Nutrients are the nourishing substances in food that are essential for the growth, development and maintenance of body functions. Essential meaning that if a nutrient is not present, aspects of function and therefore human health decline. When nutrient intake does not regularly meet the nutrient needs dictated by the cell activity, the metabolic processes slow down or even stop.

- Perspectives in Nutrition, Wardlow and Insel

In this context the amount and type of nutrients depends on the cell activity. In short the more activity the more nutrients are required. According to Teresa A. Marshall and Connie C. Mobley an improper diet and to much or less activity can cause chronic deases.