful advice from a spokesperson from Blood Pressure UK: "Until we can control the weather, we can still rely on more traditional ways of controlling our blood pressure, such as eating more fruit and vegetables, less salt and alcohol, and taking more exercise."

What kind of research was this?

The researchers say that there is growing evidence that outdoor temperature has an influence on blood pressure, with blood pressure being higher in winter and lower in summer.

This is believed to be because the constriction of blood vessels at colder temperatures increases blood pressure. However, it is unclear whether the temperature-related response differs among individuals.

The current study aimed to examine people's individual changes in blood pressure in response to a range of weather patterns. The researchers also wanted to see whether this was predictive of longer term blood pressure control and mortality.

What did the research involve?

The study included 16,010 people from the Glasgow Blood Pressure Clinic (47% male) who had been referred by their GP in order to control their high blood pressure.

Information on the monthly average weather for the west of Scotland was obtained from the UK Met Office. The Met Office has used a consistent method to analyse climate patterns since 1961, and can provide weather for square kilometre grid points across the UK. Information on four aspects of weather was used in the study:

• air frost
• air temperature
• rainfall
• sunshine

Each visit every person made to the Blood Pressure Clinic was mapped to the mean monthly weather of the west of Scotland. Mean monthly measurements for each of the four aspects of weather were ranked from the lowest to the highest measurement, and then split into four equal groups called quartiles. The lowest quartile (Q1) contained the lowest 25% of measurements and the highest quartile (Q4) contained the highest 25% of measurements.

For each person, the researchers looked at pairs of consecutive clinic visits that were at least one month apart but within the same 12-month period. They were interested in pairs of visits where weather either remained constant (both visits in the same weather quartile) or where weather was very different (one visit in the lowest quartile and one visit in the highest quartile). They categorised the weather for these clinic visits as:

• Q1 to Q4, where weather for the first clinic visit was in the lowest quartile and the subsequent visit was in the highest quartile
• Q4 to Q1, where weather for the first clinic visit was in the highest quartile and the subsequent visit was in the lowest quartile
• Qn to Qn, where both the first and the second clinic visits were in the same weather quartile – there was no change in weather patterns

For each individual, the researchers examined changes in their blood pressure and heart rate between the two visits, and looked at how the size and direction of this change (up or down) related to the change in weather.

The researchers used the General Register Office for Scotland to obtain information on deaths among the participants and causes of death. Mortality information was available up to 2011, allowing up to 35 years of follow-up.

Analyses were adjusted for factors known to influence blood pressure (confounders), including:

• age
• smoking
• alcohol
• high body mass index (BMI)
• kidney disease

What were the basic results?

The average age of individuals at their first clinic visit was 51 years, and most were overweight (mean BMI was 28). The average length of follow-up for each person was 6.5 years.

The researchers found that when there was consistent weather between the two clinic visits (Qn to Qn), there was:

• an average 2.1% decrease in systolic blood pressure (the upper figure of a blood pressure measurement) with consistent air frost
• a 2.2% decrease with consistent temperature
• a 1.7% decrease with consistent rainfall
• a 2.2% decrease with consistent sunshine

For change from high to low weather extremes, there was:

• about a 2% increase in systolic blood pressure with a decrease in temperature and sunshine
• no significant change in systolic blood pressure with a decrease in air frost and rainfall

For change from low to high weather extremes, there was:

• a 1.4% increase in systolic blood pressure with an increase in air frost
• a 0.8% increase in blood pressure for an increase in rainfall
• there was no consistent pattern in blood pressure with a change in temperature from low to high 

When the researchers compared the blood pressure changes seen with consistent weather patterns, a change in the weather from the highest to lowest quartile was associated with about a 6% increase in systolic blood pressure when there was a decline in temperature and sunshine, and about a 4% increase in systolic blood pressure when there was a decline in air frost.

Compared with consistent weather, a change from the lowest to highest quartile was associated with 2-6.6% increases in systolic blood pressure for all four weather characteristics assessed.

