The Truth About Vitamin D: 10 Facts on Levels, Benefits and Safety
Last reviewed and updated: October 30, 2026
Vitamin D is one of the most talked-about supplements of the last decade, and also one of the most misunderstood. Some people take far too little, a few take far too much, and many are confused by headlines that seem to change every year.
Here is the truth about vitamin D: what it is, how your body makes it, who tends to run low, what the best research actually shows, and how to raise your levels safely.
What Is Vitamin D?
Vitamin D is a fat-soluble vitamin that acts more like a hormone in the body. Your skin makes it when exposed to the sun’s ultraviolet B (UVB) rays, and you can also get it from food and supplements.
There are two main forms:
- Vitamin D3 (cholecalciferol): the form your skin makes and the form found in animal foods such as fish and egg yolks.
- Vitamin D2 (ergocalciferol): the form made by plants and fungi, such as UV-exposed mushrooms.
Your liver converts both forms into 25-hydroxyvitamin D, the form your doctor measures in a blood test. Your kidneys and other tissues then convert it into the active hormone, calcitriol. A meta-analysis of controlled trials found that vitamin D3 raises blood levels more effectively than D2, which is why most practitioners prefer D3.5 I covered newer research on this in Vitamin D3 vs D2.
The Truth About Vitamin D: 10 Facts Worth Knowing
1. Low vitamin D is common, especially in winter
A large share of adults have vitamin D levels below what most doctors consider ideal, and levels drop further in fall and winter. In much of the northern United States, the winter sun sits too low in the sky for your skin to make much vitamin D at all, no matter how long you are outside.
Fall and winter are also the seasons when colds and flu tend to spread, which is one reason researchers have looked so closely at vitamin D and immunity.
2. Modern life works against vitamin D
Our grandparents spent much of their day outdoors. Today most of us work, drive, and relax indoors. Factors linked with lower vitamin D levels include:
- Limited sun exposure: indoor jobs, long winters, and covering up outdoors.
- Darker skin: more melanin means the skin makes less vitamin D from the same amount of sun.
- Age: older skin makes vitamin D less efficiently, and older adults often spend less time outside.
- Digestive conditions: problems absorbing fat, such as celiac disease or Crohn’s disease, can reduce vitamin D absorption.
- Excess body weight: vitamin D is stored in fat tissue, so people with more body fat tend to have lower blood levels.
- Sunscreen: sunscreen is important for protecting your skin, and it also reduces vitamin D production.
- Few food sources: very few foods naturally contain much vitamin D.
3. Vitamin D is often lower in people who are overweight
Because vitamin D is fat-soluble, it gets distributed into fat tissue. When there is more fat tissue, the vitamin D in your body is spread across a larger volume, so blood levels tend to read lower. People carrying extra weight often need more vitamin D to reach the same blood level.
The good news is that healthy weight loss tends to help. If that is a goal for you, my article on how you can lose weight with a ketogenic diet is a good place to start.
4. The recommendations have changed, and they changed again in 2024
This is where much of the confusion comes from. In 2011, the Endocrine Society published a guideline that defined deficiency as a blood level below 20 ng/mL and suggested aiming for at least 30 ng/mL.2 For more than a decade, many doctors used 30 ng/mL as the target.
In 2024, the Endocrine Society issued a new guideline focused on preventing disease in people without a known vitamin D problem.1 The key changes:
- For most healthy adults under 75, it recommends getting the standard daily intake from the Institute of Medicine (600 IU for adults up to 70 and 800 IU over 70), rather than higher doses.
- It suggests extra vitamin D for certain groups: children and teens, adults over 75, pregnant women, and adults with prediabetes.
- It no longer recommends routine vitamin D blood testing for healthy people and does not endorse a specific target blood level.
My recommendation is different. In my practice, I recommend 5,000 to 10,000 IU of vitamin D3 a day for most adults, taken with a meal and with vitamin K2. For patients with weakened immunity, I may use 20,000 IU a day for a period of time. These amounts are above the official upper limit of 4,000 IU a day, so take them with your own doctor’s guidance, get your 25-hydroxyvitamin D level checked before you start, and recheck it so your level stays in a healthy range.
5. Vitamin D may modestly support respiratory health
Researchers have run dozens of trials on vitamin D and respiratory infections. A 2017 analysis in the BMJ, which pooled individual data from about 11,000 participants in 25 trials, found that vitamin D supplementation reduced the risk of acute respiratory infections overall, with the biggest benefit in people who started out with very low levels.3
An updated 2021 meta-analysis that added newer trials found a smaller but still statistically significant protective effect. Importantly, the benefit showed up with daily doses, not with large occasional “bolus” doses.4
So the honest summary is this: vitamin D is not a magic shield, but steady daily intake that corrects a shortfall may modestly support your body’s defenses, especially if you were low to begin with.
6. Vitamin D plays a direct role in the immune system
Many immune cells carry vitamin D receptors and can activate vitamin D themselves. Vitamin D helps the body produce antimicrobial peptides, such as cathelicidin, which are part of the first line of defense. It also helps balance inflammatory signaling. This is why vitamin D status matters for overall immune function.
