7-Dehydrocholesterol has significant absorption of UV light at the lowest wavelengths of light found in sunlight (290-310 nm). Hence the need for full-spectrum lamps to emit UV-B radiation (UV-B light is defined here to be light between 290-315 NM).
In rare cases some vegans might not get enough fat or calories to produce adequate amounts of steroid hormones (which are produced from cholesterol). Two studies have shown vegans to have sex hormones on par with meat-eaters (1, 2), but one report showed vegan women to have lower levels of estrogen (3).
If you eat eggs, dairy products, and/or fish, you get enough fat. Weakness on a vegan diet 2 traps to avoid : I know it sounds silly, but when you think you're eating enough (and the majority of people have no idea how many calories they consume a day, vegan or Being vegan doesn't have to mean counting every calorie.