Looking at longer term changes over five or more years, people whose blood pressure changed when there was a decline in temperature experienced a 2.68mmHg increase in their systolic blood pressure, and a 1.84mmHg increase in their diastolic blood pressure (the lower figure in a blood pressure measurement), compared with people whose blood pressure seemed insensitive to temperature change. 

A similar 1.31mmHg increase in systolic blood pressure and a 1.22mmHg increase in diastolic blood pressure was seen for people who were sensitive to a decline in sunshine.

Looking at survival data, people who were insensitive to temperature or sunlight change seemed to have longer survival than people who were sensitive to a decline in temperature or sunlight.

There were no significant longer term differences in blood pressure or survival between people insensitive to temperature or sunlight change, or in people sensitive to an increase in weather extremes.

How did the researchers interpret the results?

The researchers have concluded that for the first time they have demonstrated the extent of alterations in blood pressure between consecutive clinic visits associated with changes in weather in people with high blood pressure.

They have extrapolated that knowing a person's blood pressure response to weather could help prevent doctors making unnecessary changes to blood pressure medication.

Conclusion

This study has used a complex method of analysis in order to look at how individuals' blood pressure at consecutive visits within a one-year period varied according to changes in the weather.

The study benefits from its large population sample and long follow-up. The blood pressure measurements taken at this specialist clinic are also likely to be reliable.

Our bodies do respond to changes in temperature and it is biologically plausible that temperature can affect our blood pressure. The researchers have adjusted for many factors known to influence blood pressure, such as age, high BMI and kidney disease.

However, it is still difficult to say with certainty that all blood pressure changes seen in people between clinic visits were solely down to changes in the weather. For example, the researchers did not have complete information about the blood pressure medications being used by the patients, or their levels of physical activity. These factors could also be influencing the findings.

Another limitation is that blood pressure would have been recorded inside the clinics and may not be representative of what blood pressure would have been if it had been taken outside, with full exposure to the weather.

The research was conducted in individuals from the Glasgow area and it is difficult to say whether similar responses would be seen in people in other locations, particularly people living in vastly different climates.

Similarly, the study only looked at people with high blood pressure. It is not clear whether people with normal blood pressure also experience similar changes in their blood pressure in response to weather changes.

The individuals in the study seem to have been variably sensitive to different changes in the weather. It is not yet clear exactly how a person's blood pressure treatment could be individualised according to their sensitivity to weather change, and whether this would successfully reduce blood pressure variability.

One final important point to make is that although we have no control over the weather, we can control a wide range of factors that contribute towards high blood pressure, such as:

• the amount of exercise you take
• diet – if your blood pressure is high, you should cut down on salt, saturated fat and sugar, and eat plenty of fruit and vegetables
• quitting smoking, if you smoke
• the amount of alcohol you drink

Read more about proven lifestyle changes you can make to reduce your blood pressure risk.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.

Links To The Headlines

Bad weather could raise your blood pressure and even kill you. Daily Mail, May 21 2013

Chilly days can kill by raising blood pressure. Daily Express, May 22 2013

Links To Science

Aubinière-Robb L, Jeemon P, Hastie CE, et al. Blood Pressure Response to Patterns of Weather Fluctuations and Effect on Mortality. Hypertension. Published online May 6 2013

“Mothers’ diets may harm IQs of two babies in three,” warns The Independent. The newspaper reports on its front page that iodine deficiency is widespread among pregnant women.

Iodine is recognised to play a role in the healthy development of the baby’s brain and nervous system while in the womb and the World Health Organization recommends that pregnant women eat iodine-rich foods.

Severe lack of iodine is one of the leading causes of brain damage in the developing world. But a new study, reported in most of the media today, suggests that even mild-to-moderate iodine deficiency during pregnancy may be associated with poorer cognitive function in the child.

In this large study, the iodine levels of pregnant women were measured, and their child’s IQ at age eight and reading ability at age nine were tested.

The researchers found that children of women who didn’t get enough iodine were more likely to be in the lowest quartile for verbal IQ, reading accuracy and reading comprehension. However, there was no significant difference in overall IQ.

