Cold weather may increase blood pressure
'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.
