Nutrition and Body Weight

Body Weight Regulation

Body weight is maintained when energy intake equals energy output, reflecting a homeostatic set point. Heredity accounts for 30-50% of individual variation, while eating and exercise habits contribute to the rest.

• Homeostatic set point: The body's tendency to maintain a stable weight.

• Heredity: Genetic factors influence body weight.

• Behavioral factors: Diet and physical activity are major contributors.

Calories and Nutrients

Caloric Content and Types of Nutrients

Calories measure the energy content of food. One calorie is the heat required to raise 1 g of water by 1°C. Fats provide about 9 kcal/g, while carbohydrates and proteins provide about 4 kcal/g. Nutrients are classified as macronutrients (needed in large amounts) and micronutrients (needed in small amounts).

• Macronutrients: Carbohydrates, fats, proteins

• Micronutrients: Minerals, vitamins

• Essential nutrients: Cannot be synthesized by the body; must be obtained from diet

• RDA: Recommended daily allowances are safe estimates for daily intake

Food Guide Pyramid

The food guide pyramid illustrates recommended proportions of different food groups for a balanced diet.

Carbohydrates

Structure and Function

Carbohydrates, also known as saccharides, are organic compounds with the formula (CH2O). They are present in the body as blood glucose and glycogen, a glucose polymer used for energy storage. Blood glucose is regulated by insulin and glucagon.

• Blood glucose: C6H12O6

• Glycogen: Polymer of glucose, stored in liver and muscles

• Regulation: Insulin lowers blood glucose; glucagon raises it

Dietary Sources and RDA

Most dietary carbohydrates come from plants, including grains, legumes, fruits, and root vegetables. Intake should favor complex carbohydrates over simple sugars.

• Complex carbohydrates: Provide additional nutrients and fiber

• Simple sugars: Sucrose (table sugar), fructose (fruits, corn syrup), maltose (cereal), lactose (milk)

• RDA: Carbohydrates have the highest recommended daily intake among nutrients

Carbohydrate Metabolism

Dietary carbohydrates are metabolized rapidly after absorption. The main reaction is cellular respiration, which transfers energy from glucose to ATP.

• Cellular respiration equation:

• Energy transfer: Glucose is broken down in a series of steps to produce ATP

Cellular Respiration Steps

Cellular respiration consists of three main reactions: glycolysis, Krebs cycle, and electron transport chain. These steps efficiently transfer energy from glucose to ATP.

• Glycolysis: Occurs in cytosol; breaks glucose into pyruvic acid

• Krebs cycle: Occurs in mitochondria; further breaks down pyruvic acid

• Electron transport chain: Produces most ATP via oxidative phosphorylation

Glycogenesis, Glycogenolysis, and Gluconeogenesis

Excess glucose is stored as glycogen (glycogenesis), stimulated by insulin and epinephrine. Glycogenolysis is the breakdown of glycogen to glucose, stimulated by glucagon during fasting. Gluconeogenesis is the synthesis of glucose from noncarbohydrate sources when glycogen stores are depleted.

• Glycogenesis: Glucose → Glycogen

• Glycogenolysis: Glycogen → Glucose

• Gluconeogenesis: Formation of glucose from fats and amino acids

Dietary Fiber

Dietary fiber is fibrous material that resists digestion. It is important for digestive health, softening stool, increasing bulk, stimulating peristalsis, and lowering blood cholesterol.

• Sources: Oats, beans, peas, carrots, brown rice, fruits

• RDA: 30 g/day

• Benefits: Absorbs water, increases transit time, lowers cholesterol

Lipids

Types and Functions

Lipids include fats, phospholipids, and steroids. They are the body's main energy storage, are hydrophobic, and provide compact energy storage. Lipids spare protein from being used as fuel and are necessary for absorption of fat-soluble vitamins (A, D, E, K).

• Structural: Phospholipids and cholesterol are components of plasma membranes

• Chemical precursors: Cholesterol is a precursor for steroids, bile salts, and vitamin D; fatty acids are precursors for prostaglandins

• Average body fat: Adult male 15%, female 25%

Fat Requirements and Sources

Most fatty acids are synthesized by the body, but essential fatty acids must be consumed. Saturated fats are of animal origin, while unsaturated fats are found in nuts, seeds, and vegetable oils. Cholesterol is found in animal products.

• Saturated fats: Meat, egg yolks, dairy

• Unsaturated fats: Nuts, seeds, vegetable oils

• Cholesterol: Egg yolks, cream, shellfish, organ meats

• Dietary fat: Should be less than 30% of daily calorie intake

Lipid Metabolism: Lipolysis and Lipogenesis

Triglycerides are stored in adipocytes. Lipolysis is the breakdown of fat for fuel, while lipogenesis is the synthesis of fat from amino acids and sugars. These pathways are important for energy balance and are common in diabetics.