A study of this kind has limitations, for example the fact that it relies on measurements being taken at a single point in time. Also, although the researchers adjusted for many factors that may have influenced the relationship (for example, parental lifestyle and socioeconomic factors), the study cannot prove a direct cause and effect relationship between a mother’s iodine consumption during pregnancy and her child’s cognitive ability. It is also not clear whether the differences seen in the children’s verbal and reading skills would translate into ‘real-world’ problems for these children.

Nevertheless, the study does highlight the need for pregnant women to get enough iodine during pregnancy.

Where did the story come from?

The study was carried out by researchers from the University of Surrey and the University of Bristol. No specific funding was reported for the current study, but researchers were supported by the Waterloo Foundation, the Commission of the European Communities, the US National Oceanographic and Atmospheric Administration and Wassen International. The latter is a company that makes and sells iodine supplements. However, none of these organisations had any role in how the study was conducted or how the collected data was interpreted.

This study used information taken from a much larger ongoing cohort study known as the Avon Longitudinal Study of Parents and Children (ALSPAC), which is looking at the health outcomes of children born during the 1990s. The ALSPAC study is supported by the Medical Research Council, the Wellcome Trust and the University of Bristol.

The study was published in the peer-reviewed medical journal The Lancet.

The media reporting is generally representative of the study, although the Mail Online headline writers got into a serious muddle. When they first published the story they used the headline “Drinking organic milk in pregnancy is 'vital for the baby's future brain power'". This was then changed later in the day – "Drinking organic milk in pregnancy could be harming baby’s IQ".

Neither claim is supported by this study. The study did not assess women’s dietary iodine intake from different sources. So it is not possible to say how many women drank organic milk and whether those who did were more likely to be in the iodine deficient group.

What kind of research was this?

The researchers say that the World Health Organization considers iodine deficiency to be “the single most important preventable cause of brain damage” worldwide. Iodine has a role in regulating the thyroid gland, and thyroid hormones have a role in brain and nervous system development. 

The researchers say that changes to dairy farming after the 1930s increased the amount of iodine in milk in the UK. After this and due to the reduction in cases of goitre associated with thyroid problems in the UK it was considered that iodine intake in the UK was sufficient.

However, some more recent UK studies have suggested that mild iodine deficiency may be quite common among adolescent schoolgirls and pregnant women.

The current study used data collected from participants in the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort study to see whether there was an association between pregnancy iodine levels and child cognitive performance. The researchers speculated that women with lower iodine levels during pregnancy would have children with poorer cognitive outcomes.

What did the research involve?

The ALSPAC cohort was eligible to all pregnant women in southwest England with a due date between April 1991 and December 1992.

A total of 14,541 pregnant women were enrolled and 13,988 of their children survived for at least 12 months.

The researchers selected 1,040 women for whom they could measure iodine in the first trimester of pregnancy (up to 12 weeks) and their child’s IQ when they were eight years old.

Iodine was measured in a single urine sample. Urinary iodine levels are said to be a good indicator of iodine levels in the body as 90% of ingested iodine is excreted into the urine. However, the results would have been more accurate if the researchers had been able to measure iodine based on 24-hour urine collection.

To try to reduce the impact of this issue, the researchers looked at the iodine-to-creatinine ratio, which is said to be a good way to get a more accurate iodine measurement. The researchers defined adequate iodine as an iodine-creatinine ratio of 150 micrograms or more per litre. Iodine deficiency was sub-categorised as mild-to-moderate (50 to 150) or severe (less than 50).

Child IQ at the age of eight was assessed using a validated scale (the Wechsler Intelligence Scale for Children). At the age of nine psychologists also assessed children’s reading speed, accuracy and comprehension. 

The researchers looked at the association between pregnancy iodine status and IQ at the age of eight and reading at the age of nine. They adjusted analyses for a wide range of confounders including:

• mother’s age
• mother’s ‘parenting score’ (assessed by looking at cognitive stimulation of the baby, parental education and socioeconomic status)
• home environment, including baby’s emotional and cognitive environment
• family adversity
• stressful events during pregnancy
• infant birth weight and prematureness
• breastfeeding history
• maternal smoking and alcohol intake
• other dietary factors during pregnancy, including intakes of omega-3 fatty acids and iron

What were the basic results?

