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Atwater factors assign 4 kcal/g to protein, 4 kcal/g to carbohydrate, and 9 kcal/g to fat. The calculation is (20 × 4) + (30 × 4) + (10 × 9) = 80 + 120 + 90 = 290 kcal.
Essential amino acids cannot be synthesized in adequate amounts and must be supplied by the diet. Leucine is one of the nine essential amino acids, whereas alanine, glutamine, and serine are nonessential.
Cereal proteins such as wheat are typically low in lysine relative to human needs, making lysine the limiting amino acid. Legumes complement cereals because they are richer in lysine.
PDCAAS compares a food's amino acid pattern with a reference pattern and then corrects the score for true fecal digestibility. Scores are truncated at 1.0 because values above that do not confer additional protein quality benefit.
Humans lack the desaturases needed to introduce double bonds beyond the ninth carbon, so linoleic acid (omega-6) and alpha-linolenic acid (omega-3) must come from the diet. Other fatty acids can be synthesized or are not dietary essentials.
VLDL is assembled in the liver and transports endogenous triglycerides to tissues, where lipoprotein lipase releases fatty acids. Chylomicrons carry dietary fat, LDL carries cholesterol, and HDL participates in reverse cholesterol transport.
Viscous soluble fibers such as oat beta-glucan trap bile acids and increase their fecal excretion, prompting the liver to use cholesterol for bile acid synthesis. Insoluble fibers such as cellulose and wheat bran mainly add bulk.
The standard glycemic index method gives 50 g of available (digestible) carbohydrate from the test food and from a reference food, usually glucose. Fiber is excluded because it does not raise blood glucose in the same way.
Salivary alpha-amylase hydrolyzes alpha-1,4 glycosidic bonds in starch, producing dextrins and maltose. Pancreatic amylase continues this work in the small intestine, while maltase and sucrase act at the brush border.
Bile salts are amphipathic and emulsify large fat droplets into smaller ones, increasing the surface area for pancreatic lipase. They do not hydrolyze fat or carry fatty acids into cells.
Bile salts form micelles that ferry fatty acids, monoglycerides, and fat-soluble vitamins to the brush border for absorption. Chylomicrons and lipoproteins are formed inside enterocytes or circulate in lymph and blood.
Vitamin C maintains iron in the ferrous state and is required for proline and lysine hydroxylation, which stabilizes collagen triple helices. Carboxylation of glutamate is vitamin K-dependent.
Parietal cells secrete intrinsic factor, which binds vitamin B12 and allows its receptor-mediated absorption in the terminal ileum. Lack of intrinsic factor causes pernicious anemia.
Folate coenzymes transfer one-carbon units needed for thymidylate and purine synthesis, so deficiency impairs DNA synthesis and cell division. This explains the megaloblastic anemia seen in folate deficiency.
Vitamin D3 is hydroxylated at carbon 25 in the liver and then at carbon 1 in the kidney to form 1,25-dihydroxyvitamin D (calcitriol). The kidney step is regulated by parathyroid hormone and is the final activation.
Ascorbic acid reduces ferric iron to ferrous iron and forms a soluble complex, enhancing nonheme iron absorption. Phytic acid, tannins, and calcium inhibit absorption.
Zinc is required for cell proliferation, immune function, and taste and smell acuity, so deficiency causes poor wound healing, taste alteration, and growth retardation. Night blindness reflects vitamin A deficiency, and goiter reflects iodine deficiency.
Iodine is incorporated into thyroxine (T4) and triiodothyronine (T3) in the thyroid gland. Deficiency leads to goiter and, in pregnancy, impaired fetal neurodevelopment.
ADH inserts aquaporin-2 channels in the collecting duct, allowing water to be reabsorbed down the medullary osmotic gradient. The proximal tubule reabsorbs most water constitutively, not under ADH control.
Metabolic acidosis lowers blood pH, stimulating peripheral chemoreceptors and increasing ventilation. The resulting fall in PaCO2 shifts the bicarbonate buffer equation and raises pH within minutes; renal bicarbonate reabsorption is a slower compensation.
Lean body mass is the strongest determinant of basal metabolic rate because muscle and organ tissue are metabolically active. BMR declines with age and energy restriction and is slightly lower in females on average, largely due to body composition.
The thermic effect of food is the increase in energy expenditure after eating, reflecting digestion, absorption, transport, and metabolism of nutrients. It is distinct from basal metabolism and activity energy expenditure.
During prolonged fasting or low carbohydrate intake, hepatic fatty acid oxidation produces excess acetyl-CoA, which is converted to acetoacetate and beta-hydroxybutyrate. Glucose and glycerol are gluconeogenic substrates rather than direct ketone precursors.
Alanine released from muscle is taken up by the liver and converted to pyruvate, which enters gluconeogenesis through the alanine-glucose cycle. Fatty acids and ketone bodies cannot be converted to glucose in humans because pyruvate dehydrogenase is irreversible.
Trans fatty acids raise LDL cholesterol and lower HDL cholesterol, producing a more atherogenic lipid profile than saturated fat at equivalent intakes. They also increase inflammation and endothelial dysfunction.
