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Could Vitamin D Help You Avoid the Flu?

A sure sign of fall is the plethora of flu shot advertisements popping up everywhere. Though the flu shot may reduce the likelihood of contracting flu in some people or potentially reduce the symptom severity, there are other ways to reduce the chance of getting sick.

The flu shot, of course, does not guarantee one will not get the flu, nor do the best hygiene or nutritional habits. 

However, a low-risk nutritional choice you can make to support your immune system is to optimize your levels of vitamin D.

A growing amount of research affirms vitamin D's critical contribution for a healthy immune system, along with its other benefits such as helping to lower visceral fat levels, improve strength and balance, and lower levels of inflammation.

Vitamin D is produced when the skin is exposed to sunlight. Many people know this, but what they don’t realize is that to produce sufficient amounts of vitamin D, the sunlight needs to be at peak times with a majority of the skin exposed. Light is not direct enough if your sun exposure comes from going for a run after work. If you go for a walk over lunch, the sunlight may be direct enough, but it’s not likely you’ll have enough skin exposed.

Vitamin D and Flu Season

R. Edgar Hope-Simpson, a British family practice doctor, was the first to propose the idea that the yearly flu outbreak was related to the seasons of the year. He showed that flu outbreaks peaked during the two months surrounding the winter solstice. This can be observed in both hemispheres and around the world.[i]

During the summer months in both hemispheres, influenza is virtually nonexistent. Aside from the flu, the common cold, which is actually a variety among more than 200 different viruses, also has a peak during the winter months.

Though people may be less likely to spend leisure time outdoors during the colder months, it’s unlikely that the higher rates of sickness are caused by being around others more. Instead, it’s more likely that the lower exposure to sunlight causes a physiological change in the body.

The temperatures become cooler as certain hemispheres get less direct sunlight. This causes people to spend more time indoors, and even when they’re outside they do not get UVB light like they do during summer months. 

Vitamin D levels in many people fall considerably in the fall and winter months. For those who do not supplement with vitamin D, vitamin D levels in the winter can become quite deficient. Even those who supplement with a regular dose throughout the year may find their winter vitamin D levels become too low. Vitamin D triggers the body to make its own antibiotics called antimicrobial peptides (AMPs). These AMPs have been shown to inactivate the flu virus.[ii],[iii]

Although it’s a northern climate country, Norway maintains the highest levels of vitamin D in Europe due to the population’s high levels of year-round fish consumption. There is also little rise in flu outbreaks in Norway compared with other European countries like Great Britain.

To make things even more interesting and show the need for UVB exposure to help increase vitamin D levels, there is a relationship between occurrence of flu and solar flares. Solar flares cause changes in ozone which block UVB light waves from penetrating the earth’s surface. Reduced UVB and reduced vitamin D: increased occurrence of the flu.

Researchers have also shown that when individuals are inoculated with the flu virus at various times of the year, they show evidence of the flu far more often in the winter than in the summer; 6.7% of people show evidence of the flu during the winter, and only 0.8% show evidence of the flu during the summer.[iv]

Does this mean vitamin D will help cure the flu? Probably not. In fact, it’s possible that if you take extra vitamin D once you’ve gotten sick, it may not have much of an effect. But in regards to flu prevention, you should look at optimizing vitamin D.

In this way, once your body is exposed to the flu virus, your immune system will be prepared to properly attack it. Though there is much more research that needs to be done, the evidence to date creates a compelling case to say that if you optimize your vitamin D levels, it’s less likely you’ll get sick when your immune system encounters a pathogen. This leads us to a question. How much vitamin D is necessary to reduce the likelihood of getting sick?

How Much Vitamin D?

Rather than asking the question, “How much vitamin D should I take?,” a better frame might be “What level should my blood level of vitamin D be?” When you consider how little vitamin D people get through their diets and how little direct sunlight they’re exposed to, it’s easy to suggest they supplement with “more” vitamin D, but how much more requires personalization. And personalization requires a bit of trial and error, including occasional blood testing to check your blood levels of vitamin D. We’ll talk about that shortly.

Most people, given the choice, would like to do more than avoid sickness but would rather experience optimal health. When it comes to blood levels of vitamin D, there’s quite a range between the levels necessary to avoid clinical deficiency and the levels required to achieve optimal health.

Blood vitamin D levels above 10 ng/mL may help one avoid rickets and osteomalacia, but it takes more than 20 ng/mL to suppress parathormone levels. Levels of more than 30 ng/mL are necessary to increase calcium absorption in the intestine. Levels greater than 50 ng/mL have been shown to improve physical performance, especially in the elderly. The Vitamin D Council suggests an optimal range of 50–80 ng/mL.

Part of the difficulty in convincing the public about the prevalence of vitamin D deficiency revolves around the inconsistent opinion on what blood level determines deficiency. Studies vary regaring what researchers deem a vitamin D deficiency.