The researchers found that, overall, the women in the study had an average (median) urinary iodine concentration of 91 micrograms per litre, and average iodine-to-creatinine ratio of 110 micrograms per litre. About two-thirds of women in the study (67%) were iodine deficient in pregnancy. None of the women was using an iodine supplement.

Compared with mothers with adequate pregnancy iodine, those with iodine deficiency were significantly younger and less educated, but had less exposure to stressful life events in pregnancy.

Compared with children of women with adequate pregnancy iodine levels and after adjustment for confounders, children of women with iodine deficiency were at significantly higher risk of:

• having a verbal IQ score in the lowest quartile (odds ratio 1.58, 95% confidence interval (CI) 1.09 to 2.30)
• having a reading accuracy score in the lowest quartile (odds ratio 1.69, 95% CI 1.15 to 2.49)
• having a reading comprehension in the lowest quartile (odds ratio 1.54, 95% CI 1.06 to 2.23)

However, there was no significant association between pregnancy iodine deficiency and performance IQ or overall IQ score – only verbal IQ. There was also no significant association between iodine deficiency and reading score or number of words read per minute – only reading accuracy and comprehension.

How did the researchers interpret the results?

The researchers say that their results demonstrate the importance of having adequate iodine intake during early pregnancy. They say that the results “emphasise the risk that iodine deficiency can pose to the developing infant, even in a country classified as only mildly iodine deficient”. The researchers consider iodine deficiency during pregnancy to be an important public health issue that needs attention.

Conclusion

This is a valuable study that demonstrates that in this subsample of a large cohort of pregnant women in the UK, the majority had inadequate iodine levels during pregnancy.

They also found that this deficiency was associated with poorer verbal IQ in their children at the age of eight, and reading accuracy and comprehension at the age of nine.

The study benefits from its relatively large sample size, from the fact that it followed participants up over time and from the fact that it adjusted for extensive confounding factors.

However, there are some limitations to this study:

• As the researchers say, several 24-hour urine collections would have been the ideal way to measure iodine levels, rather than a single measure, but this would be impractical in a large-scale study.
• It would also be useful to continue to reassess the children’s IQ and reading performance at different time points, particularly as the associations were only found for certain measures of IQ and reading ability. Related to this, it is also unclear what impact these differences in verbal IQ and reading accuracy and comprehension would have had on the children’s learning and school performance. Children’s IQs are not thought to be fixed for life but can change over time.
• Studies in other population samples from other countries would be valuable.

The researchers note that a randomised controlled trial assessing the effect of iodine supplementation in pregnant women on child cognitive ability in areas with mild-to-moderate iodine deficiency would be valuable. They say that they hope to run such a trial in the UK, as current evidence from trials in this area is weak. 

Overall, the study highlights the need for pregnant women to obtain sufficient iodine during pregnancy. The World Health Organization recommends that pregnant and breastfeeding women consume 250 micrograms of iodine a day.

Dietary sources of iodine include dairy products and fish. Pregnant or breastfeeding women who are unable or unwilling to eat these types of iodine-rich dietary sources may need supplements.

If you are pregnant or breastfeeding and are concerned about your iodine levels, speak to your GP or midwife before taking supplements. Supplements will not be suitable for every woman.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.

Links To The Headlines

Mothers' diets may harm IQs in two-thirds of babies. The Independent, May 22 2013

Iodine deficiency 'may lower UK children's IQ'. BBC News, May 22 2013

Women who drink organic milk in pregnancy could be harming their baby's IQ. Mail Online, May 22 2013

Enough iodine vital in pregnancy – study. The Guardian, May 22 2013

Links To Science

Bath SC, Steer CD, Golding J, et al. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC) . The Lancet. Published online May 22 2013

'Should you be taking vitamin B to protect against Alzheimer's?,' asks the Daily Mail.

