The need for carbohydrate and fat

Although there is a requirement for energy sources in the diet, it does not matter unduly how that requirement is met. There is no requirement for a dietary source of carbohydrate — as discussed in section 5.7, the body can make as much carbohydrate as is required from proteins. Similarly, there is no requirement for a dietary source of fat, apart from the essential fatty acids (section 4.3.1.1), and there is certainly no requirement for a dietary source of alcohol. However, as discussed in section 7.3.2, diets that provide more than about 35—40% of energy from fat are associated with increased risk of heart disease and some cancers, and there is some evidence that diets that provide more than about 20% of energy from protein are also associated with health problems. Therefore, as discussed in section 7.3, the general consensus is that diets should provide about 55% of energy from carbohydrates, 30% from fat and 15% from protein.

Although there is no requirement for fat in the diet, fats are nutritionally important and, as discussed in section 1.3.3.1, there is a specific mechanism for detecting the taste of fats in foods.

Table 1.1 The energy yield of metabolic fuels

kcal/g

kJ/g

Carbohydrate

4

17

Protein

4

16

Fat

9

37

Alcohol

7

29

1 kcal = 4.186 kJ or 1 kJ = 0.239 kcal

  • It is difficult to eat enough of a very low-fat diet to meet energy requirements. As shown in Table 1.1, the energy yield per gram of fat is more than twice that of carbohydrate or protein. The problem in many less developed countries, where undernutrition is a problem (see Chapter 8), is that diets provide only 10—15% of energy from fat, and it is difficult to consume a sufficient bulk of food to meet energy requirements. By contrast, the problem in Western countries is an undesirably high intake of fat, contributing to the development of obesity (see Chapter 6) and the diseases of affluence (see section 7.3.1).
  • Four of the vitamins, A, D, E and K (see Chapter 11), are fat soluble, and are found in fatty and oily foods. More importantly, because they are absorbed dissolved in fat, their absorption requires an adequate intake of fat. On a very low-fat diet the absorption of these vitamins may be inadequate to meet requirements.
  • There is a requirement for small amounts of two fatty acids which are required for specific functions; these are the so-called essential fatty acids (section 4.3.1.1). They cannot be formed in the body, but must be provided in the diet.
  • In many foods, a great deal of the flavour (and hence the pleasure of eating) is carried in the fat.
  • Fats lubricate food, and make it easier to chew and swallow.
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