Most experts agree it's not. The majority of athletes – like the majority of people in the general population – are deficient in vitamin D.

Treating this deficiency can help athletes prevent stress fractures as well as maintain a healthy vitality. If this also happens to improve the athlete's reaction time, muscle strength, speed, and endurance, well, that's just a very nice side effect of getting adequate vitamin D.

So vitamin D has been making waves in the athletic community since at least 1927, but it's also becoming a hot topic in another field: life extension. Add to this some evidence that it could help with fat loss and strength gains, and you just might have...

Dr. Jonny Bowden calls vitamin D the most underrated "vitamin" on the planet.

Dr. Ziegenfuss, a researcher and sports nutritionist to elite athletes, tests himself often to make sure he's getting enough. He even tests his kids for it and supplements them as needed.

Coach Eric Cressey says vitamin D might just be the next fish oil. He makes sure the athletes under his care get plenty of it. Charles Poliquin does the same.

And finally, medicinal chemist Bill Roberts says that you should absolutely be taking vitamin D.

What about the stuffy and often behind-the-times nutritional organizations and agencies? Well, the FDA has stated that they're likely going to up their vitamin D recommendations the next time they release new standards.

In 2008, the American Academy of Pediatrics doubled the amount of D they recommend for kids (from 200 IU per day to 400 IU per day).

And the Department of Family and Consumer Sciences has recommended that sports nutritionists assess levels of vitamin D in their athletes. If they're getting too little, they contend it will compromise the athlete's ability to train.

From government agencies to in-the-trenches trainers, the trend is clear: vitamin D is important. And if you think you're getting enough of it from natural foods, fortified foods, and sunlight, then think again, Sunshine.

Three reasons: Longevity, performance, and lookin' good naked.

Let's break those down:

1. Longevity

You know what really gets in the way of building muscle, losing fat, and benching a ton?

Death.

The New England Journal of Medicine recently warned that the number of diseases associated with vitamin D deficiency is growing. And who's deficient? Most people, the studies seem to be saying, including otherwise nutrition-conscious athletes and gym rats.

In one mind-blowing study using population data, researchers found that total mortality was 26% higher in those with the lowest D levels compared with the highest.

And a meta-analysis of 18 randomized controlled trials found that supplemental vitamin D significantly reduced total mortality. That means quite simply this: vitamin D supplementation prolongs life.

Here's just a handful of examples:

• According to the Vitamin D Council, current research has implicated vitamin D deficiency as a major factor in the pathology of at least 17 varieties of cancer.
• Vitamin D may protect against both Type I and Type II diabetes.
• Low D may contribute to chronic fatigue, depression, and seasonal affective disorder.
• Parkinson's and Alzheimer's sufferers have been found to have lower levels of D.
• Low levels of vitamin D may contribute to "syndrome X" with associated hypertension, obesity, diabetes, and heart disease.
• Administration of dietary vitamin D has been shown to lower blood pressure and restore insulin sensitivity.

In short, if you care about living a good long life, then vitamin D looks like it could certainly help with that goal.

2. Performance

Studies on vitamin D, sunlight, and performance go back for decades. Russian studies in the 1930's showed that 100 meter dash times improved in irradiated athletes vs. non-irradiated athletes undergoing the same training – 7.4% improvement vs. 1.4% improvement.

German studies in the 1940's showed that irradiation lead to a 13% improvement in performance on the bike ergometer vs. no improvement in the control group.

In the 1950's researchers saw a convincing effect on athletic performance after treating athletes at the Sports College of Cologne. Findings were so convincing that they notified the Olympic Committee.

At one point, even school children were irradiated and given large doses of vitamin D in 1952 Germany. Treated children showed dramatic increases in overall fitness and cardiovascular performance. UV radiation was also shown to improve reaction times by 17% in a 1956 study.

In the 1960s, a group of American college women were treated with a single dose of ultraviolet irradiation. The results: improvements in strength, speed, and endurance.

Other studies showed "distinct seasonal variation" in the trainability of musculature. Athletes performed better and got stronger in the late summer due to their greater exposure to the sun and subsequent vitamin D production.

Vitamin D has also been shown to act directly on muscle to increase protein synthesis. Deficient subjects administered vitamin D showed improvement in muscle protein anabolism and an increase in muscle mass.

Improvements in neuromuscular functioning have also been seen. People with higher levels of vitamin D generally have better reaction time and balance.

3. Looking Good Naked

If vitamin D improves the effects of training and helps to stave off illnesses, then it's easy to see how this can translate into an improved aesthetic: you're healthier, you feel better, you get more out of your training, and you end up looking better when you make sexy-time.

But there could be a more direct effect as well. Research shows that adding vitamin D to a reduced-calorie diet may lead to better, faster weight loss. In one study, excess body fat came off faster when plenty of D3 was present, but it also came off the abdominal area.

The icing on the cake? The same D-supplemented subjects retained muscle mass while losing the fat.

In other studies, subjects receiving vitamin D therapy lost weight, lost their sugar cravings, and saw a normalization in blood sugar levels.

Before we get to the recommendations, let's review some vitamin D basics and some little known facts:

• There is no RDA for Vitamin D due to "insufficient evidence." But there is an AI or Adequate Intake recommendation:

• Ages 19-50: 200 IU
• Ages 51-70: 400 IU
• Over age 70: 600 IU

That means this is the amount assumed to ensure nutritional adequacy: sufficient to maintain bone health and normal calcium metabolism in healthy people. Suffice it to say, these are bare minimums that newer evidence suggests are way too conservative.

