Nutrients and Metabolism

• Understanding the process of how the body converts nutrients into energy helps in advising patients on dietary choices that allow their bodies to operate at peak performance.

Nutrients

• Nutrients are substances in food needed for growth, maintenance, and repair.
• There are five categories of nutrients.
• Macronutrients, including carbohydrates, lipids, and proteins, make up most of the diet.
• Micronutrients, such as vitamins and minerals, are equally important, and requirements are small.
• Most nutrients serve as metabolic fuel and some are for building molecules and cells.
• Water, which accounts for approximately 60% of the food volume, is also needed.
• Essential nutrients, about 40 molecules, must be provided by the diet.
• Cells, especially liver cells, can convert one type of molecule to another, allowing adjustment to varying food intakes through interconversions.
• The energy value of nutrients is measured in kilocalories (kcal).
• One kcal is the amount of heat needed to raise the temperature of 1 kg of H₂O by 1°C; 1 kcal is equal to one calorie (C).
• Carbohydrates and proteins provide 4 kcal/g, while lipids provide almost 9 kcal/g.
• USDA's MyPlate guidelines are represented as portions on a dinner plate, including fruits, vegetables, grains, protein, and dairy.
• Basic dietary principles include only eating what is needed, consuming plenty of fruits, vegetables, and whole grains, and avoiding junk food.

Carbohydrates

• Dietary sources include mostly plants, except for milk sugar (lactose) and small amounts of glycogen.
• Sugars (mono- and disaccharides) are found in fruits, sugarcane, sugar beets, honey, and milk.
• Starch (polysaccharide) is present in grains and vegetables.
• Insoluble fiber (cellulose), found in vegetables, provides roughage that increases stool bulk and facilitates defecation.
• Soluble fiber (like pectin in apples and citrus) reduces blood cholesterol levels.
• Glucose is the carbohydrate molecule cells use to make ATP.
• Fructose and galactose are converted to glucose by the liver before entering circulation.
• While many cells utilize fats for energy, neurons and RBCs rely almost entirely on glucose.
• Excess glucose is converted to glycogen or fat for later use.
• Other uses of carbohydrates include building nucleic acids (with pentose sugars) and cell's glycocalyx (with short chain sugars).
• The recommended daily intake is 45–65% of total calories; the typical American adult consumes about 46%.
• The diet should consist mostly of complex carbohydrates (whole grains and vegetables) rather than simple sugars (monosaccharides and disaccharides).
• Large amounts of refined, sugary foods ("empty calories") can lead to obesity and nutritional deficiencies.
• Because starchy foods (rice, pasta, breads) are less expensive than high-protein foods, carbohydrates often make up a greater percentage of the diet in low-income groups.

Lipids

• They are primarily triglycerides (neutral fats), including saturated fats in meat, dairy, and tropical plants, trans fats in hydrogenated oils, and unsaturated fats in seeds, nuts, olive oil, and most vegetable oils.
• Cholesterol is found in egg yolk, meats, organ meats, shellfish, and milk products.
• The liver makes approximately 85% of blood cholesterol.
• Liver cannot synthesize the two essential fatty acids, but they are found in most vegetable oils.
• Linoleic acid is an omega-6 fatty acid, a component of lecithin.
• Linolenic acid is an omega-3 fatty acid.
• Adipose tissue provides protective cushioning, insulation, and energy storage.
• Phospholipids are essential components of myelin sheaths (neurons) and cell membranes.
• Cholesterol supports the stability of cell membranes and it is a precursor of bile salts and steroid hormones.
• Prostaglandins are regulatory molecules made from linoleic acid, playing a role in smooth muscle contraction, regulation of blood pressure, and inflammation.
• Triglycerides are a major energy source for skeletal muscle and liver cells.
• Lipids help the body absorb fat-soluble vitamins.
• Fats should constitute 20-35% of total caloric intake (though it is higher in a typical American diet, at >40%).
• Dietary intake should limit saturated fats to 10% or less of the total fat intake.
• It is not required in diet because cholesterol can be synthesized to meet needs.
• Many recommend a low intake, especially for those with high blood cholesterol levels, as it is associated with cardiovascular disease.

Proteins

• Dietary sources include animal products (eggs, milk, fish, most meats) and soybeans, which provide complete proteins-meeting all amino acid requirements.
• Legumes, nuts, and cereal grains contain incomplete proteins-low in one or more essential amino acids.
• Cereal grains and legumes, when ingested together, provide all essential amino acids.
• Proteins are structural materials such as keratin (skin), collagen, and elastin (connective tissue), and muscle proteins.
• Functional molecules like enzymes and protein hormones are Proteins that control activities.
• Multiple factors determine whether amino acids in a cell are used to synthesize new proteins or used for energy (to make ATP).
• All amino acids needed to build a particular protein must be present at the same time, according to the all-or-none rule.
• If one or more amino acids are insufficient, the protein can't be made, and its amino acids are instead used as energy or converted to carbs or fats.
• Adequacy of caloric intake: If carbohydrate or fat calorie is insufficient for ATP needs, proteins are used as energy.
• Anabolic hormones, like GH and gonadal steroids, promote protein synthesis; other hormones, such as glucocorticoids, promote protein breakdown and conversion of amino acids to glucose.
• Nitrogen balance is a homeostatic state where the rate of protein synthesis equals the rate of breakdown and loss, and the amount of nitrogen ingested (via protein) equals the amount excreted.
• synthesis exceeds breakdown is identified as Positive nitrogen balance and normal in growing children and pregnant women and tissue repair.
• breakdown for energy exceeds synthesis is a Negative nitrogen balance, which occurs during stress, burns, infection, injury, or a diet with low quality or quantity of protein, or starvation.
• Dietary requirements include supplying essential amino acids and making nonessential ones.
• The amount of protein needed depends on age, size, metabolic rate, and current nitrogen balance.
• Guidelines says a daily intake of 0.8 g per kg of body weight.

