Does Vitamin D Make You Lose Weight?

Our ancestors exposed their naked skin to the sun to get the UV radiation they needed to synthesize vitamin D in their bodies (note how this nice-looking Neanderthal girl exposes her skin). Did they get a “Low Vitamin D signal” when the sun was low on the horizon, indicating that it was time to get fat in preparation for winter? Unlikely, and it doesn’t seem that vitamin D is the way to lose weight for us.

You know how fond I am of evolutionary explanations and how I think that natural selection is the key to almost everything in the biosphere. So, I was immediately interested in a suggestion I received from Tom Lundgren in a comment on a previous post. He pointed out a recent book by Jeff Bowles, The Winter Gene, where he argues that the modern obesity epidemic is primarily due to low vitamin D, from sun avoidance and sunscreens.

Vitamin D is an important element in human evolution. Humans are one of a few “naked” species of mammals, that is nearly hairless. They have the peculiarity of not being able to synthesize vitamin D without exposing their skin to UV radiation from the sun. That creates problems for modern humans who tend to spend time indoors and block sunlight using clothes, and recently also using sunscreens. That’s especially true for people living in Northern latitudes, where the sun stays low and even disappears completely in Winter. For them, the main source of vitamin D is from fish, but in modern times people’s diet switched to agricultural products, and that made the lack of vitamin D a health problem.

For centuries, rickets, caused by a lack of vitamin D, was an epidemic problem in Northern European regions. In a book that I wrote with my coworker (The Empty Sea) Ilaria Perissi, we examined this story, and we proposed that the fashion of women’s decolletages in Europe arose when natural selection favored women who exposed more of their skin to the sun because it stimulates the production of vitamin D.

So Bowles’ idea is straight in the evolutionary camp. He proposes that in ancient times, our ancestors got a “signal” from low vitamin D levels caused by the low sun. It told their bodies that it was time to build up fat to prepare for the coming winter. And this ancient signal is still present in our bodies and so it is the lack of vitamin D that makes us fat the year round. Fascinating idea. But does it work?

Something smelled wrong with this book. I confess I didn’t buy it. The claim that low vitamin D is the cause of “autoimmune disease, cancer, diabetes, obesity, mental illness, and heart disease” was a little too much. As a general rule, you shouldn’t believe in people who sell you a single cure for all diseases.

I investigated the claims of the book, and I think I found enough data to confirm my suspicions. The idea is nice, but the literature data are fully against it. Randomized supplementation trials found that giving people vitamin D produces no weight loss (1). I can confirm this from my own experience: reasonably high vitamin D doses make my fingernails stronger. Too high doses give me constipation. Neither caused any weight loss that I could detect.

One problem with Bowles’ idea is that the mechanism he proposes would have worked only for those ancestors of ours who migrated into the Northern Eurasian and American regions. But that was a relatively recent phenomenon — probably not ancient enough to justify the large modifications that Bowles’ proposed mechanism implies. Besides, it would have affected only a small fraction of our ancestors. Most humans lived, and still live, in tropical regions. It is unlikely that such a mechanism would affect all human beings living today.

Besides, there is no biochemical evidence that a “low vitamin D signal” regulates the adaptation of people living in Northern regions. Not even bears use vitamin D as a signal for when it is time to go into hibernation. Bowles’ idea that the widespread use of sunscreens was the cause of the obesity epidemics is just a hypothesis, supported by no specific data. Even the idea that sunscreens reduce vitamin D production is debatable.

Bowles’ book also has strong marketing elements. He proposes high-dose vitamin D as preventing or treating MS, asthma, seventeen kinds of cancer, lupus, arthritis, heart disease, obesity, depression, Parkinson’s, and more, and he also publishes a book called The Vitamin D Conspiracy, exposing a trillion-dollar pharma-politics-media system. Conspiracies do exist, but invoking conspiracy doesn’t make one’s idea necessarily right.

Besides, who is Jeff Bowles? Straight from his own “About” page: he has no degree in any health-related field — his degrees are in real estate finance, a CPA, and an MBA from Northwestern University. He earned those by age 24 and went into real estate development in Chicago, starting his own company at 25. So his credentials are in finance and property development (2). His claim to authority rests on three other things: First, self-directed reading. Second, lineage rather than training — his grandfather and father were Stanford-educated MDs, which he offers as evidence that medicine “runs through my blood and veins.” Third, self-experimentation: the origin of the whole enterprise is his own n=1 subjects vitamin D trial, starting at 20,000 IU/day, then 50,000, then 100,000 IU/day.

I am no MD, either, but Bowles’ protocol looks dangerous to me. He proposes a high-dose D3 of roughly 10,000–50,000+ IU per day, and elsewhere he cites protocols running to 100,000, 200,000, even up to 1,000,000 IU per day. The listed tolerable upper intake is around 4,000 IU/day. Try overdosing yourself with vitamin D (I did, by mistake); you will survive, but you won’t like the results. The fact that Bowles sells a companion book titled How Not To Die With True High-Dose Vitamin D Therapy should tell us something.

And yet, Bowles managed to become a genuinely best-selling self-published author — over 500,000 books sold. It is true that the pharmaceutical industry is overselling drugs to you, but at least they have to maintain a certain semblance of decency and to be based on real data. But people are so desperate to solve their obesity problem, and all the associated troubles, that they will believe in almost anything that frees them from their accumulated layers of fat.

Returning to our (Ilaria and I) idea that generous decolletages help women be healthier, it may be wrong, but at least it is hard to imagine it could do any damage to anyone!

(1) Pathak K, Soares MJ, Calton EK, Zhao Y, Hallett J. "Vitamin D supplementation and body weight status: a systematic review and meta-analysis of randomized controlled trials." Obesity Reviews 2014;15(6):528–537. doi:10.1111/obr.12162 (PMID 24528624)

(2) Bowles’s story much resembles that of Tomas Pueyo and his viral 2020 essay on how to stop the Coronavirus pandemic by “flattening the curve,” something that I defined “the worst model in history.” Just like Bowles, Pueyo had no qualifications in medicine or epidemiology. Yet, his ideas affected the whole world. There is something badly wrong in how we disseminate scientific ideas worldwide.

Acknowledgement: this post was prepared with the assistance of Claude Opus 4.8