Atwater factors assign 9 kcal per gram to fat, 4 kcal per gram to carbohydrate and protein, and 7 kcal per gram to alcohol. These values reflect metabolizable energy after accounting for digestion and absorption losses.
Cereal proteins contain relatively little lysine, so lysine is the amino acid that limits protein synthesis when grains are the main protein source. Legumes, by contrast, are typically limiting in the sulfur amino acid methionine.
Potassium is the dominant intracellular cation, maintained inside cells largely by the sodium-potassium pump. Sodium is the principal extracellular cation, with chloride as its main accompanying anion.
Retinal, derived from vitamin A, is required to form rhodopsin in the rod cells that mediate vision in dim light. Depletion of rhodopsin therefore impairs dark adaptation before other deficiency signs appear.
The RDA is defined as the EAR plus two standard deviations of the requirement distribution. This level is intended to cover the needs of about 97 to 98 percent of healthy individuals in a group.
The adult protein RDA is 0.8 g per kilogram of body weight per day, based on nitrogen balance studies in healthy adults. Higher intakes are recommended during pregnancy, lactation, and recovery from illness or injury.
Negative nitrogen balance occurs when protein catabolism exceeds intake, as in the marked catabolic stress of major burns. Growth and pregnancy are anabolic states that ordinarily produce positive nitrogen balance.
Lactase is a brush-border hydrolase of the small intestinal villi that splits lactose into glucose and galactose for absorption. Reduced brush-border lactase activity produces lactose malabsorption and osmotic symptoms.
Alpha-linolenic acid (18:3 n-3), commonly called linolenic acid, cannot be synthesized by humans because they lack the desaturase needed to introduce a double bond beyond carbon 9. Linoleic acid is the essential omega-6 fatty acid.
Thiamin pyrophosphate is the coenzyme for oxidative decarboxylation reactions, including the pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase complexes. This is why thiamin deficiency impairs glucose oxidation and produces neurologic and cardiac signs.
Vitamin K is the cofactor for gamma-glutamyl carboxylase, which carboxylates glutamate residues on clotting factors II, VII, IX, and X so they can bind calcium and phospholipid. Without this modification, these factors are secreted in an inactive form.
Selenium is incorporated as selenocysteine into glutathione peroxidase and other selenoproteins that reduce hydrogen peroxide and lipid peroxides. It also supports the deiodinases that convert thyroxine to its active form.
Rapid refeeding of a malnourished patient drives insulin release, which shifts phosphate, potassium, and magnesium into cells. The resulting hypophosphatemia is the defining biochemical feature and can cause cardiac and respiratory failure.
Transthyretin, measured as prealbumin, has a half-life of about two days, so it responds quickly to changes in protein intake and inflammatory status. Albumin has a half-life of roughly 20 days and changes too slowly to reflect recent nutrition.
Fructose enters the enterocyte by facilitated diffusion through GLUT5 on the brush-border membrane. Glucose, by contrast, is taken up by the sodium-dependent transporter SGLT1, and both sugars exit the basolateral side via GLUT2.
The respiratory quotient for fat oxidation is about 0.70, for protein about 0.82, and for carbohydrate 1.00. A value of 0.70 therefore indicates that fat is the dominant fuel source.
Leucine is degraded entirely to acetyl-CoA and acetoacetate, so it is the only purely ketogenic amino acid along with lysine. Isoleucine yields both acetyl-CoA and propionyl-CoA and is therefore both ketogenic and glucogenic.
A deficit of 500 kcal per day totals 3,500 kcal over 7 days, which approximates the energy in 0.5 kg (1 lb) of body fat. Deficit-based predictions assume adherence and stable metabolic rate, so actual loss varies.
For a normal prepregnancy BMI of 18.5 to 24.9, the recommended total gain in a singleton pregnancy is about 11.5 to 16 kg, with the gain accelerating in the second and third trimesters. Ranges shift upward or downward according to prepregnancy BMI category.
A daily supplement of 400 mcg of folic acid begun before conception and continued through early pregnancy lowers the risk of neural tube defects. Higher doses are reserved for women with a prior affected pregnancy or specific risk factors.
Exclusive breastfeeding is recommended for about the first 6 months, after which iron-rich complementary foods are introduced while breastfeeding continues. Starting solids much earlier or later than this window raises nutritional and developmental concerns.
Breast milk contains little vitamin D, so 400 IU per day is recommended beginning soon after birth for exclusively breastfed infants. Vitamin K is given as a one-time intramuscular dose at birth rather than daily.
The protein allowance for healthy adults is about 0.8 g per kg of body weight per day. Requirements rise with catabolic illness, wounds, and some training states.
Clear liquids are transparent at room temperature and leave little residue, so strained juices, broth, and gelatin are appropriate. Milk-based and fiber-containing items are not clear liquids.
DASH emphasizes potassium-rich fruits and vegetables, low-fat dairy, whole grains, and limited sodium, saturated fat, and sweets. It is not an extreme macronutrient-restriction diet.