Vitamin D keeps your bones strong, helps your immune system function properly, supports muscle strength, and plays a role in mood regulation. Its most critical job is making sure your body can absorb calcium from food. Without enough vitamin D, you absorb only a fraction of the calcium you eat, which sets off a chain reaction that weakens bones, drains energy, and leaves your immune defenses compromised.
How It Helps You Absorb Calcium
Vitamin D’s primary function is enabling your small intestine to pull calcium and phosphorus out of the food you eat. When active vitamin D reaches the cells lining your intestine, it switches on proteins that act like a shuttle system: one protein opens a channel in the cell wall to let calcium in, a second protein carries the calcium safely through the cell, and a third pumps it out the other side into your bloodstream. Without vitamin D driving this process, calcium passes through your gut largely unused.
This absorption system also works on phosphorus, though the mechanism is less well understood. Both minerals are essential raw materials for building and maintaining bone tissue, which is why vitamin D deficiency ripples outward into so many other problems.
Bone Strength and What Happens Without It
Because vitamin D controls calcium absorption, it is the gatekeeper of bone health. When your vitamin D levels drop too low, your body can’t get enough calcium from food, so it starts pulling calcium from your skeleton instead. Your parathyroid glands ramp up production of a hormone that signals bone-dissolving cells to break down bone tissue and release its stored calcium into the blood, keeping your heart and nerves functioning at the expense of your skeleton.
In children, prolonged severe deficiency causes rickets, a condition where bones become soft and bend under the body’s weight. In adults, the equivalent condition is called osteomalacia, where bones soften and ache. Even moderate, long-term deficiency accelerates bone loss and increases fracture risk, particularly in older adults. This is why major osteoporosis organizations recommend maintaining blood levels of at least 30 ng/mL to minimize fall and fracture risk.
Immune System Regulation
Vitamin D does more than build bones. Immune cells carry vitamin D receptors, and some can even convert inactive vitamin D into its active form on their own. This allows vitamin D to directly influence how your immune system responds to threats.
One of its key effects is dialing down overactive inflammatory responses. Vitamin D shifts the balance of your T cells (a major type of immune cell) away from aggressive, inflammation-promoting activity and toward a more measured, anti-inflammatory response. In a randomized controlled trial, high-dose vitamin D supplementation reduced the activation of CD4+ T cells, the immune cells involved in coordinating the body’s defense. Other studies have found that correcting a vitamin D deficiency increases regulatory T cells by about 1.5% and boosts production of anti-inflammatory signaling molecules.
This immunomodulatory role helps explain why low vitamin D levels are linked to higher rates of autoimmune conditions like type 1 diabetes, inflammatory bowel disease, and multiple sclerosis. In those conditions, the immune system attacks the body’s own tissue, and vitamin D appears to help keep that response in check.
Muscle Function and Balance
Muscle cells have their own vitamin D receptors, and the vitamin influences muscle performance through both direct effects on muscle tissue and indirect effects on the nervous system. Genetic variations in the vitamin D receptor gene are associated with differences in muscle strength. In one study of non-obese women aged 70 and older, those with a particular receptor gene variant had 7% greater grip strength and 23% greater quadriceps strength than those with an alternative variant.
Animal research has revealed another layer: mice engineered to lack vitamin D receptors showed poor balance on rotating platforms and tilting surfaces, and they were significantly worse swimmers. This suggests vitamin D also supports the vestibular system, the inner-ear mechanism that governs balance. For older adults, this combination of weaker muscles and impaired balance is a practical explanation for why low vitamin D levels are consistently linked to a higher risk of falls.
Brain Health and Mood
Vitamin D receptors are present throughout the brain, and evidence points to a role in regulating serotonin, the neurotransmitter most closely associated with mood. Vitamin D appears to increase serotonin availability in the gaps between nerve cells, which is essentially the same goal as SSRI antidepressants, though through a different mechanism. This connection is still an active area of investigation, but it offers a biological explanation for the well-documented association between low vitamin D and higher rates of depression.
How Much You Need
The recommended daily intake varies by age. Infants up to 12 months need 400 IU (10 mcg). Adults aged 19 to 70 need 600 IU (15 mcg). Adults over 71 need 800 IU (20 mcg). These recommendations assume minimal sun exposure.
A simple blood test measures your level of 25-hydroxyvitamin D, the circulating form your body uses. Below 20 ng/mL is generally considered deficient. Between 20 and 30 ng/mL is insufficient. Above 30 ng/mL is considered adequate, and that is the threshold most organizations recommend for protecting bone health and reducing fall risk.
Getting Vitamin D From Sunlight
Your skin produces vitamin D when exposed to UVB rays, but the amount depends heavily on your skin tone, latitude, and how much skin you expose. People with lighter skin in the UK need roughly 9 minutes of midday sun with forearms and lower legs exposed during summer months to meet daily needs. People with darker skin (type V on the Fitzpatrick scale) need about 25 minutes under the same conditions, and up to 40 minutes at more northern latitudes. Sunscreen, cloud cover, window glass, and winter sun angles all reduce or eliminate UVB exposure.
For many people, especially those living above the 37th parallel (roughly the latitude of San Francisco or Athens), sunlight alone is not a reliable year-round source.
Food Sources
Very few foods naturally contain significant vitamin D. The richest natural sources are fatty fish and fish liver oils. Here’s what a single serving provides:
- Cod liver oil (1 tablespoon): 1,360 IU, more than double the daily recommendation
- Rainbow trout, cooked (3 oz): 645 IU
- Sockeye salmon, cooked (3 oz): 570 IU
- UV-exposed white mushrooms (½ cup): 366 IU
- Fortified milk (1 cup): 120 IU
- Fortified plant milks (1 cup): 100 to 144 IU
- One scrambled egg: 44 IU
In most American diets, fortified foods like milk and cereal provide the bulk of dietary vitamin D. If you don’t eat fatty fish regularly or drink fortified milk, reaching 600 IU from food alone is difficult. This is why supplementation is common, particularly during winter months or for people who spend most of their time indoors.
Too Much Vitamin D
Vitamin D toxicity is rare but possible, almost always from supplements rather than food or sunlight. Because vitamin D increases calcium absorption so effectively, taking excessive amounts can flood your bloodstream with calcium, a condition called hypercalcemia. Symptoms include nausea, vomiting, weakness, and in severe cases, kidney damage. Your body has a built-in safeguard for sun exposure: it stops producing vitamin D in the skin once it has made enough. No such safeguard exists for supplements, which is why sticking to recommended doses matters unless a healthcare provider has checked your levels and advised otherwise.