Vitamins

• Vitamins are organic compounds the body requires in minute amounts.
• Vitamins are not an energy source themselves, but needed to use macronutrients-dietary carbohydrates, proteins, and fats cannot perform without vitamins.
• Most vitamins are coenzymes (or parts of coenzymes) that act with an enzyme to carry out a particular reaction. For example, when glucose is used to make ATP, B vitamins act as coenzymes.
• Most vitamins must be ingested, except for vitamin D (made in skin) and vitamins B and K (synthesized by intestinal bacteria).
• The body can convert beta-carotene (orange pigment in carrots) to vitamin A.
• A balanced diet is the best way to avoid deficiencies, as no single major food group contains all vitamins.
• Water-soluble vitamins: B complex and C are absorbed with water.
• B12 absorption needs intrinsic factor (secreted from stomach glands).
• There is no significant storage in the body, therefore absorbed vitamins not used by cells are excreted in urine, making problems from excessive intake are rare.
• Fat-soluble vitamins: A, D, E, and K are absorbed with lipids in the gut. Problems with lipid absorption can interfere with uptake of fat-soluble vitamins.
• It is stored in the body the exception of vitamin K. Excessive intake can cause health problems.
• Free radicals (molecules with an unpaired electron) are generated during normal metabolism.
• Vitamins A, C, and E, and the mineral selenium, are antioxidants, participating in antioxidant reactions that neutralize dangerous free radicals.
• Megadoses of vitamins are not beneficial and may cause serious health problems, especially with fat-soluble vitamins.

Minerals

• Seven minerals are required in moderate amounts, plus trace amounts of others: calcium, phosphorus, potassium, sulfur, sodium, chlorine, and magnesium.
• Like vitamins, minerals work with nutrients for proper body functioning.
• Minerals are incorporated into structures to make them stronger; for example, calcium, phosphorus, and magnesium salts harden teeth and strengthen bone.
• Most minerals are ionized in body fluids or bound to organic compounds to form phospholipids, hormones, and various proteins.
• Iron is an essential part of oxygen-binding heme of hemoglobin.
• Sodium and chloride are major electrolytes in blood.
• Iodine is necessary for thyroid hormone synthesis.
• Uptake and excretion are balanced to prevent toxic overload.
• Natural sodium in foods poses little to no health risk, compared to the large amounts added to processed foods and sprinkled on food, which may cause fluid retention and high blood pressure.
• Mineral-rich foods include legumes and other vegetables, milk, and some meats.

Metabolism

• Metabolism is the sum of all biochemical reactions in the body involving nutrients.
• Substances are constantly built up (anabolism) and broken down (catabolism).
• The body uses lots of energy for essential activities, even at rest, like breathing and absorbing nutrients from food.

Anabolism and Catabolism

• Anabolism refers to reactions that build larger molecules or structures from smaller ones, such as the synthesis of proteins from amino acids.
• Catabolism refers to reactions that break down more complex structures into simpler ones, such as the hydrolysis of proteins into amino acids.
• Stage 1 of processing energy-containing nutrients: digestion and absorption in the gastrointestinal tract.
• Stage 2, which takes place in the cytoplasm, involves newly delivered nutrients either being built into lipids, proteins, and glycogen via anabolic pathways or broken down by catabolic pathways into smaller fragments, like pyruvate.
• Stage 3 occurs in the mitochondria and involves the complete breakdown of stage 2 products, typically converted into acetyl CoA, which uses oxygen and produces carbon dioxide, water, and significant amounts of ATP.
• Cellular respiration is a group of catabolic reactions, including glycolysis, the citric acid cycle, and oxidative phosphorylation, to convert some of the chemical energy of nutrients (like glucose) into a form of chemical energy (ATP) that cells can use to do work.
• Phosphorylation transfers a high-energy phosphate group from ATP to another molecule.
• The transfer primes the molecule; this transfer of a high-energy group changes the molecule in a way that increases its activity, produces motion, or does work.
• The body stores energy as glycogen and triglycerides, then breaks them down later to produce ATP for cellular use.

Oxidation-Reduction Reactions and the Role of Coenzymes

• Many reactions in cells are oxidation reactions
• Oxidation is the gain of oxygen or the loss of hydrogen atoms (with their electrons).
• Oxidized substance always loses (or nearly loses) electrons as they move to (or toward) a substance that more strongly attracts them
• Substances catalyze removal of hydrogen atoms, called Dehydrogenases.
• substances catalyze transfer of oxygen, called Oxidases.
• Two important coenzymes of the oxidative pathway: Nicotinamide adenine dinucleotide (NAD+) and Flavin adenine dinucleotide (FAD).

ATP Synthesis

• Two mechanisms capture (as ATP) some of the energy released via cellular respiration
• Direct transfer of high-energy phosphate group from substrate to ADP refers to Substrate-level phosphorylation.
• Its Occurs twice in glycolysis (enzymes in cytosol) and once in Krebs cycle (enzymes in mitochondria).
• Most of the ATP. is produced by Oxidative phosphorylation.
• ~50% of energy released via nutrient oxidation used to pump H+ across inner membrane, creating steep [H+] gradient refers to Electron transport chain.
• Diffusion of H+ across inner membrane through protein ATP synthase provides energy to attach phosphate groups to ADP, making ATP refers to Chemiosmosis.