Vitamin D and weight loss – not what you think!

Vitamin D has had a great decade in the press. Some of the enthusiasm is earned. Some of it has tipped into the kind of religious devotion that should be reserved for things that actually pan out in large trials. I have read Dr. Holick’s book on vitamin D twice over, and my read is that his actual message has been distorted by the internet gurus louder than he is. So let me give you my position on vitamin D, with one specific application: weight loss.

The dose question, briefly

In 30 years of medicine I have seen exactly one case of vitamin D toxicity and many, many cases of deficiency. Toxicity is rare. Deficiency is the default state for most of the indoor working population. So the bias should generally lean toward supplementation, but not without checking.

Here is where I differ from some of the more aggressive voices. I do not think a flat 10,000 IU per day is the right dose for everyone. It probably will not cause toxicity in most people, but the plasma 25 hydroxy vitamin D curve is not linear. The data suggest a U shape, where outcomes improve from deficient up to roughly 45 to 70 ng per mL, then plateau, then turn negative at substantially higher levels. Mortality data at very high plasma D levels, while not yet conclusive, are enough to recommend caution against the more is better default.

Practical rule. If you supplement vitamin D, test your 25 hydroxy level once or twice a year. Aim for 45 to 70 ng per mL. Adjust dose accordingly. Most adults land in the right range somewhere between 2,000 and 5,000 IU per day.

Vitamin D and body weight

Two observations from the literature are worth knowing.

First, there is a steady inverse association between plasma vitamin D and body weight across many observational studies. The lower the level, in general, the higher the weight. Whether vitamin D deficiency contributes to weight gain, or whether higher adiposity sequesters vitamin D in fat tissue and lowers circulating levels, or both, is not fully resolved. Both directions appear to be real.

Second, and more interesting from a practical standpoint, pre diet vitamin D levels appear to predict dietary success. Subjects entering caloric restriction with higher baseline vitamin D levels lost more weight, on average, than those entering with lower levels. The effect is statistically meaningful in several trials.

So is vitamin D a weight loss aid? Not really, not on its own. But sufficiency before you start a weight loss effort appears to set you up for better results. Combined with everything else vitamin D does, hitting your level is one of the smaller cost, higher return items on the list.

Why omega 3 status matters here too

Vitamin D works through receptors that sit in cell membranes. Vitamin D has to be transported into the cell and then into the nucleus to exert its epigenetic effects. That transport depends on healthy, fluid cell membranes. The fluidity of those membranes depends on the fatty acid composition, which depends on dietary fat intake, which for most Americans means inadequate marine omega 3 fatty acids and excess omega 6.

In other words, vitamin D works best when the membranes it has to cross are built well. Adequate omega 3 status, evidenced by an omega 3 index of 8 percent or higher, supports vitamin D function the same way it supports a long list of other receptor based biology.

This is one reason I rarely recommend a single supplement in isolation. The interactions are usually more important than the headline molecules.

What to do

• Test your 25 hydroxy vitamin D level every 6 to 12 months.
• Aim for 45 to 70 ng per mL.
• 5,000 IU per day is a reasonable adult starting dose if you are deficient. Adjust based on retest.
• Get some sun. Sun synthesis is still meaningful even in temperate latitudes during summer, with the obvious caveat about skin cancer and judgment about exposure time.
• Test your omega 3 index. Aim for 8 percent or higher. Membrane biology underwrites a lot of receptor function, including vitamin D.
• If you are planning a serious weight loss effort, hit your vitamin D level before you start.

Vitamin D is not the magic story the internet wants it to be. It is also not the disposable supplement the conservative medical community sometimes treats it as. Test, dose, retest, hold the level steady. That is the boring version, and it is the version that actually pays.

— Doc