No sunlight leads to rickets due to vitamin D deficiency, vitamin D prevents rickets. No fruit leads to scurvy due to vitamin C deficiency, vitamin C prevents scurvy. If you eat well, and include sufficient fruit, meat, eggs and milk, you won't be deficient in anything and don't need vitamins. If you work night shifts, live in a cloudy country or eat a retard diet like vegans, you probably need vitamins to prevent floppy bones or death.