Vitamin D: Why So Many People Are Deficient

Vitamin D deficiency is one of the most common nutrient shortfalls in the world, affecting an estimated 35-40% of adults in some studies — far more than most people expect from a single vitamin.

Why deficiency is so common

Unlike most vitamins, your body can produce vitamin D from sunlight exposure on skin, which is also why it's sometimes called the 'sunshine vitamin.' Modern indoor lifestyles, sunscreen use, higher latitudes, darker skin tone (which produces vitamin D less efficiently from the same sun exposure), and aging skin (which synthesizes it less efficiently) all reduce how much your body makes on its own — and few foods naturally contain much vitamin D to fill the gap.

How much you need

The NIH recommends 600 IU per day for most adults up to age 70, and 800 IU per day for adults over 70. Levels up to 4,000 IU per day are generally considered a safe upper limit for supplementation without medical supervision.

Signs of deficiency

Mild deficiency often causes no obvious symptoms, which is part of why it's so widespread and underdiagnosed. More significant or prolonged deficiency can contribute to fatigue, bone pain, muscle weakness, and over the long term, reduced bone density.

Best sources

Fatty fish (salmon, mackerel, sardines), egg yolks, and fortified foods like milk and some cereals are the main dietary sources, though few foods provide enough on their own — which is why moderate sun exposure and, for many people, a supplement are common ways to close the gap.