CHAPTER 27Digestion, Absorption, & Nutritional Principles 463balance between caloric intake and energy output. If the calor-
ic content of the food ingested is less than the energy output,
that is, if the balance is negative, endogenous stores are uti-
lized. Glycogen, body protein, and fat are catabolized, and the
individual loses weight. If the caloric value of the food intake
exceeds energy loss due to heat and work and the food is prop-
erly digested and absorbed, that is, if the balance is positive,
energy is stored, and the individual gains weight.
To balance basal output so that the energy-consuming tasks
essential for life can be performed, the average adult must take
in about 2000 kcal/d. Caloric requirements above the basal level
depend on the individual’s activity. The average sedentary stu-
dent (or professor) needs another 500 kcal, whereas a lumber-
jack needs up to 3000 additional kcal per day.
NUTRITION
The aim of the science of nutrition is the determination of the
kinds and amounts of foods that promote health and well-be-
ing. This includes not only the problems of undernutrition
but those of overnutrition, taste, and availability (Clinical Box
27–4). However, certain substances are essential constituents
of any human diet. Many of these compounds have been
mentioned in previous sections of this chapter, and a brief
summary of the essential and desirable dietary components is
presented below.
ESSENTIAL DIETARY COMPONENTS
An optimal diet includes, in addition to sufficient water (see
Chapter 38), adequate calories, protein, fat, minerals, and
vitamins.
CALORIC INTAKE & DISTRIBUTION
As noted above, the caloric value of the dietary intake must be
approximately equal to the energy expended if body weight is
to be maintained. In addition to the 2000 kcal/d necessary to
meet basal needs, 500 to 2500 kcal/d (or more) are required to
meet the energy demands of daily activities.
The distribution of the calories among carbohydrate, pro-
tein, and fat is determined partly by physiologic factors and
partly by taste and economic considerations. A daily proteinFIGURE 27–9 Correlation between metabolic rate and body
weight, plotted on logarithmic scales. The slope of the colored line
is 0.75. The black line represents the way surface area increases with
weight for geometrically similar shapes and has a slope of 0.67.
(Modified from Kleiber M and reproduced with permission from McMahon TA: Size
and shape in biology. Science 1973;179:1201. Copyright © 1973 by the American
Association for the Advancement of Science.)
105104103102101100
10 −^310 −^210 −^1100101102103104
Body weight (kg)Heat production (kcal/d)
MouseRat Guinea pigCats MacaqueRabbitsSheepSteer
GoatChimpanzeeCowElephant
CLINICAL BOX 27–4The Malabsorption Syndrome
The digestive and absorptive functions of the small intestine
are essential for life. However, the digestive and absorptive
capacity of the intestine is larger than needed for normal
function (the anatomic reserve). Removal of short segments
of the jejunum or ileum generally does not cause severe
symptoms, and compensatory hypertrophy and hyperplasia
of the remaining mucosa occur. However, when more than
50% of the small intestine is resected or bypassed, the ab-
sorption of nutrients and vitamins is so compromised that it
is very difficult to prevent malnutrition and wasting (malab-
sorption). Resection of the ileum also prevents the absorp-
tion of bile acids, and this leads in turn to deficient fat ab-
sorption. It also causes diarrhea because the unabsorbed bile
salts enter the colon, where they activate chloride secretion
(see Chapter 26). Other complications of intestinal resection
or bypass include hypocalcemia, arthritis, hyperuricemia,
and possibly fatty infiltration of the liver, followed by cirrho-
sis. Various disease processes can also impair absorption
without a loss of intestinal length. The pattern of deficiencies
that results is sometimes called the malabsorption syn-
drome. This pattern varies somewhat with the cause, but it
can include deficient absorption of amino acids, with marked
body wasting and, eventually, hypoproteinemia and edema.
Carbohydrate and fat absorption are also depressed. Be-
cause of the defective fat absorption, the fat-soluble vita-
mins (vitamins A, D, E, and K) are not absorbed in adequate
amounts. One of the most interesting conditions causing the
malabsorption syndrome is the autoimmune disease celiac
disease. This disease occurs in genetically predisposed indi-
viduals who have the major histocompatibility complex
(MHC) class II antigen HLA-DQ2 or DQ8 (see Chapter 3). In
these individuals gluten and closely related proteins
cause intestinal T cells to mount an inappropriate im-
mune response that damages the intestinal epithelial cells
and results in a loss of villi and a flattening of the mucosa.
The proteins are found in wheat, rye, barley, and to a lesser
extent in oats—but not in rice or corn. When grains contain-
ing gluten are omitted from the diet, bowel function is gen-
erally restored to normal.