Page 239 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

10 Nutrition and Digestion

vide the largest portion of the energy require-
Nutrition
ment. They are basically interchangeable
An adequate diet must meet the body’s energy sources of energy. The energy contribution of
requirements and provide a minimum of car- carbohydrates can fall to about 10% (normally
bohydrates, proteins (incl. all essential amino 60%) before metabolic disturbances occur.
acids) and fats (incl. essential fatty acids). Min- Fat is not essential provided the intake of
erals (incl. trace elements), vitamins, and suffi- fat-soluble vitamins (vitamins E, D, K and A)
cient quantities of water are also essential. To and essential fatty acids (linoleic acid) is suffi-
ensure a normal passage time, especially cient. About 25–30% of dietary energy is sup-
through the colon, the diet must also provide a plied by fat (one-third of which is supplied as
sufficient amount of roughage (indigestible essential fatty acids; ! A), although the pro-
plant fibers—cellulose, lignin, etc.). portion rises according to energy require-
The total energy expenditure (TEE) or total ments (e.g., about 40% during heavy physical
metabolic rate consists of (1) the basal meta- work). Western diets contain generally too
bolic rate (BMR), (2) the activity energy costs, much energy (more fats than carbohydrates!)
and the (3) diet-induced thermogenesis (DIT; considering the generally low level of physical
! p. 228, 231 A). TEE equals BMR when activity of the Western lifestyle. Alcohol also
measured (a) in the morning (b) 20 h after the contains superfluous energy (ca. 30 kJ/g =
last meal, (3) resting, reclining, (4) at normal 7.2 kcal/g). The excessive intake of dietary
body temp., and (5) at a comfortable ambient energy leads to weight gain and obesity
temp. (! p. 224). The BMR varies according to (! p. 230).
sex, age, body size and weight. The BMR for a An adequate intake of minerals (inorganic
young adult is ca. 7300 kJ/day (! 1740 kcal/ compounds), especially calcium (800 mg/day;
day; see p. 374 for units) in men, and ca. 20% ! p. 290ff.), iron (10–20 mg/day; ! p. 90) and
lower in women. During physical activity, TEE iodine (0.15 mg/day; ! p. 288), is essential for
increases by the following factors: 1.2-fold for proper body function. Many trace elements
sitting quietly, 3.2-fold for normal walking, (As, F, Cu, Si, V, Sn, Ni, Se, Mn, Mo, Cr, Co) are
and 8-fold for forestry work. Top athletes can also essential. The normal diet provides suffi-
perform as much as 1600 W (= J/s) for two cient quantities of them, but excessive intake
hours (e.g., in a marathon) but their daily TEE is has toxic effects.
much lower. TEE also increases at various Vitamins (A, B 1, B 2, B 6, B 12, C, D 2, D 3, E, H
degrees of injury (1.6-fold for sepsis, 2.1-fold (biotin), K 1, K 2, folic acid, niacinamide, pan-
for burns). 1!C of fever increases TEE 1.13-fold. tothenic acid) are compounds that play a vital
Protein, fats and carbohydrates are the three role in metabolism (usually function as
basic energy substances (! B). coenzymes). However, the body cannot pro-
An adequate intake of protein is needed to duce (or sufficient quantities of) them. A defi-
maintain a proper nitrogen balance, i.e., ciency of vitamins (hypovitaminosis) can lead
balance of dietary intake and excretory output to specific conditions such as night blindness
of nitrogen. The minimum requirement for (vit. A), scurvy (vit. C), rickets (vit. D = cal-
protein is 0.5 g/kg BW per day ( functional min- ciferol; ! p. 292), anemia (vit. B 12 = cobalamin;
imum). About half of dietary protein should be folic acid; ! p. 90), and coagulation disorders
animal protein (meat, fish, milk and eggs) to (vit. K; ! p. 104). An excessive intake of certain
ensure an adequate supply of essential amino vitamins like vitamin A and D, on the other
acids such as histidine, isoleucine, leucine, ly- hand, can be toxic (hypervitaminosis).
sine, methionine, phenylalanine, threonine,
tryptophan and valine (children also require
arginine). The content of most vegetable pro-
teins is only about 50% of animal protein.
Carbohydrates (starch, sugar, glycogen) and
226
fats (animal and vegetable fats and oils) pro-
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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