• Lipolysis: Fat breakdown; glycerol enters glycolysis

• Lipogenesis: Fat synthesis from other sources

Ketosis

Ketosis occurs during fasting or low carbohydrate intake, when the liver produces ketone bodies from fatty acids. This state is common in diabetics who cannot absorb glucose.

• Ketone bodies: β-hydroxybutyric acid, acetoacetic acid, acetone

Serum Lipoproteins

Lipids are transported in blood as lipoproteins, which are hydrophobic and coated in protein and phospholipid shells. Lipoproteins are categorized by density: chylomicrons, VLDL, LDL, HDL.

• Chylomicrons: Form in small intestine, transport dietary fats

• VLDL: Produced by liver, transport lipids to adipose tissue

• LDL: Deliver cholesterol to cells

• HDL: Remove cholesterol from arteries, deliver to liver for elimination

Desirable Lipoprotein Levels

High HDL levels indicate cholesterol removal from arteries, while high LDL levels correlate with cholesterol deposition and increased risk of cardiovascular disease.

• Total cholesterol: Should be less than 200 mg/dL

• Recommendations: Exercise, avoid smoking, saturated fats, coffee, and stress

Proteins

Functions and Requirements

Proteins make up 12-15% of body mass, mostly in skeletal muscles. They are essential for muscle contraction, cell membranes, fibrous and globular proteins, and plasma proteins.

• RDA: 44-60 g/day

• Essential amino acids: 8 cannot be synthesized and must be obtained from diet

• Complete proteins: Animal proteins supply all essential amino acids

• Plant proteins: Must be combined for complete amino acid profile (e.g., beans and rice)

Protein Metabolism

Amino acids are used to synthesize new proteins. For use as fuel, they must be deaminated, producing ammonia, which is converted to urea by the liver and excreted in urine.

• Deamination: Removal of NH2 group

• Urea: Waste product excreted in urine

Minerals

Functions and Dietary Sources

Minerals are essential for bone and teeth formation, enzyme cofactors, hemoglobin and myoglobin, stomach acid, and electrolyte balance. Sources include vegetables, legumes, milk, eggs, fish, and animal tissues.

• Calcium and phosphorus: Bones and teeth

• Iron: Hemoglobin and myoglobin

• Chlorine: Stomach acid (HCl)

• Electrolytes: Nerve and muscle function

• Sodium intake: Recommended 1.1 g/day; typical American diet 4.5 g/day

Vitamins

Types and Absorption

Some vitamins are synthesized from precursors. Water-soluble vitamins (C, B) are absorbed with water, while fat-soluble vitamins (A, D, E, K) are absorbed with dietary lipids.

• Water-soluble: Vitamin C, B-complex

• Fat-soluble: Vitamins A, D, E, K

Appetite Regulation

Hormonal Control

Appetite is regulated by hormones such as leptin, ghrelin, peptide YY (PYY), insulin, and GLP-1. Leptin decreases appetite, ghrelin increases hunger, PYY and GLP-1 promote satiety, and insulin has a weaker effect similar to leptin.

• Leptin: Secreted by adipocytes, decreases appetite

• Ghrelin: Produced by parietal cells, increases hunger

• PYY: Secreted by ileum, increases satiety

• GLP-1: Increases satiety, promotes insulin secretion

Obesity

Genetic and Environmental Factors

Heredity plays a greater role in weight than height. Childhood obesity increases adipocyte proliferation, making weight loss more difficult. Evolutionarily, humans are programmed not to lose weight easily.

• Adipocyte hyperplasia: Increase in fat cell number during childhood

• Weight loss: More difficult than weight gain

Absorptive and Postabsorptive States

Absorptive State

The absorptive state lasts about 4 hours during and after a meal, when nutrients are absorbed and used for energy. Carbohydrates are used for ATP synthesis, excess is stored as glycogen or fat, fats are taken up by fat cells, and amino acids are used for protein synthesis.

• Regulation: Insulin increases cellular uptake of glucose, stimulates glycolysis, glycogenesis, lipogenesis, and protein synthesis

Postabsorptive State

In the postabsorptive state, glucose is obtained from glycogen reserves or synthesized from other compounds. Fat and protein are used as fuel when glycogen is depleted. Homeostasis of blood glucose is critical, especially for the brain.

• Regulation: Glucagon stimulates glycogenolysis and gluconeogenesis

• Weight loss: Increased postabsorptive state (e.g., not eating after 6 PM) promotes weight loss

Metabolic Rate

Definition and Factors

Metabolic rate is the amount of energy used by the body in a given period (kcal/hr or kcal/day). Basal metabolic rate (BMR) is measured in a relaxed, awake, fasting state at comfortable temperature. Adult male BMR is about 2000 kcal/day.

• Factors affecting metabolic rate: Size, gender, age, pregnancy, anxiety, fever, eating, thyroid hormones, depression, exercise, muscle/fat mass, activity level, body temperature