It's the vitamin associated with the sun that we absorb through our skin, and increasingly block out when we use sunblock, to avoid getting skin cancer.
VOA: standard.2010.05.28
Those strong enough to go on will usually travel only at night, hiding by day under mesquite brush to avoid the sun and the spotter aircraft of the Border Patrol.
Getting people to exercise regularly, avoid cigarettes, sun exposure, and other known carcinogens would have a much bigger and immediate impact in reducing cancer cases.