Could the reason why some people put weight on in the winter be down to vitamin D?

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I was having a casual conversation with a lady a few weeks ago on a cold and dull day. She mentioned that she was training for the London marathon (and racking up some significant mileage by the sounds of things) and eating what seemed to me to be a genuinely healthy diet (not rammed full of starchy carbs, for instance). Yet, she was putting on weight. I suppose it’s always possible, though unlikely, to build significant quantities of muscle through running. But, by her own admission, this lady felt that the additional weight was coming in the form of fat: she felt she could pinch more fat than before.

This lady was looking for an explanation, and one of the things that occurred to me at the time is that this lady’s paradoxical weight gain might have not much to do with her exercise and eating regime, but perhaps some more external facture. It was the depth of winter, after all, and it’s not so unusual for individuals to find they put on weight during the colder months.

There are several potential reasons for this. Some suggest, for instance, that during the winter we tend to gravitate to heavier, more calorific food, and at the same time are generally less active than when the weather is more agreeable. Also, one could argue that there is less ‘pressure’ to not be carrying excess weight during the winter, as it’s less likely our body will be revealed to others in public, like on the beach.

These things may be true, but is it possible that the weather itself has some role to play in weight? This may sound far-fetched, but a couple of studies recently have found that lower levels of vitamin D (as found during the winter) are associated with higher levels of body fat. One of these studies has yet to be published but can be read about here. The other was published in January’s edition of the Journal of Clinical Endocrinology and Metabolism [1]. The first study used teenagers as its subject, while the other used females aged 16-22.

Obviously, it’s possible that these results will not be seen in older individuals. And both these studies were ‘epidemiological’ in nature, and can only really tell us that there’s an association between vitamin D levels and body fat. It might be that vitamin D has no direct effect on body fat levels. One way to test this would be to see what effect boosting vitamin D levels (e.g. through food, supplements or sunlight exposure) has on body fat in the context of a randomised controlled trial. To my knowledge, no such studies have been published.

However, I was interested to read about a recently-published article in the journal Medical Hypotheses that puts forward the theory that vitamin D deficiency may actually cause obesity [2]. The idea put forward concerns the concept that there are evolutionary advantages to accumulating fat in the winter.

For a start, fat is a fuel store for the body, and this may have been especially important in evolutionary terms in the winter, when food is likely to have been in shorter supply than during warmers times of the year. Also, subcutaneous fat affords the body some insulation, which enhances the chance of surviving the harsh weather conditions winter can bring. Putting on weight also reduces the surface area to volume ratio of the body, which helps the body in terms of maintaining internal temperature and reduces the risk of hypothermia.

The author of this article suggests that lower levels of vitamin D may provide a signal to the body that winter is coming or has come, and triggers a switch to a ‘winter metabolism’ which predisposes to, among other things, weight gain. The suggestion is that ensuring optimal vitamin D levels may help to combat obesity.

To test this theory properly intervention studies are required. However, advances in medical science generally start as a hypothesis, and the one that vitamin D deficiency triggers fatty accumulation in the body seems reasonable. It might explain how someone training for a marathon during the winter months could end up putting on weight, rather than losing it.


1. Richard Kremer, et al. Vitamin D Status and Its Relationship to Body Fat Final Height, and Peak Bone Mass in Young Women. J Clin Endocrinol Metab 2009;94:67″73

2. Foss YJ. Vitamin D deficiency is the cause of common obesity. Medical Hypotheses 2009; 72(3):314-321

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