Its question is prompted by new research into whether a daily dose of vitamin B could reduce the loss of brain tissue in people with mild cognitive impairment. Mild cognitive impairment is thought to be a risk factor for developing Alzheimer’s disease.

The researchers were particularly interested in the effects of B vitamins on ‘grey matter’ – brain tissue. Grey matter consists of a complex mixture of nerve cells and is found in regions of the brain associated with higher cognitive functions, such as memory and reasoning. Previous studies have found that in people with Alzheimer’s disease, certain regions of grey matter begin to shrink, and this may contribute towards the symptoms of the disease.

This research clearly shows that grey-matter loss in certain regions of the brain was reduced with B vitamin treatment – and the results were particularly striking in patients with high levels of an amino acid called homocysteine.

However, whether the reduction in grey matter shrinkage caused by the vitamin B treatment reduced the likelihood of participants developing Alzheimer’s disease, is unknown.

Until further trials have confirmed the benefits of B vitamin supplements and found that they outweigh any potential harms, the best way to keep healthy in mind and body is to eat a balanced diet, control your weight and blood pressure, and to take some exercise.

Where did the story come from?

The study was carried out by researchers from the University of Oxford, the University of Warwick, and the University of Oslo, Norway. It was funded by a wide range of charitable organisations and research institutes.

The study was published in the peer-reviewed Proceedings of the National Academy of Sciences of the United States of America (PNAS).

The researchers hold patents on the use of B vitamins to treat Alzheimer’s disease or mild cognitive impairment, meaning they could benefit financially if vitamin B treatments were licensed for this use.

This story was widely reported in the media. The Daily Express went with the headline “The daily vitamin B pill that halts the ravages of dementia” and The Daily Telegraph with “Vitamin B could stave off Alzheimer’s”. Unfortunately, these headlines are a little optimistic, as although the study found that vitamin B reduced loss of grey matter in certain parts of the brain, especially in older people with high levels of the amino acid homocysteine, the effects this reduction had on an individual’s risk of developing Alzheimer’s disease were not assessed. 

What kind of research was this?

This was a randomised controlled trial that aimed to determine whether B vitamins are effective in preventing the shrinkage of grey matter in areas of the brain known to be vulnerable to Alzheimer’s disease, especially those regions linked to mental processes.

This was a secondary analysis of data collected in a previous study which found that B vitamins reduce whole volume brain shrinkage.

A randomised controlled trial is the best type of study design to address this question.

What did the research involve?

The researchers randomised 156 elderly volunteers with memory complaints who fulfilled criteria for mild cognitive impairment to receive B vitamin treatment (folic acid 0.8mg/day, vitamin B12 0.5mg/day, vitamin B6 20mg/day) or placebo for 24 months.

Images of the participants’ brains were taken at the start and end of the study using magnetic resonance imaging (MRI). The researchers compared the images to see whether B vitamins prevented shrinkage of grey matter in areas of the brain affected by the Alzheimer’s disease, especially those regions linked to mental processes.

What were the basic results?

Grey matter volumes were similar at the start of the study in both groups. Over the course of the study, areas of grey matter shrunk in both the placebo and B vitamin groups. However, participants who received B vitamins had less shrinkage of certain areas of grey matter than participants who received placebo.

The researchers report that significant reductions in grey matter loss were seen in some of the regions most affected in Alzheimer’s disease.

The researchers drew on the results of previous research, which has found that levels of an amino acid called homocysteine may play a role in cognitive impairment, Alzheimer’s disease and vascular dementia.

They found that participants with higher homocysteine levels had smaller brain volume, and a faster reduction in brain size.

B vitamin treatment had no effect in participants who had homocysteine levels below the median (average), but significantly reduced grey matter loss in participants with homocysteine levels above the median.    

The researchers also monitored changes in participants’ scores on a variety of neuropsychological scales. They found that scores were correlated with grey matter loss in certain regions, some of which shrunk less with vitamin-B treatment than placebo in participants with high homocysteine levels.

Based on these findings, the researchers suggest that changes in vitamin B12 levels that occur with B vitamin treatment leads to a reduction in homocysteine levels. This decreases the rate of grey matter loss. This in turn affects neuropsychological functioning.

