The integumentary system is the scientific name for your skin and its accessories, including hair and nails. Your integumentary system provides a physical barrier to injury and infection, it helps you maintain your body temperature and it senses your environment. You need certain nutrients to maintain your integumentary system.
Of all the components of your integumentary system, the most complex is the skin. Skin consists of several layers, the outermost of which are the epidermal layers. You have five layers of epidermal cells, or skin cells -- the lower layer reproduces itself and generates new cells that then migrate progressively outward. The outermost layer continually sloughs off, or sheds, which is why constant production of new skin cells is so important to maintaining the epidermis.
While the lower layer of the epidermis is a layer of living cells, and while the dermis, which lies below the epidermis and contains blood vessels and nerves, is living, many components of the integumentary system aren't alive. For instance, the portion of your hair that you can touch, the portion of the nails that lies above the surface of the skin and the surface of the skin itself all consist of "dead" cells, meaning cells that have lost all metabolic function.
While the integumentary system cells need nutrients, since the portions of the system that you can touch aren't living, you can't nourish your skin, nails or hair by rubbing nutrients on them. These cells have no mechanism for absorbing energy or vitamins. Some cells, including outer skin cells, benefit from moisture, which is why you use lotion, but the cells don't actually derive nutrition from topically applied rubs or creams.
Like all cells, the living cells of the integumentary system -- including those of the dermis and lower epidermis, the nail beds, and the hair follicles -- need energy-providing nutrients. The energy-providing nutrients include carbohydrates, proteins and fats. When you eat, some of the nutrient molecules from your food travel through the bloodstream to provide for the needs of the living integumentary system cells.
Specific vitamins help to support the integumentary system. For instance, you use vitamin C to generate collagen, which is an elastic protein that helps keep the skin and connective tissue flexible. Without sufficient vitamin C in your diet, your skin can become brittle or weak. Your skin actually helps in the production of one important vitamin -- vitamin D. In sunlight, the skin transforms cholesterol in your blood into vitamin D for the rest of your body cells.