7. Vitamin D is essential for bones and muscles
Vitamin D helps your gut absorb calcium and phosphorus. Without enough of it, the body pulls calcium from the bones, which over time weakens them. Severe deficiency causes rickets in children and osteomalacia (soft bones) in adults. Adequate vitamin D also supports normal muscle function, which matters for balance and fall prevention as we age.
8. Vitamin D is not a cure-all
Low vitamin D has been linked in observational studies with many chronic health problems, as well as low moods. But linked is not the same as caused. Poor health can also lower vitamin D, for example by keeping people indoors.
The large VITAL trial gave about 25,000 generally healthy adults 2,000 IU of vitamin D3 daily or a placebo for about five years. Vitamin D did not lower the overall rates of cancer or heart attack and stroke compared to placebo.6 For most people, the goal is to avoid deficiency, not to expect a high dose to prevent disease on its own.
9. Your body makes vitamin D from sunlight, but it may not be enough
Sensible sun exposure is the most natural source of vitamin D. In the warmer months, many people with lighter skin can make a meaningful amount with about 10 to 15 minutes of midday sun on the arms and legs a few times a week, while people with darker skin need longer. Be careful never to burn.
In winter, at northern latitudes, or if you spend most of your day indoors, sunlight alone often will not keep levels where they should be. Spending more time outside has other benefits too. See my article on daylight and dementia risk.
10. More is not always better
Because vitamin D is stored in the body, it is possible to take too much. The upper limit set by the Institute of Medicine for adults is 4,000 IU per day unless your doctor prescribes more for a specific reason. Vitamin D toxicity is rare and almost always comes from taking very high doses over a long period.
The main problem with too much vitamin D is a buildup of calcium in the blood (hypercalcemia), which can cause nausea, vomiting, weakness, confusion, frequent urination, and over time kidney stones and kidney damage. If you take more than the standard amount, ask your doctor to check your blood levels.
How to Raise Your Vitamin D Levels Naturally
If your levels are low, here are practical steps:
- Get outside. Enjoy brief, regular midday sun in the warmer months, without burning.
- Eat vitamin D foods. Fatty fish such as wild salmon, sardines, herring, and mackerel are the best food sources. Egg yolks and UV-exposed mushrooms add smaller amounts. Try my Keto Zone salmon chowder.
- Work toward a healthy weight. This can help your body make better use of the vitamin D you get.
- Consider a supplement. If you supplement, D3 is the preferred form, and taking it with a meal that contains fat helps absorption. A quality multivitamin can also cover the standard daily amount. For example, Dr. Colbert’s Enhanced Multivitamin provides 800 IU of vitamin D3 along with vitamin K2. Read more in 10 Reasons to Take a Daily Multivitamin.
- Talk with your doctor about whether testing makes sense for you, and before taking higher doses.
“But unto you that fear my name shall the Sun of righteousness arise with healing in his wings.”
Malachi 4:2 (KJV)
Bottom Line
Vitamin D is a remarkable vitamin and hormone that supports your bones, muscles, and immune system. Low levels are common, especially in winter, in older adults, in people with darker skin, and in those carrying extra weight. The newest guidelines encourage most healthy adults to simply meet the standard daily intake, while certain groups may need more. Get some safe sun, eat vitamin D foods, keep doses sensible, and work with your doctor if you think you may be low.
For more ways to stay strong as you age, visit my Healthy Aging Start Here guide.
Frequently Asked Questions
What is a normal vitamin D level?
Most labs report 25-hydroxyvitamin D in ng/mL. Below 20 ng/mL is widely considered deficient, and many doctors have used 30 ng/mL as a minimum goal. The 2024 Endocrine Society guideline does not set a specific target, so ask your doctor what level is right for you.
How much vitamin D should I take daily?
The official recommended intake is 600 IU per day for adults up to age 70 and 800 IU over 70, with an upper limit of 4,000 IU unless your doctor prescribes more. Dr. Colbert recommends 5,000 to 10,000 IU of vitamin D3 a day for most adults, with vitamin K2, and up to 20,000 IU for a period of time for people with weakened immunity, with a doctor’s guidance and blood tests to monitor levels.
Is vitamin D3 better than D2?
Research shows vitamin D3 raises blood levels more effectively than vitamin D2, which is why most practitioners prefer D3 for supplements.
What are the signs of low vitamin D?
Many people with low vitamin D have no obvious symptoms. When symptoms do occur, they can include fatigue, bone pain, muscle weakness or aches, and low mood. A blood test is the only way to know your level.
Can you get enough vitamin D from the sun?
In the warmer months, many people can make a good amount with short, regular periods of midday sun. In winter, at northern latitudes, with darker skin, or if you are mostly indoors, sunlight alone often is not enough.
Can you take too much vitamin D?
Yes. Taking very high doses for a long time can raise blood calcium too high, which can cause nausea, weakness, frequent urination, and kidney problems. If you take more than 4,000 IU a day, do it with your doctor’s guidance and periodic blood tests.
References
- Demay MB, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024.
- Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011.
- Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017.
- Jolliffe DA, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021.
- Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012.
- Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019.
This article is for educational purposes only and is not medical advice. Talk with your own doctor before changing your diet, starting a supplement, or changing any medication. These statements have not been evaluated by the Food and Drug Administration. Products mentioned are not intended to diagnose, treat, cure, or prevent any disease.
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