How did the researchers interpret the results?

The researchers conclude that, “our results show that B-vitamin supplementation can slow the atrophy [shrinkage] of specific brain regions that are a key component of the Alzheimer’s disease process and that are associated with cognitive decline.”

They go on to suggest that “further B vitamin supplementation trials focusing on elderly subjects with high homocysteine levels are warranted to see if progression to dementia can be prevented.”

Conclusion

This two-year long randomised controlled trial found that B vitamin treatment significantly reduces loss of grey matter in certain regions of the brain in elderly volunteers with mild cognitive impairment. The researchers report that these regions are specifically vulnerable to Alzheimer’s disease. B vitamin treatment was beneficial for the subgroup of participants who had higher than average levels of an amino acid called homocysteine.

This research clearly shows that grey matter loss in certain regions of the brain was reduced with B vitamin treatment. This follows on from the researchers’ previous findings that B vitamin treatment slows brain shrinkage.

However, it is less clear whether the reduction in grey matter actually had any real health impact on individual people. Although the researchers report that loss of grey matter was linked to declining neuropsychological scores, they do not specifically report that participants who received the B vitamins improved their brain function scores. Whether the B vitamin treatment actually prevented Alzheimer’s disease is also unknown.

B vitamins are a recurring focus of Alzheimer’s disease research, and they have been studied in both the prevention and treatment of the disease. This may partly be because vitamin B deficiency can have an effect on brain function.

For more background information about vitamin B, Alzheimer’s, and how it has been reported in the news, read the Behind the Headlines special report on ‘Alzheimer's in the news’

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.

Links To The Headlines

Should you be taking vitamin B to protect against Alzheimer's? Daily Mail, May 21 2013

Vitamin B could stave off Alzheimer's. The Daily Telegraph, May 20 2013

The daily vitamin B pill that fights dementia. Daily Express, May 21 2013

Links To Science

Douaud G, Refsum H, de Jager CA, et al. Preventing Alzheimer's disease-related gray matter atrophy by B-vitamin treatment. PNAS. Published online May 20 2013

"Bed-sharing 'raises cot death risk fivefold'," BBC News reports. The news has featured in much of the media, with headlines based on a large analysis of previous studies into the risk of cot death, or sudden infant death syndrome (SIDS), associated with bed-sharing.

Bed-sharing is where babies sleep in the same bed as their parents. It has long been known that bed-sharing with a parent who smokes or has consumed drugs or alcohol increases the risk of SIDS.

This new study aimed to see if bed-sharing still increased the risk of SIDS in the absence of these risk factors.

The researchers found a fivefold increase in the risk of SIDS associated with bed-sharing in breastfed babies who were under three months old, had parents who did not smoke, and when the mother had not had any alcohol or drugs.

The risk of SIDS was even higher when the parents smoked, drank or used drugs.

It is important to note that the researchers did not conclude that babies should not be brought into their parents' bed for comfort and feeding, only that they should not sleep in the same bed as their parents.

It is also important to be aware that the overall risk of SIDS is very small and it is a rare condition. However, there are simple steps parents can take to reduce the risk of SIDS (see box), and it is well worth following this potentially life-saving advice.

Where did the story come from?

The study was carried out by an international team of researchers from the London School of Hygiene and Tropical Medicine, the University of Glasgow and the Medical Research Council, UK; the Children's University Hospital, Ireland; the University of Auckland, New Zealand; and the University of Muenster, Germany.

No external sources of funding were reported, although the original research this report is based on was funded by a number of governmental bodies, charities and trusts.

The study was published in the peer-reviewed open access medical journal, BMJ Open.

The story was well reported by the media. 

What kind of research was this?

This study combined information from individuals in five large case-control studies. These studies had collected data on babies who died from SIDS (cases) and babies of a similar age who were still alive (controls).

The researchers were interested in bed-sharing specifically. The research aimed to resolve the uncertainty over whether there is a risk of SIDS associated with bed-sharing in breastfed babies where neither parent smoked. 

What did the research involve?

