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International Journal of Obesity
and related metabolic disorders

COLD BABIES MAKE FAT ADULTS

The idea that your metabolism could be determined by your star sign is unlikely to be taken seriously by even the most ardent disciple of astrology, and yet two scientists have just published some data suggesting that your chances of being obese could be determined by the season of your birth. David Phillips at the University of Southampton and Jim Young of Northwestern University in Chicago have just published a paper in the International Journal of Obesity in which they analyse the birth-weight and current weight and obesity of 1750 men and women born in the English county of Hertfordshire between 1920 and 1930. Obese adults were defined as those having a Body Mass Index greater than 30 (BMI = weight divided by height squared; kg/m²). As in studies by other workers, they found that BMI increased with increasing birth weight - i.e. big babies tend to make big adults. However, this was only true for those born in the first 6 months of the year, and there was no relation between birth weight and BMI in those born July-December. Moreover, this trend for higher birth weight and BMI in those born January-June meant that the prevalence of obesity was much greater in this group.

It is well known that our body weight is influenced by our genes and that body size tends to track throughout life, but the observation that the relation between birth weight and adult weight only applies to those born in the winter months indicates that there also must be an early environmental, non-genetic influence on the factors controlling body weight throughout the next 60-70 years of life. This early seasonal effect prompted Phillips and Young to analyse data from the Meteorological Office for the decade 1920-1930 to see if some winters were colder than others, and if the average temperature in December or January was above or below 4°C, winters were classified as mild or cold, respectively. Analysing the data in this way showed that the relationship between birth weight, BMI and obesity in subjects born in the first 6 months of the year only applied to those born following a cold winter. In other words, those babies born in the first 6 months of the year when the winter was cold were much more likely to be obese than those born in the first 6 months following a mild winter or in the last 6 months of any year.
This study does not prove that environmental temperature is the cause of the seasonal influence on adult BMI, but it is consistent with some animal data showing increased adiposity and weight gain in young animals exposed to the cold. It is not known if this seasonal imprinting or programming is due to increased food intake, decreased metabolism or a bit of both, and it will be very difficult to find out. Whatever the explanation, obesity now joins a long list of other disorders (e.g. diabetes, heart disease, asthma, arthritis, schizophrenia and manic-depressive disorder) that exhibit differences in prevalence according to the season of birth.

These seasonal effects provide a new and intriguing area for further research, but one cannot help wondering how long it will be before the astrologers catch-on to the idea of offering people medical profiles based on their star sign!

Further information may be obtained from Dr J B Young (fax: +1 312 503 8104; email: jbyoung@nwu.edu) or Dr D I W Phillips (Fax: +44 (0)1703 704021; email: diwp@mrc.soton.ac.uk).

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