For example, the National Institutes of Health (NIH) views a blood level under 12 ng/mL as a deficiency and suggests that a level over 20 ng/mL is sufficient for most healthy adults. Though a level above 20 ng/mL may help to avoid severe deficiency conditions, growing bodies of research say this level is far from optimal. A 2011 European study showed that 80% of adolescents had suboptimal levels of vitamin D - less than 30 ng/mL.

Whatever one chooses to use as a level of deficiency, it’s clear that the majority of people have far from optimal levels of circulating vitamin D.

How Much Vitamin D Should I Take?

The amount of vitamin D one can safely take varies dramatically from one individual to the next. Personally, I get 1000 IU per day from the Men’s Performance Multivitamins, another 5000 IU per day from the Douglas Labs vitamin D I take once per day, and 150 IU from the Cal/Mag 1001 I take twice per day. There’s a small amount in food, so my daily intake keeps me around 6200-6500 IU per day. That keeps my blood level at about 80 ng/mL, but I make a point of getting it checked each year with the Premium Longevity and Vitality Test. The vitamin D levels from my most recent Longevity and Vitality test are shown below.

I’ve found an amount that works for me, but I have a friend who was taking almost the exact amount I was and his vitamin D levels were higher than 120 ng/mL, much higher than necessary. Another friend was taking about 12,000 IU per day because he knew some extra vitamin D was good for him, but when he had his blood tested, his values exceeded 120 ng/mL as well. In both cases, they reduced their daily intake and will get retested in the future to see if their levels have come down to a more optimal level.

Many studies have been completed with older adults who took very high amounts of vitamin D and saw much smaller rises in blood levels. For example, elderly adults were given 50,000 IU per day for 10 days and saw a modest rise in blood levels. Some elderly patients are treated with vitamin D injections to quickly get their blood levels up. One study used injections of 600,000 IU and found the injections were safe.

Looking at more typical supplemental levels, among a group using 2000 IU per day for a year, 40% were unable to get levels above 32 ng/mL. Another group supplemented with 4000 IU per day for six months and got to an average of 44 ng/mL.

The bottom line is this: if you don’t get your vitamin D levels checked, you really won’t know how much you need to take to achieve optimal levels. Around 1000–2000 IU per day may be helpful and lead to better levels than most people have, but it doesn’t mean it will be enough to get to optimal levels. If you’ve had most of your other blood work done recently, you can order a comprehensive vitamin D test by itself. Of course, if you get it as part of a larger test package, you can save some money overall and find out a lot more about the health of your metabolism.

If you are lucky enough to get quality sunlight throughout most of the year, you may not need to supplement with vitamin D, but most people don’t get enough - even in southern states. We also tend to overestimate how much sun we actually get. Since many people do not supplement with enough vitamin D and they don’t get enough quality sun, it leaves them deficient in this important nutrient. As you contemplate the importance of supplementing with vitamin D, let me leave you with the conclusion of researchers who published an article in the European Journal of Clinical Nutrition:

“Increasing serum 25(OH)D levels is the most cost-effective way to reduce global mortality rates, as the cost of vitamin D is very low and there are few adverse effects from oral intake and/or frequent moderate UVB irradiance with sufficient body surface area exposed.”

Sun and supplementation are the main ways to get blood levels of vitamin D high enough in most people. Just remember that more of either isn’t necessarily better. The only way to know how your habits are affecting your blood levels is to get your blood tested. If you keep your levels in the optimal range, most research shows it can have a significant effect on reducing the occurrence of flu - and provide a variety of other health benefits.

Have you had your vitamin D levels checked? Talk with a club dietitian or your physician about testing. Thanks for reading.

Written by Tom Nikkola – Former Director of Nutrition and Weight Management


This article is not intended for the treatment or prevention of disease, nor as a substitute for medical treatment, nor as an alternative to medical advice. Use of recommendations in this and other articles is at the choice and risk of the reader.

[i] Cannell JJ, Vieth R, Umhau JC, et al. Epidemic influenza and vitamin D. Epidemiol. Infect. 2006;134:1129-1140

[ii] Reddy KV, Yedery RD, Aranha C. Antimicrobial peptides: premises and promises. International Journal of Antimicrobial Agents. 204;24:536-547

[iii] Daher KA, Selsted MD, Lehrer RI. Direct inactivation of viruses by human granulocyte defensins. Journal of Virology. 1986;60:1068-1074

[iv] Shadrin AS, Marinich IG, Taros LY. Experimental and epidemiological estimation of seasonal and climato-geographical features of non-specific resistance of the organism to influenza. J Hyg, Epidem, Micro, and Immuno. 1977; 21:155-161

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Reader Comments (2)

I had my Vitamin D checked last fall and it showed a 29.9 ng/dl. I think sun is the best way to increase my levels to 50. Vitamins are chemically produced and are not natural in any way. Stick to the sun.

September 10, 2012 | Unregistered CommenterBeytZim

very good post, I actually love this information,this is what I looking for. thanks

September 13, 2012 | Unregistered Commenterslimfast
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