What happens to the food we eat?

The nutrients in food are used by our bodies in varying ways. Each nutrient plays some part in normal body function (that is, body physiology).

Not all components of food are nutrients, and not all nutrients are essential to life. Some nutrients may have important physiological functions without being absorbed from the gut, for example dietary fibre. Dietary fibre, while not being essential to life, is associated with health because of its role in the correct functioning of the bowel. Its absence may increase the risk of bowel disease.

Although recommendations about nutrient intakes are made in terms of daily intakes, not all nutrients are needed every day. Most can be stored to a lesser or greater extent in our bodies. The macronutrients that provide energy are stored in the liver and muscles in the form of a carbohydrate called glycogen, and in fatty tissues (adipose tissue) and muscles in the form of fats, known as triglycerides. The situation for micronutrients varies. Fat-soluble vitamins, such as vitamin A, tend to be stored in liver and fat and do not need to be replenished every day; they can last for weeks if the stores are good. In general, water-soluble components like vitamin C are quickly excreted in the urine. But some water-soluble nutrients such as vitamin B-12 and folacin (folic acid) can be stored for years and months, respectively, in the liver. Some nutrients, for example calcium and phosphorus, are in effect stored in bone.

How often a nutrient needs to be ingested depends on how all of the various processes shown in Figure 4 operate for the nutrient in question.

Although not every nutrient is needed every day, it is safest to 'top up' regularly through a consistent pattern of eating. Nevertheless, undue concern about particular food preferences, for example about children who seem to want one kind of food and not another for a few days. is probably unnecessary. As long as a variety of foods are eaten in the longer term, the range of essential nutrients should be provided.