An interesting idea was presented recently in Medical Hypotheses, postulating that common obesity (i.e., fatness that does not have an underlying medical cause) is due to a lack of vitamin D.

Most of us experience weight gain over winter, and I had always assumed that it was due to the changes in lifestyle that the winter months necessitate. Physical activity is curtailed because of bad weather; gymming is often eschewed in favor of curling up at home; and heavy, energy-dense food somehow becomes incredibly appetizing. In the northern hemisphere, winter also brings holiday celebrations and wanton feasting.

Certainly, these factors contribute to the winter bulge, but the new idea presented by Dr. Y.J. Foss is that winter weight-gain is an actual physiological, adaptive response by the body to the hardships of winter, and that it’s controlled by the body’s vitamin D status.

Although most people tend to think that the cause of obesity has solely to do with the consumption of excess food and a lack of exercise, there are still factors that defy explanation.

For example, if we assume that the cause of obesity is a mismatch between genes and environment – i.e., some people have a ‘thrifty’ genotype, one that prefers to conserve energy, and they live in the modern western world, where there is an overabundance of food and little need for physical activity, it remains unknown why there is such a difference between different individuals in the same environment. However, if we invoke genetic differences between people as the explanation for individual differences, there is no explanation for why the initiation of weight gain occurs at different points in the course of a person’s life, nor is there an explanation for large-scale trends towards weight gain depending on geographical location, industrialization, and other societal differences.

Also, it remains that most interventions that focus on altering the lifestyle of the overweight individual fail in the long term, which suggests the existence of an as-yet-unexplained causative factor for weight gain.

Therefore, the hypothesis that vitamin D deficiency might be the missing link has been presented.

The body has a seasonal cycle that tends to favor energy storage during the cold, winter months. This ‘winter response’ is thought to be controlled by the levels of circulating vitamin D, which is synthesized by the skin in response to UV light.

So think about it:

• Even fat people usually don’t tend to constantly increase in weight – even though they experienced weight gain, there is some kind of brain-controlled mechanism that keeps their weight stable, even if excessive
• Animals also have a tendency to be larger in cooler climates, indicating weight-gain is a controlled, adaptive response to the environment
• What we consider to be ‘metabolic syndrome’ is actually very similar, if not identical, to the “winter metabolism”

So, it now remains to be tested whether obese individuals have lower plasma calcidiol levels (the form of vitamin D that fluctuates most according to exposure to sunlight). It’s a very exciting theory, because vitamin D deficiency has the potential to explain both large scale trends and individual variation with regard to obesity. It can be inferred that on a population-wide scale, vitamin D status has fallen in line with the increase in obesity – both obesity and low vitamin D status are both artifacts of the urban-industrial lifestyle. Whether the two are linked will be investigated in future study.

One wonders if our friend in the picture above would do well to make a habit of what he’s currently doing.

Reference: Foss YJ, Vitamin D de?ciency is the cause of common obesity, Med Hypotheses (2008), doi:10.1016/j.mehy.2008.10.005
Photo credit: Kyle May on Flickr