The long, dark days of winter bring with them a distinct lack of vitamin D. Dubbed the “sunshine vitamin”, it works in the body as a hormone, explains Alice Mackintosh, nutritionist and co-founder of EQUI London. “The best way to increase vitamin D is via sun exposure – 20 minutes of summer-strength sunlight on bare skin (without SPF) is a good goal,” says Mackintosh, explaining that the back, legs and arms are the best parts to expose, for maximum sunlight absorption. “Those most at risk include strict vegans, people with dairy allergies (since dairy products are a key source of vitamin D), individuals with darker skin tones, and people living with obesity,” explains Mackintosh.