• There aren't that many foods in nature containing high amounts of vitamin D. The best source is halibut liver oil, followed by cod liver oil, salmon, tuna, and mackerel.

And by the way, farm-raised salmon has been shown to have 25% less vitamin D than wild salmon. And cod liver oil? Good source of D but also high in vitamin A, which can be toxic if over-consumed. Do not use cod liver oil alone to boost your vitamin D intake!

Beef liver, cheese, and egg yolks contain a smidge. Foods like milk do contain vitamin D but only because manufacturers add it into fortified milk, fortified cereal and the like. Milk was fortified back in the 1930's to combat rickets, and it worked.

Despite all of this, those who wish to maximize the benefits of a higher vitamin D intake wouldn't be able to get enough through food sources alone. And of course a lot of that "fortified" food is still make-you-fat food, avoided by most physique athletes.

• Most people get their D through sunlight. The basic intake guidelines are: 5-30 minutes of sun exposure between 10 AM and 3 PM at least twice a week without sunscreen. But much depends on where you live, the pollution levels, cloud cover, age, the season of the year, your natural cutaneous melanin content.
• Another factoid: While it's possible to get too much vitamin D, you can't get too much from the sun, only from over-supplementing.
• What about tanning beds? Moderate use of commercial tanning beds that emit 2-6% UVB radiation can help, but of course there's that whole skin cancer thing to consider.
• Sunlight that comes through glass doesn't count. Most UVB radiation doesn't penetrate glass, corner-office boy.
• If using the sun to get your D, remember that cholesterol-containing body oils are critical to the absorption process.

Some experts say that because the body needs 30 to 60 minutes to absorb these vitamin-D-containing oils, it's best to delay showering for about an hour after sun exposure. And don't jump right into the pool either as these natural oils can be stripped by chlorine.

In researching this article, I looked to find a consensus among the experts. Here's what I've found:

• As a general rule, Dr. Clay Hyght recommends 1,000 IU per day. This represented the low end amongst our experts, but note that it's still way over current government guidelines.
• One of the world's foremost experts on vitamin D, Dr. Reinhold Vieth, says levels should be in the range of 4,000 IU from all sources.
• Dr. Jonny Bowden has recommended 2,000 IUs per day.
• Dr. Ziegenfuss personally keeps his levels of 25-hydroxy D at 50 to 100 ng/mL. That means he uses around 4000 IU per day. He notes that when he took 1000 to 2000 IU per day his levels rarely hit 40.
• Bill Roberts has noted that 4,000 IU a day can be a substantial help to fat loss.
• The Vitamin D Council says that those who rarely get sunlight need to supplement with 5,000 IU per day. Note that this would take 50 glasses of fortified milk a day or 10-12 standard multivitamins, hence the need for targeted supplementation.
• Dr. Robert P. Heaney of Creighton University estimates that 3,000 IU per day is required to assure that 97% of Americans obtain levels greater than 35 ng/mL.

So the government says 200 to 400 IU for most of us, but even they admit that's low. Those more in-the-know suggest anywhere from 1000 to even 5000 IU per day.

But this may depend on how much sunlight you get and your ethnicity. Some estimate that dark-skinned individuals may need double the amount of D that a pasty white guy needs.

T Nation will leave your personal dosage choice up to you and maybe your physician (if he knows a damn). If you really want to dial this in, we suggest getting tested.

1. When looking for a vitamin D supplement, choose the D3 form. Gel caps are probably best. Liquids are favored by some. Since D is fat soluble, take with foods containing a little fat to optimize absorption. Polyunsaturated and monounsaturated fatty acids are best.
2. Get some sun when you can, but don't burn. The occasional use of tanning beds is also fine, particularly in the winter.
3. If in doubt, test. The test you want to ask for is 25 (hydroxy) D. That's 25-hydroxyvitamin D, not 1,25-dihydroxyvitamin D. The Vitamin D Council says you should shoot for blood levels between 50-80 ng/mL. The average American in late winter averages about 15 to 18 ng/ml, which would be considered a serious deficiency. Your doctor can give you this test and some home testing kits are available (around $65 each).
4. It's wise to ensure adequate calcium intake when increasing your intake of vitamin D.

Yes. But it's unlikely. Some doctors suggest that critical toxicity may occur at doses of 20,000 IU daily (for many months), and that the Upper Limit (UL) of safety be set at 10,000 IU, rather than the current 2,000 IU.

So while toxicity issues exist, you probably won't have to worry about it when staying at 5000 IU per day or less according to most forward-thinking researchers and nutrition experts.

The Public Health Agency of Canada says that vitamin D may offer protection against viruses like Swine Flu.

In short, if you get plenty of vitamin D and catch the flu, it's a mild illness. If you're lacking – and most people are, especially in the winter – then you're more likely to develop full-blown symptoms.

The message is loud and clear: It's time to start "doping" with vitamin D.

1. Melamed ML et al. 25-Hydroxyvitamin D levels and the risk of mortality in the general population. Arch Intern Med. 2008;168(15):1629-37. PubMed.
2. Autier P et al. Vitamin D supplementation and total mortality: a meta-analysis of randomized controlled trials. Arch Intern Med. 2007;167(16):1730-7. PubMed.
3. Cannell JJ et al. Athletic Performance and Vitamin D. Med Sci Sports Exerc. 2009 May;41(5):1102-10. PubMed.