Why Almost Everyone Is Vitamin D Deficient (And What Actually Works)

Vitamin D has become the most awkward vitamin at the dinner table. Everyone has heard of it. Everyone assumes they are low. Half the people you know are popping pills the size of Chiclets, the other half are convinced it is a scam cooked up by Big Supplement, and your doctor keeps circling numbers on a lab report like they are reading tea leaves. Welcome to the modern Vitamin D mess.

Here is the short version, then we will slow it down and tell the truth. Most people really are low in vitamin D. The reasons have nothing to do with moral failure or weak willpower. The “right amount” depends on age, body fat, sun exposure, health status, and how broken your metabolism already is. Vitamin D can be helpful, boring, or harmful depending on how casually you treat it. As with most things in health, the danger lies not in deficiency or supplementation. The danger is oversimplification.

Vitamin D is technically a hormone, not a vitamin in the classic sense. Your body produces it when ultraviolet B light hits your skin, converting cholesterol into vitamin D3. That alone should tell you something important. This is not kale dust. This is a system that evolved when humans spent most of their time outdoors, half-naked by modern standards, not under fluorescent lights and SPF 50.

So why does everyone seem deficient?

First, we stopped going outside. Not occasionally. Systemically. We work indoors, drive everywhere, exercise inside climate-controlled buildings, and spend our evenings staring at screens that slowly flatten our circadian rhythm. Even people who “go outside” often do it early in the morning or late in the day when UVB exposure is minimal. Add sunscreen, long sleeves, hats, and the very real fear of skin cancer, and you have effectively shut down the body’s primary vitamin D factory.

Second, we got older and heavier. Both matter. As we age, the skin becomes less efficient at producing vitamin D. Additionally, vitamin D is fat-soluble, which means it gets stored in fat tissue. The more body fat you carry, the more vitamin D gets locked away instead of circulating where it can actually do something useful. This is why two people taking the same dose can have wildly different blood levels.

Third, modern diets do not help. Outside fatty fish, fortified dairy, and a few mushrooms, vitamin D does not naturally occur in food in meaningful amounts. Fortification was never meant to replace sunlight. It was meant to prevent rickets in children living in polluted industrial cities. We are now asking it to prop up an entire indoor civilization.

Fourth, we test for it now. This matters. We did not routinely measure vitamin D. Once you shine a flashlight into a dark room, you notice dust everywhere. Some of the “epidemic” is real. Some of it is awareness. Both can be true at the same time.

So what does vitamin D actually do?

It plays a role in calcium absorption and bone health, which is the part everyone knows. It also affects muscle function, immune signaling, regulation of inflammation, insulin sensitivity, and even mood. Low vitamin D levels are associated with increased fracture risk, muscle weakness, increased fall risk in older adults, and impaired immune response. Associations exist with cardiovascular disease, autoimmune conditions, depression, and certain cancers, but associations are not the same thing as causation. That distinction matters, especially when supplement companies start promising miracles.

Now let’s talk numbers, because this is where things get muddy.

Vitamin D status is measured by serum 25-hydroxyvitamin D (25(OH)D). Most labs report it in ng/mL. According to the National Institutes of Health, levels below 20 ng/mL are considered deficient, 20 to 30 ng/mL insufficient, and 30 ng/mL or higher generally adequate for bone and overall health. The Endocrine Society tends to recommend slightly higher targets, often 30 to 50 ng/mL, particularly for older adults and people with obesity or malabsorption issues.

This is where people start arguing online.

Some clinicians push for levels of 60 or even 80 ng/mL. Others say anything above 30 is plenty, and chasing higher numbers is unnecessary. The truth, as usual, is boring. There is good evidence that very low levels are harmful. There is decent evidence that correcting deficiencies improves bone health and muscle function, especially in older adults. There is weak evidence that pushing levels very high provides additional benefit for the average person.

More is not automatically better. Vitamin D is fat-soluble. Your body does not just pee out the excess. Too much can lead to hypercalcemia, which means elevated blood calcium levels. That can cause nausea, kidney stones, arrhythmias, confusion, and in extreme cases, kidney damage. Toxicity is rare, but it almost always comes from high-dose supplements taken chronically without monitoring, not from sunlight or food (NIH Office of Dietary Supplements).

So what is the “right amount”?

For most adults, the Recommended Dietary Allowance is 600 IU per day up to age 70, and 800 IU per day over 70 (NIH). That is a minimum to prevent deficiency, not an optimal dose for everyone. Many adults, especially older adults, people with obesity, darker skin pigmentation, limited sun exposure, or autoimmune conditions, require more to reach adequate blood levels.

Ordinary supplemental doses range from 1,000 to 2,000 IU per day. This is generally considered safe for long-term use in most adults. Some clinicians prescribe 4,000 IU or more for individuals who are deficient, often for a limited period followed by retesting. The tolerable upper intake level for adults is set at 4,000 IU per day by the Institute of Medicine, although short-term higher doses are sometimes used under medical supervision.

Sun exposure complicates this further. A fair-skinned person can generate thousands of IU of vitamin D with 10 to 20 minutes of midday sun exposure on arms and legs. Darker-skinned individuals require more prolonged exposure. Latitude, season, cloud cover, and air pollution all affect this process. In much of the United States, from roughly October through March, UVB rays are insufficient to produce meaningful vitamin D from sunlight.

Now let’s talk about the good.

Adequate vitamin D supports bone density and reduces fracture risk when combined with sufficient calcium intake. It improves muscle strength and balance in older adults, which lowers fall risk. It plays a role in immune function, particularly in respiratory infections. There is evidence that correcting deficiency improves markers of inflammation in specific populations. It is cheap, widely available, and generally safe when used appropriately.

Now the bad.

Vitamin D has been oversold. It is not a cure for everything. Supplementing does not magically undo poor sleep, chronic stress, a sedentary lifestyle, or an ultra-processed diet. Many observational studies linking low vitamin D to disease cannot prove that low vitamin D causes the disease. It may be a marker of poor health, limited mobility, or inflammation. People who are sick tend to be indoors more. People who are healthier tend to spend more time outdoors. Correlation is not causation, no matter how much we want a simple fix.

High-dose supplementation without testing is another problem. Chasing “optimal” numbers without context can cause harm. Vitamin D interacts with calcium, magnesium, and vitamin K2. Taking one without considering the others can create imbalances. This is not a reason to panic. It is a reason to stop treating supplements like candy.

So what should a sane boomer actually do?

Get tested, especially if you are over 50, carry excess body fat, have osteoporosis or an autoimmune disease, or have limited sun exposure. Aim for adequacy, not heroics. A blood level somewhere in the 30 to 50 ng/mL range is reasonable for most people. Use moderate supplementation if needed. Recheck levels after a few months if you are correcting a deficiency. Do not assume that more is better or that a podcast host knows more than your labs.

Vitamin D is not magic. It is maintenance, like changing the oil. Ignore it long enough, and things start grinding. Obsess over it and you miss the bigger picture.

We did not evolve to live indoors, sit all day, fear the sun, and then hope a capsule fixes the downstream effects. Vitamin D deficiency is not a personal failure. It is a predictable outcome of modern life. Fixing it helps. Pretending it fixes everything is where people go off the rails.

References

• National Institutes of Health, Office of Dietary Supplements. “Vitamin D Fact Sheet for Health Professionals.”

• Institute of Medicine. “Dietary Reference Intakes for Calcium and Vitamin D.”

• Endocrine Society Clinical Practice Guidelines on Vitamin D Deficiency.