Food must be adequate in both quantity and quality in order to sustain normal growth and development. There is an obligatory requirement for essential amino acids, nitrogen, glucose, certain fatty acids, vitamins and minerals. If energy intake is inadequate, protein will oxidised for energy rather than used as a source of amino acids for growth.
The question is how much of each is adequate, and what are the consequences of too little or too much? We also need to know which foods supply which nutrients and what constitutes a 'balanced' diet.
Is Alice's diet adequate? Adults eat about 8000 to 12,000 kJ/day, roughly equivalent to their energy expenditure, assuming they are maintaining constant weight. Children and adolescents need to eat more energy than they expend in order to grow (i.e lay down new tissue). The energy in food comes from three sources: carbohydrate, fat and protein.
Carbohydrate and fat usually show a reciprocal relationship: a high fat diet is low in carbohydrate and vice versa. Carbohydrate includes sugars, starches and dietary fibre. The brain and red cells can use only glucose as their source of fuel (under normal circumstances), and there is an obligatory requirement for about 50 g carbohydrate per day.
In practice, Australians eat about 200-250 g carbohydrate/day, representing 45% of total energy intake (1 g of carbohydrate yields 16 kJ energy or 4 calories when oxidised). A higher carbohydrate intake is considered desirable (i.e. 55% of energy) but less (40% of energy) may be helpful in individuals with diabetes. Cereals, fruit and starchy vegetables are good sources of carbohydrate.
A patient with anorexia nervosa (AN) often eats a very low fat-high carbohydrate diet but total energy intake is inadequate to cope with the demand of both a growing body and excessive exercise that is characteristic of these patients.
Does Alice eat enough fat? In western countries fat supplies about 30-40% of our total energy needs (1 g fat yields 37 kJ or 9 calories) but apart from small amounts of the essential fatty acids (linoleic acid, linoleic acid and very long chain polyunsaturated fatty acids), most fat is just a concentrated source of energy and oxidised in the muscles and other tissues.
Fat makes food taste good and easy to over consume. On the other hand, a very low fat diet (<20% energy) can be bulky and unpalatable and difficult for children to consume in appropriate amounts. In western countries, we eat too much saturated fat and this is associated with high rates of degenerative disease such as obesity and cardiovascular disease. Health authorities recommend that we eat less than 30% energy as fat and less than 10% energy as saturated fat.
Fat comes in the form of butter, margarine and oils as well as "hidden" such as in fried foods, fatty meat, full-fat dairy products and confectionery. Patients with AN usually stay clear of fatty foods in order to lose weight and maintain the low weight status. In some cases, there are show signs of essential fatty acid deficiency (look them up!).
Is Alice's weight loss a sign of protein deficiency? Tissues of the human body incorporate about 20 different sorts of amino acids. Only 8 or 9 are essential in the sense they can't be synthesised in the body and must be supplied preformed in the diet. The protein in food supplies the essential amino acids.
Inorganic nitrogen can be used to synthesise the nonessential amino acids, but in practice most of these are supplied by foods as well. In industrialised countries we eat more than enough protein and the excess is oxidised as a source of energy (unlike fat, we cannot store protein for a rainy day). Australians eat about 1 g protein/kg body weight, or about 13-18% or total energy intake (1 g protein yields 17 kJ or 4 calories).
The minimum recommended amount of protein is 0.5 g/kg body weight for adults and up to 2 g /kg body weight in young infants. The exact minimum amount depends on the type of protein consumed. Meat, milk, eggs and legumes are rich sources of protein. Cereal foods, while not as high, are major contributors of protein to the diet. Patients with AN usually eat a high protein diet e.g. their weight loss is not a sign of protein deficiency (what are the signs of protein deficiency?).
Is Alice's weight loss a sign of vitamin or mineral deficiency? Vitamins are the co-factors and co-enzymes that are required by the body to synthesise new compounds and oxidise the fuels of the tissues.
They are needed in very small amounts (measured in micrograms and milligrams) but they are not evenly distributed among the different sorts of food we eat. For example, animal foods are the highest sources of bioavailable iron, zinc, vitamin A and iodine. Fruit and vegetables are rich sources of vitamin C while animal foods contain none. Although we need vitamin A, it is also possible to eat toxic quantities.
The lower the energy intake, the more likely there will be inadequate intake of vitamins and minerals. A higher proportion of energy from animal foods ensures better micronutrient intake in energyted diets.-restric
In Alice's case, the problem is a voluntary reduction in food intake i.e. inadequate quantity rather than quality. Dietary guidelines have been produced to guide people with the right food choices.
The 12345+ approach recommends 1 serve of meat, 2 serves of dairy products, 3 serves of fruit, 4 serves of vegetables and 5 or more serves of cereal foods per day for the average person, with additional serves for more active, growing individuals.
The patient with AN often knows a lot about nutrition (possibly more than the doctor) and chooses foods which are nutritious but low in fat and energy. If she/he ate more of the same, they would achieve and maintain their ideal weight.
Digestion smp.sydney.edu.au/compass/teachingactivity/view/id/796
Nutrient absorption and transport smp.sydney.edu.au/compass/teachingactivity/view/id/797
Truswell, A. S., 2003, ABC of nutrition, 4th edn. BMJ., London.
This is a very readable introduction to human nutrition written especially for general practitioners.