# 13 - SECTION 2 Nutrition

## SECTION 2 Nutrition

Flum DR: Acute appendicitis—appendectomy of the “antibiotics first” 

strategy. N Engl J Med 372:1937, 2015.
Khan S et al: Endoscopic retrograde appendicitis therapy: Is it really a 
need of the hour. Ann Surg 277:e1, 2023.
Moris D et al: Diagnosis and management of acute appendicitis adults. 
JAMA 326:2299, 2021.
Ohle R et al: The Alvarado score for predicting acute appendicitis: A 
systematic review. BMC Med 9:139, 2011.
Talan DA, DiSaverio S: Treatment of acute uncomplicated appendi­
citis. N Engl J Med 385:1116, 2021.
Vons C et al: Amoxicillin plus clavulanic acid versus appendicectomy 
for treatment of acute uncomplicated appendicitis: An open-label, 
non-inferiority, randomised controlled trial. Lancet 377:1573, 2011.
Section 2	 Nutrition
Johanna T. Dwyer

Nutrient Requirements 

and Dietary Assessment
PART 10
Disorders of the Gastrointestinal System
Nutrients are substances that are essential but not synthesized in suf­
ficient amounts in the body and therefore must be supplied by the 
diet. Nutrient requirements for groups of healthy persons have been 
determined experimentally. The absence of essential nutrients leads 
to growth impairment, organ and metabolic dysfunction, and failure 
to maintain nitrogen balance or adequate status of protein and other 
nutrients. For good health, we require energy-providing nutrients (pro­
tein, fat, and carbohydrate), vitamins, minerals, and water. Require­
ments for organic nutrients include 9 essential amino acids, several 
fatty acids, glucose, 4 fat-soluble vitamins, 10 water-soluble vitamins, 
dietary fiber, and choline. The inorganic nutrients include 4 minerals, 
7 trace minerals, 3 electrolytes, and the ultra trace elements that must 
also be supplied by diet. Individuals of different ages and physiologic 
states differ in the amounts of nutrients they require.
Conditionally essential nutrients are not required in the diet but 
must be supplied to certain individuals such as those with genetic 
defects; pathologies such as infection, disease, or trauma with nutri­
tional implications; and some developmentally immature infants 
who need them because they do not synthesize inositol, taurine, 
arginine, and/or glutamine in adequate amounts. Many other organic 
and inorganic bioactive compounds that are present in foods and 
dietary supplements may also have health effects, including pesticides, 
heavy metals like lead, phytochemicals, zoochemicals, and microbial 
products.
■
■ESSENTIAL NUTRIENT REQUIREMENTS
Energy 
The estimated energy requirement (EER) is the predicted 
average requirement to maintain energy balance and health in an adult 
of a defined age, sex, weight, height, level of physical activity, and life 
stage. For weight to remain stable, energy intake must match total 
energy expenditure (TEE). The major components of TEE are resting 
energy expenditure (REE) and the physical activity level (PAL), and 
minor components include the energy cost of metabolizing food (ther­
mic effect of food, or specific dynamic action) and cold-induced (shiv­
ering) thermogenesis. The average energy intake is ~2600 kcal/d for 
American men and ~1800 kcal/d for American women, but individuals 
vary with body size and activity level. Although estimates introduce 
considerable error, energy intakes are usually calculated from formulas 

rather than measuring TEE directly. In individuals whose weights are 
stable, for males, REE = 900 + 10m, and for females, REE = 700 + 7m, 
where m is mass in kilograms. The calculated REE is then multiplied 
by the appropriate PAL to account for physical activity that ranges from 
1.4 (inactive) for individuals who perform only essential activities of 
daily living (e.g., 30 minutes walking and 90 minutes light to moderate 
activity) to 1.6 (low active), 1.75 (active), and 2.05 (very active). The 
result is the estimated energy (EER) or intake. When describing their 
PALs, sedentary, inactive, and low active individuals tend to overes­
timate, and so in practice, use of PALs of 1.2 to 1.4 may be closer to 
their actual activity levels. The EER provides a rough target for plan­
ning caloric needs for a person of a certain age, sex, weight, height, 
and physical activity level who is neither gaining nor losing weight. 
For further discussion of energy balance in health and disease, see 
Chap. 345.
Protein 
Dietary protein consists of both “essential” (e.g., not 
synthesized endogenously and must be supplied by diet) and “nones­
sential” (e.g., synthesized endogenously or obtained from diet) amino 
acids that are all required for protein synthesis. The nine essential 
amino acids are histidine, isoleucine, leucine, lysine, methionine/
cystine, phenylalanine/tyrosine, threonine, tryptophan, and valine. 
Several amino acids, such as alanine, arginine, aspartic acid, glutamic 
acid, glutamine, and glycine can also be converted to glucose and used 
for energy and gluconeogenesis. When energy intake is inadequate, 
protein intake must be increased, because ingested amino acids are 
diverted into pathways of glucose synthesis and oxidation. In extreme 
energy deprivation, protein-calorie malnutrition may ensue (Chap. 345).
For adults, the recommended dietary allowance (RDA) for protein is 
~0.8 g/kg desirable body mass per day, assuming that energy needs are 
met and that the protein is of relatively high biological value. Current 
recommendations for a healthy diet call for at least 10–14% of calories 
from protein. Most American diets provide at least those amounts. Bio­
logic value tends to be highest for animal proteins, followed by proteins 
from legumes (beans), cereals (rice, wheat, corn), and roots. Combina­
tions of plant proteins that complement one another in their essential 
amino acid profiles or combinations of animal and plant proteins can 
increase biologic value and lower total protein intakes necessary to 
meet requirements. In healthy people with adequate diets, the timing 
of protein intake over the course of the day has little effect.
Protein needs increase during growth, pregnancy, lactation, and 
rehabilitation after injury or undernutrition. Tolerance to dietary pro­
tein is decreased in renal insufficiency (with consequent uremia) and 
in liver failure. Usual protein intakes can precipitate encephalopathy in 
patients with cirrhosis of the liver.
Fat and Carbohydrate 
Carbohydrate (4 kcal/g), fat (9 kcal/g), and 
protein (4 kcal/g) all provide energy. So does alcohol (ethanol) (7 kcal/g), 
but it is not a nutrient. Fats are a concentrated source of energy and 
constitute, on average, 34% of calories in U.S. diets. However, for 
optimal health, fat intake should total no more than 30% of calories. 
Saturated fat and trans-fat should be limited to <10% of calories and 
polyunsaturated fats to <10% of calories, with monounsaturated fats 
accounting for the remainder of fat intake. At least 45–55% of total 
calories should be derived from carbohydrates, with <10% and prefer­
ably <6% from added sugars. The brain requires ~100 g of glucose per 
day for fuel; other tissues use ~50 g/d. Some tissues (e.g., brain and 
red blood cells) rely on glucose supplied either exogenously or from 
muscle proteolysis. Over time, during hypocaloric states, adaptations 
that lower carbohydrate needs are possible.
Water 
For adults, 1–1.5 mL of water per kilocalorie of energy 
expenditure is sufficient under usual conditions to allow for normal 
variations in physical activity, sweating, and the diet’s solute load. 
Water losses include 50–100 mL/d in the feces; 500–1000 mL/d 
by evaporation or exhalation; and, depending on the renal solute 
load, ≥1000 mL/d in the urine. If external losses increase, intakes must 
increase accordingly to avoid underhydration. Fever increases water 
losses by ~200 mL/d per °C; diarrheal losses vary but may be as great 
as 5 L/d in severe diarrhea. Heavy sweating, vigorous exercise, and