Info

V >

ß3-adrenergic

Heat loss by evaporation

Blood vessels

Brown fat

Brown fat

ß3-adrenergic

Somatic nervous system

Hi >

cholinergic

Non-shivering thermogenesis (in infants)

Heat production by shivering

10 Nutrition and Digestion

Nutrition

An adequate diet must meet the body's energy requirements and provide a minimum of carbohydrates, proteins (incl. all essential amino acids) and fats (incl. essential fatty acids). Minerals (incl. trace elements), vitamins, and sufficient quantities of water are also essential. To ensure a normal passage time, especially through the colon, the diet must also provide a sufficient amount of roughage (indigestible plant fibers—cellulose, lignin, etc.).

The total energy expenditure (TEE) or total metabolic rate consists of (1) the basal metabolic rate (BMR), (2) the activity energy costs, and the (3) diet-induced thermogenesis (DIT; ^ p. 228, 231 A). TEE equals BMR when measured (a) in the morning (b) 20 h after the last meal, (3) resting, reclining, (4) at normal body temp., and (5) at a comfortable ambient temp. (^ p. 224). The BMR varies according to sex, age, body size and weight. The BMR for a young adult is ca. 7300kJ/day 1740 kcal/ day; see p. 374 for units) in men, and ca. 20% lower in women. During physical activity, TEE increases by the following factors: 1.2-fold for sitting quietly, 3.2-fold for normal walking, and 8-fold for forestry work. Top athletes can perform as much as 1600 W (=J/s) for two hours (e.g., in a marathon) but their daily TEE is much lower. TEE also increases at various degrees of injury (1.6-fold for sepsis, 2.1-fold for burns). 1 °C of fever increases TEE 1.13-fold.

Protein, fats and carbohydrates are the three basic energy substances (^ B).

An adequate intake of protein is needed to maintain a proper nitrogen balance, i.e., balance of dietary intake and excretory output of nitrogen. The minimum requirement for protein is 0.5 g/kg BW per day (functional minimum). About half of dietary protein should be animal protein (meat, fish, milk and eggs) to ensure an adequate supply of essential amino acids such as histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine (children also require arginine). The content of most vegetable proteins is only about 50% of animal protein.

Carbohydrates (starch, sugar, glycogen) and fats (animal and vegetable fats and oils) pro vide the largest portion of the energy requirement. They are basically interchangeable sources of energy. The energy contribution of carbohydrates can fall to about 10% (normally 60%) before metabolic disturbances occur.

Fat is not essential provided the intake of fat-soluble vitamins (vitamins E, D, K and A) and essential fatty acids (linoleic acid) is sufficient. About 25-30% of dietary energy is supplied by fat (one-third of which is supplied as essential fatty acids; ^ A), although the proportion rises according to energy requirements (e.g., about 40% during heavy physical work). Western diets contain generally too much energy (more fats than carbohydrates!) considering the generally low level of physical activity of the Western lifestyle. Alcohol also contains superfluous energy (ca. 30kJ/g = 7.2 kcal/g). The excessive intake of dietary energy leads to weight gain and obesity (^ p. 230).

An adequate intake of minerals (inorganic compounds), especially calcium (800mg/day; ^ p. 290ff.), iron (10-20 mg/day; ^ p. 90) and iodine (0.15 mg/day; ^ p. 288), is essential for proper body function. Many trace elements (As, F, Cu, Si, V, Sn, Ni, Se, Mn, Mo, Cr, Co) are also essential. The normal diet provides sufficient quantities of them, but excessive intake has toxic effects.

Vitamins (A, B1, B2, B6, B12, C, D2, D3, E, H (biotin), Ki, K2, folic acid, niacinamide, pan-tothenic acid) are compounds that play a vital role in metabolism (usually function as coenzymes). However, the body cannot produce (or sufficient quantities of) them. A deficiency of vitamins (hypovitaminosis) can lead to specific conditions such as night blindness (vit. A), scurvy (vit. C), rickets (vit. D = calciferol; ^ p. 292), anemia (vit. B12 = cobalamin; folic acid; ^ p.90), and coagulation disorders (vit. K; ^ p. 104). An excessive intake of certain vitamins like vitamin A and D, on the other hand, can be toxic (hypervitaminosis).

I— A. Energy content of foodstuffs and energy requirement

Fats

Proteins

Carbohydrates

Requirement (g/day)

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