Carbohydrates, Proteins and Lipids: Sources, Functions and Deficiency
Carbohydrates
Monosaccharides (glucose, fructose, galactose), disaccharides (sucrose, lactose, maltose), polysaccharides (starch, glycogen, cellulose). Digestible carbohydrate gives 4 kcal/g. Fibre is largely non-digestible: soluble fibre lowers blood cholesterol and slows glucose absorption; insoluble fibre prevents constipation. Deficiency signs: hypoglycaemia, fatigue, ketosis on very low intake, constipation with low fibre. Excess: obesity, dental caries, hypertriglyceridaemia.
Proteins
Nine amino acids are essential. Complete proteins supply all of them in adequate amounts; incomplete proteins are limited by one amino acid. Cereal (limiting amino acid lysine) plus pulse (limiting methionine) forms a complementary combination. Protein gives 4 kcal/g and provides growth, repair, enzymes, hormones, transport proteins, antibodies and osmotic balance. Deficiency gives kwashiorkor (oedema, hypoalbuminaemia, skin and hair changes) or marasmus (severe wasting without oedema).
Lipids
Saturated, monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids; linoleic (omega-6) and linolenic (omega-3) acids are essential. Trans fats raise LDL and lower HDL. Fat gives 9 kcal/g, carries fat-soluble vitamins, insulates, forms myelin, and cholesterol is the precursor of steroid hormones and bile acids. Essential fatty acid deficiency causes dry scaly dermatitis and poor growth.
Worked example
A meal contains 60 g carbohydrate, 20 g protein and 15 g fat. Energy = (60 × 4) + (20 × 4) + (15 × 9) = 240 + 80 + 135 = 455 kcal. Reasoning: Atwater factors of 4, 4 and 9 are applied to the macronutrient that actually yields them; applying 9 to protein or 4 to fat is the commonest arithmetic error.
Traps that cost marks
Excess protein is not stored as protein - it is deaminated, the nitrogen excreted as urea, and the carbon skeleton oxidised or stored as fat. Fibre is a carbohydrate but not an energy source for humans. A 'fat-free' food is not necessarily low in calories.