What is Vitamin D?

Vitamin D controls how much calcium and phosphate the gut absorbs from food, which is what allows bone to mineralise and hold its strength. Sustained deficiency shows up in bone and muscle, as rickets in children and osteomalacia in adults.

Calcium and Phosphate are the minerals whose absorption vitamin D governs, and all three are read together as one account of supply to bone. ALP is an enzyme that bone-building cells release as they work, so it rises when bone is actively laying down the minerals vitamin D has supplied.

Sunlight on skin does most of the work, and in the UK it is strong enough for that only between about April and September. Skin tone, clothing, time outdoors and sunscreen all change how much of that sunlight reaches the skin. Food contributes modestly, mainly oily fish, eggs and fortified products, alongside any supplements taken. Body weight matters too: vitamin D dissolves in fat, so the more fat tissue there is, the more thinly the same supply is spread.

The circulating amount climbs through summer and drains away through winter, moving across weeks of changed sun exposure rather than days. Measuring about every four months lands at different points in that cycle, so the year's swing becomes visible instead of one season standing in for the whole. This is Do Health's monitoring schedule, not a general recommendation for everyone.