A new study published in the British Medical Journal December 2019 suggests a novel risk factor for heart disease – the season in which the individual is born. The large study carried out in the US shows that people born in either spring or summer had a significant, though slight, increase in the risk of fatal heart disease, when compared to the autumn-born. Why this should be so is unknown as of now.
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However, this confirms older studies that have also shown a higher risk of death for those born in these seasons in the northern hemisphere, with those born in November showing the lowest risk. In the southern hemisphere, the situation is opposite.
Swedish studies including a total of 10 million people, for instance, have shown that those born in November are at the lowest risk for cardiovascular death, but those born between January and August had the highest, especially during March/April. Similar findings have come from the US, Denmark, Germany, Japan and Lithuania.
These studies were limited by their failure to adjust for confounding factors such as a low socioeconomic position, a family history of risk factors for potentially fatal illnesses, and so on.
The current study
To avoid this issue in the present study, the researchers took data obtained from over 115,000 registered nurses in the US, as part of the Nurses’ Health Study carried out in 1976. They looked ae overall death rates, and specifically at risk of death from cardiovascular disease, in association with the seasonal timing of birth.
The participants were 30-55 years old when they started the study. Each of them filled out a detailed form on their health and lifestyle, which was repeated at intervals of 2 years. At present, the study is 38 years old, allowing for a long follow-up period with respect to their medical history. The researchers had access to the death and medical records of the participants over this period.
They found that over 43,000 of them died during this period. Among these deaths, almost 8,400 were due to cardiovascular disease. Women born in the winter, in the Nurses’ Health Study, had a higher body mass index.
While overall death rates failed to show any consistent linkage with birth season, there was a small but definite rise in cardiovascular deaths among those born in spring and summer. This remained significant even when factors like the family history and socioeconomic status was taken into account.
The fact that this is an observational study precludes drawing any cause-and-effect inferences from it. The study conclusions are supported by the number of participants, the long period of follow-up, and the wealth of information about the participant’s lifestyle, family history, social position and economic status.
Researchers speculate that heart disease risk could be mediated in part by the way the dietary pattern, air pollution, and the availability of sunlight both during life in utero and soon after birth vary with the seasons. For instance, fruit and vegetable intake could be much lower during certain months, while infections and inflammatory conditions could peak at other times. Sunlight exposure mediates vitamin D levels. Vitamin D has been suspected to be a factor affecting cardiovasculare health in adults.
There could perhaps have been some factors that are not recognized but have influence on the deaths in at least some cases. The researchers say, “Season and month of birth represent proxies for various complex environmental and other factors (eg, familial and socioeconomic factors) in prenatal and early postnatal life.”
Seasonal factors operating in very early life, even before birth, such as inadequate sunlight exposure, could be continuing to act in an adult to increase the risk of heart disease, stroke and other cardiovascular events. This is called the fetal origins hypothesis and is built on the concept that health and disease have developmental origins.
Another factor to be considered is that in older studies, the participants were born in or before the first part of the last century, a time characterized by greater seasonal fluctuations in food supply and in infectious diseases. In addition, climate and temperature variations, as well as changes in the social and economic conditions in various countries.
In sum, the researchers say their study adds to the proof that, “individuals born in the spring and summer have higher cardiovascular mortality than those born in autumn, but conflicts with previous findings on overall mortality.”
The study will need to be supported by more research, according to the scientists. “Further investigations are required to confirm current findings and uncover mechanisms of seasonal birth month effect in cardiovascular mortality.”
Birth month, birth season, and overall and cardiovascular disease mortality in US women: prospective cohort study BMJ 2019; 367 doi: https://doi.org/10.1136/bmj.l6058 (Published 18 December 2019), https://www.bmj.com/content/367/bmj.l6058