The researchers combined individual data from studies in the UK, Europe and Australasia. In total, information was collected for 1,472 babies who died from SIDS and 4,679 control infants, all under one year of age. Control infants were randomly selected normal infants of a similar age, from a similar location and born at a similar time.

The researchers estimated the risk associated with bed-sharing in relation to breastfeeding, smoking, and the mother's recent alcohol consumption or illegal drug use. This was after they had controlled for other important risk predictors, including:

• whether the baby slept in the parents' room or elsewhere
• the position in which the baby was put to sleep
• mother's age
• mother's relationship status
• the number of children the mother had had previously
• the baby's birth weight

What were the basic results?

The researchers found that 22.2% of cases and 9.6% of controls reported a history of bed-sharing. Bed-sharing increased the risk of SIDS fivefold (adjusted odds ratio [OR] 5.1, 95% confidence interval [CI] 2.3 to 11.4) compared with room-sharing (assuming the baby was placed on his or her back in a cot in the parents' room) when:

• neither parent smoked
• the mother did not drink
• the baby was less than three months old
• the baby was breastfed, and
• there were no other risk factors

The researchers estimate that the absolute risk of SIDS for room-sharing infants was 0.00008 (eight per 100,000) when neither parent smoked and the baby was less than three months old, breastfed, and had no other risk factors.

Bed-sharing increased the absolute risk of SIDS by 0.15 per 1,000. This means the absolute risk from bed-sharing was 0.00023 (0.23 per 1,000).

Bed-sharing, smoking and alcohol use all increased the risk of SIDS. However, the risks associated with bed-sharing decreased as the baby got older.

How did the researchers interpret the results?

The researchers concluded that, "Bed-sharing for sleep when the parents do not smoke or take alcohol or drugs increases the risk of SIDS. Risks associated with bed-sharing are greatly increased when combined with parental smoking, maternal alcohol consumption and/or drug use. A substantial reduction of SIDS rates could be achieved if parents avoided bed-sharing."

Conclusion

This large case-control study combined information from five studies to investigate the risk of sudden infant death syndrome (SIDS) associated with bed-sharing.

It found that bed-sharing is associated with a fivefold increase in the risk of SIDS compared with room-sharing for babies who were less than three months old, breastfed, sharing with non-smoking parents, and the mother had not had alcohol or drugs.

Smoking, alcohol and drug use are already recognised risk factors for SIDS and greatly increase the risk associated with bed-sharing.

The researchers have not concluded that babies should not be brought into the parents' bed for comfort and feeding. Rather, they advise that babies should not sleep in their parents' bed. It is also worth highlighting the very small actual risk of SIDS, both for non-sharing and bed-sharing babies.

Although this study was appropriately designed and controlled for, a number of potential factors that could affect the risk of SIDS and the causes of SIDS are not firmly established. This means that there may be other factors responsible for the observed increase in risk associated with bed-sharing.

To decrease the risk of SIDS, current advice recommends:

• putting your baby to sleep on their back – the safest place for them to sleep is in a cot in a room with you for the first six months
• do not smoke
• do not share a bed with your baby, particularly if you have been drinking or have taken drugs
• never sleep with your baby on a sofa
• do not let your baby get too hot and keep your baby's head uncovered
• if possible, breastfeed your baby

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.

Links To The Headlines

Bed-sharing 'raises cot death risk fivefold'. BBC News, May 21 2013

Sudden infant death risk 'is five times higher if the baby sleeps in its parents' bed'. Daily Mail, May 20 2013

Sudden infant death risk greater when parents share bed with babies. The Guardian, May 20 2013

Parents warned sharing bed with babies increases risk of cot death. The Independent, May 21 2013

Sharing bed with babies raises cot death risk. The Daily Telegraph, May 20 2013

Cot Death Risks Of Babies Bed Sharing. Sky News, May 21 2013

Links To Science

Carpenter R, McGarvey C, Mitchell EA, et al. Bed sharing when parents do not smoke: is there a risk of SIDS? An individual level analysis of five major case–control studies. BMJ Open. Published online May 20 2013