Micronutrients
Vitamin D: how much you need and why
Vitamin D behaves more like a hormone than a typical vitamin, and it's one of the few nutrients most people genuinely struggle to get enough of.
What it does
Vitamin D's best-known job is helping your gut absorb calcium — without enough of it, you can eat plenty of calcium and still absorb only a fraction of it. It also supports bone mineralization, normal muscle function, and immune signalling. Severe, prolonged deficiency causes rickets in children and osteomalacia (soft bones) in adults.[1]
How much you need
| Group | RDA | Upper limit |
|---|---|---|
| Adults 19–70 | 600 IU (15 mcg) | 4,000 IU (100 mcg) |
| Adults 71+ | 800 IU (20 mcg) | 4,000 IU (100 mcg) |
These are the U.S. Dietary Reference Intake values, set assuming minimal sun exposure.[1]
Why deficiency is common
Very few foods naturally contain meaningful vitamin D. The reliable ones are fatty fish (salmon, mackerel, sardines), cod liver oil, egg yolks in smaller amounts, and fortified foods — most notably milk and some cereals and plant milks. Your skin also makes vitamin D from UVB sunlight, but that production drops sharply with higher latitude, winter months, darker skin pigmentation, sunscreen use, and time spent indoors.[1]
Worth knowing
Vitamin D is fat-soluble, so it's stored in body tissue rather than flushed out daily. That's exactly why the upper limit matters — unlike water-soluble vitamins, it's genuinely possible to take too much over time.
Because it's fat-soluble, vitamin D is absorbed better when eaten alongside a meal containing some fat. See our guide to fat-soluble vitamins for the full picture.
Keep reading: Fat-soluble vitamins explained · Calcium and bone health · Healthy high-fat foods