Another explanation on this is by Mike Adams. Part of his explanation:
The best sunscreen, however, is an internal sunscreen built with nutrition. Eat chlorella, spirulina, goji berries, raspberries, blackberries, blueberries, astaxanthin, carrots and nutrient-rich superfoods to boost your skin's natural UV protection (takes about 30 days of nutrition to boost skin levels).
here is the article on the Sunscreen Myth and its relation to skin cancer.