Individuals at high risk for low vitamin D levels include:

Individuals who have dark skin and no sun exposure are indeed at high risk for low vitamin D levels. Vitamin D is synthesized in the skin upon exposure to sunlight, specifically ultraviolet B (UVB) rays. People with darker skin have more melanin, which provides some natural protection against UV radiation but also reduces the skin's ability to produce vitamin D. Therefore, if these individuals have little to no sun exposure, their capacity to synthesize vitamin D is significantly diminished, leading to an increased risk of deficiency.

Other factors may play a role in vitamin D levels, but in this case, the specific combination of having dark skin and limited sun exposure creates a notable risk. This is because the skin's production of vitamin D depends heavily on both melanin content and sun exposure; without sufficient exposure, even individuals with lighter skin can struggle to maintain adequate levels. While vegetarians might have some dietary limitations when it comes to vitamin D, it’s the lack of sunlight exposure in darker-skinned individuals that is most clearly linked to deficiency. Additionally, regular exercise and adequate hydration do not directly influence vitamin D synthesis, making those options less relevant to the question at hand.