Caloric Intake and Distribution

Caloric Intake and Distribution

• Dietary intake should match energy expenditure to maintain body weight.
• Daily energy needs include 2000 kcal/d for basal needs, plus 500–2500 kcal/d (or more) for daily activities.
• Calorie distribution among carbs, protein, and fat is affected by physiology, taste, and economics.
• A daily protein intake of 1 g/kg body weight supplies the eight essential amino acids.
• Essential amino acids cannot be synthesized by the body and must come from the diet.
• Grade I proteins (animal sources like meat, fish, dairy, eggs) have all amino acids needed for protein synthesis.
• Grade II proteins (plant sources) have varying amino acid proportions and may lack some essential ones.
• Fat is the most energy-dense food source, providing 9.3 kcal/g.
• A low-fat diet is acceptable if essential fatty acid needs are met, and it's desirable to limit saturated fats.
• Western diets often contain high fat amounts (100 g/d or more).
• Carbohydrates are the most affordable calorie source, often providing 50%+ of dietary calories.
• The average middle-class American diet consists of 50% carbs, 15% protein, and 35% fat.
• Dietary needs are calculated by first meeting protein requirements, then dividing remaining calories between fat and carbs based on preferences, income, etc.

Digestion and Absorption - Carbohydrates

• Dietary carbohydrates are mainly polysaccharides, disaccharides, and monosaccharides.
• Salivary α-amylase starts starch digestion in the mouth.
• The optimal pH for salivary α-amylase is 6.7.
• Salivary α-amylase remains partially active in the stomach despite its acidity due to the protection of its active site by substrate.
• Salivary and pancreatic α-amylase continue polysaccharide digestion in the small intestine.
• These enzymes hydrolyze internal α1:4 linkages, but not α1:6 or terminal α1:4 linkages.
• α-amylase digestion end products are oligosaccharides like maltose, maltotriose, and α-limit dextrins.
• Oligosaccharidases for further starch derivative digestion are located in small intestinal epithelial cell brush borders.
• Deficiency in brush border oligosaccharidases can cause diarrhea, bloating, and flatulence after sugar intake.
• Bloating and flatulence occur due to gas production (CO2 and H2) from disaccharide residues in the lower small intestine and colon.

Digestion and Absorption - Protein

• Protein digestion starts in the stomach, where pepsin breaks peptide bonds.
• Pepsin is secreted as an inactive proenzyme (pepsinogen) and activated in the GI tract.
• Pepsinogen, the pepsin precursor, is activated by gastric acid.
• Pepsin hydrolyzes bonds between aromatic amino acids (phenylalanine, tyrosine) and other amino acids, producing polypeptides of various sizes.
• Pepsin's action stops when gastric contents mix with alkaline pancreatic juice in the duodenum and jejunum, as it has a pH optimum of 1.6-3.2.
• The pH of intestinal contents in the duodenal bulb is 3.0-4.0, rising to about 6.5 in the rest of the duodenum.

Digestion and Absorption - Lipids

• Lingual lipase, secreted from Ebner glands on the tongue's dorsal surface, and gastric lipase initiate fat digestion.
• Pancreatic lipase in the duodenum is key for fat digestion.
• Pancreatic lipase hydrolyzes the 1- and 3- bonds of triglycerides (triacylglycerols) easily, but not the 2-bonds, yielding FFA and 2-monoglycerides (2-monoacylglycerols).
• Colipase, secreted in pancreatic juice, helps stabilize pancreatic lipase in its active form.
• Cholesterol esterase, another pancreatic lipase activated by bile acids, makes up about 4% of pancreatic juice protein.
• Cholesterol esterase acts on cholesterol esters, fat-soluble vitamin esters, phospholipids, and triglycerides.
• Fats are poorly soluble, limiting their ability to reach mucosal cells.
• Bile acids, phosphatidylcholine, and monoglycerides emulsify fats in the small intestine.
• High intestinal bile acid concentrations post-meal cause lipids and bile acids to form micelles spontaneously.
• Micelles solubilize lipids and enable their transport to enterocytes.

Digestion and Absorption - Minerals

• Daily mineral intake is needed for health, including trace elements found in minute tissue amounts.
• Iron deficiency leads to microcytic hypochromic anemia.
• Cobalt, part of vitamin B12, deficiency causes megaloblastic anemia.
• Iodine deficiency causes thyroid disorders.
• Zinc deficiency causes skin ulcers, depressed immunity, and hypogonadal dwarfism.
• Copper deficiency leads to anemia and bone ossification changes.
• Chromium deficiency causes insulin resistance.
• Fluorine deficiency causes dental caries.
• Na/K are essential minerals.
• Iron excess causes hemochromatosis.
• Copper excess causes brain damage (Wilson disease).

Digestion and Absorption - Vitamins

• Vitamins are organic dietary components needed for life, health, and growth. They don't supply energy and can't be synthesized by the body.
• Most vitamins are absorbed in the upper small intestine.
• Vitamin B12 is absorbed in the ileum.
• Water-soluble vitamins are easily absorbed.
• Fat-soluble vitamins (A, D, E, K) are poorly absorbed without bile or pancreatic juice, relying on micellar solubilization.
• High doses of fat-soluble vitamins can be toxic.
  • Hypervitaminosis A symptoms: anorexia, headache, hepatosplenomegaly, irritability, scaly dermatitis, hair loss, bone pain.
  • Hypervitaminosis D symptoms: weight loss, soft tissue calcification, acute kidney injury.
  • Hypervitaminosis K symptoms: gastrointestinal issues and anemia
  • Large doses of water-soluble vitamins are less likely to cause problems due to rapid clearance.
• High pyridoxine (vitamin B6) doses can cause peripheral neuropathy.