Terms to know:
- Nutrients: Chemical substances supplied from the environment that an organism requires for survival.
- Macronutrients: Nutrients needed in bulk. This category includes carbohydrates, fats, and proteins.
- Micronutrients: Essential nutrients required in small daily doses, which include vitamins and minerals.
- Essential nutrients: Nutrients that human cells cannot synthesize; they must be obtained from the diet.
18.2 Carbohydrates
Carbohydrate Sources
Polysaccharides:
- Complex carbohydrates such as starch (from plant foods) and glycogen (from meats).
Disaccharides (double sugars):
- Examples include milk sugar, cane sugar, beet sugar, and molasses.
Monosaccharides (single sugars):
- Found in honey and fruits.
- Digestion Process: Complex carbohydrates are broken down into monosaccharides, small enough for absorption.
- Cellulose: A complex carbohydrate abundant in foods that is not digested but provides dietary fiber.
- Common monosaccharides absorbed from the digestive tract include fructose, galactose, and glucose.
- Liver enzymes convert fructose and galactose into glucose.
Carbohydrate Use
Storage and Conversion:
- Excess glucose is polymerized to glycogen through a process called glycogenesis and is then stored in the liver and muscles.
- Glycogen can be rapidly converted back to glucose through glycogenolysis.
- There is a limited amount of glycogen storage; excess glucose reacts to form fat in a process called lipogenesis.
- Neurons require glucose for survival; if there is not enough available glucose, the liver converts non-carbohydrates (such as amino acids or glycerol) into glucose through gluconeogenesis.
Biochemical Importance:
- Carbohydrates are also used as starting materials for vital biochemicals such as ribose and deoxyribose (5 carbon sugars).
Carbohydrate Requirements
Carbohydrates are the primary source of fuel for cellular processes.
The need for carbohydrates varies with individual energy requirements.
The minimal requirement for carbohydrates in the human diet is unknown, but an intake of at least 125 to 175 grams/day is likely necessary.
The average diet in the United States includes approximately 200 to 300 grams/day of carbohydrates.
18.3 Lipids
Lipid Properties
Lipids are organic molecules that include fats, oils, and fat-like substances such as phospholipids and cholesterol.
The most common dietary lipids are triglycerides.
Lipid Sources
Triglycerides: Found in both plant and animal-based foods.
Saturated fats: Typically found in foods of animal origin.
Unsaturated fats: Found in seeds, nuts, and plant oils.
Cholesterol:
- Sourced from the liver and egg yolk, with smaller amounts in whole milk, butter, cheese, and meats.
- Foods of plant origin do not contain cholesterol.
Lipid Use
Lipids in food include phospholipids, cholesterol, and most commonly, triglycerides. Functions of Lipids:
- Stored as energy, insulation, and they play a role in cell structure and hormone production.
Beta Oxidation (in Mitochondria)
Activation of Fatty Acids:
- Fatty acids require ATP and thiokinases to be activated, particularly those with specific carbon chain lengths.
Fatty Acid Breakdown:
- Upon activation, fatty acid chains are broken down by fatty acid oxidases, removing two-carbon segments progressively.
- Some segments are converted by the liver to Acetyl CoA.
- Excess Acetyl CoA is transformed into ketone bodies, which can later be converted back into Acetyl CoA.
- Ketone Production:
- When ketones are produced at a rate higher than they can be decomposed, they are eliminated by the lungs and kidneys, leading to a fruity odor in breath and urine.
- The citric acid cycle can oxidize Acetyl CoA molecules.
Fatty Acid Usage by the Liver
From triglycerides and digestion:
- Products include fatty acids and glycerol, which are reassembled into triglycerides.
- Lipoproteins:
- Large complexes comprised of a phospholipid, cholesterol, and protein layer surrounding a triglyceride core.
- Vary in lipid composition based on type:
- Very-low-density lipoproteins (VLDLs) have a high concentration of triglycerides.
- Low-density lipoproteins (LDLs) have a high concentration of cholesterol.
- High-density lipoproteins (HDLs) are rich in protein and lower in lipids.
Cholesterol Functions:
- Crucial for making bile salts and serves as a structural component for cell and organelle membranes; also provides raw materials for the synthesis of certain sex hormones and adrenal cortex hormones.
Lipid Requirements
Daily caloric intake from fats should not exceed 30% of total daily calories.
Dietary fats must also provide adequate amounts of fat-soluble vitamins.
18.4 Proteins
Protein Digestion and Pathways
Proteins are broken down during digestion into amino acids, which can be further classified into:
- Structural proteins
- Enzymes
- Hormones
- Plasma proteins
- By-products include energy, carbon dioxide, and water.
Protein Sources
Foods rich in protein include:
- Meats, fish, poultry, cheese, nuts, milk, eggs, and cereal.
- Legumes also contain protein but in lesser amounts.
Essential Amino Acids: There are eight essential amino acids required by adults (and ten for growing children) that must be ingested through diet:
- Essential Amino Acids:
- Alanine: Not essential
- Glycine: Not essential
- Proline: Not essential
- Arginine: Conditional (ch)
- Histidine: Conditional (ch)
- Serine: Not essential
- Asparagine: Not essential
- Isoleucine: Essential (e)
- Threonine: Essential (e)
- Aspartic acid: Not essential
- Leucine: Essential (e)
- Tryptophan: Essential (e)
- Cysteine: Not essential
- Lysine: Essential (e)
- Tyrosine: Not essential
- Glutamic acid: Not essential
- Methionine: Essential (e)
- Valine: Essential (e)
- Glutamine: Not essential
- Phenylalanine: Essential (e)
Protein Classification:
- Complete Proteins: High-quality proteins that contain adequate amounts of essential amino acids (found in meat, milk, and eggs).
- Incomplete Proteins: Low-quality proteins that do not provide adequate amounts of essential amino acids (found in various plant proteins).
- Certain combinations of plant proteins, such as beans and rice, can provide sufficient dietary diversity of amino acids.
Nitrogen Balance
In healthy adults, proteins are continuously built up and broken down.
A state of dynamic equilibrium results when overall gain of nitrogen equals overall loss.
Protein Requirements
In addition to providing essential amino acids, proteins supply nitrogen and other elements necessary for producing non-essential amino acids and other non-protein nitrogenous substances.
Dietary protein intake varies according to body size, metabolic rate, and nitrogen balance.
It is typically recommended that daily protein intake be 10% of a person's diet, which equates to 0.8 g/kg of body weight or 0.4 g/lb. of body weight.
18.5 Energy Expenditures
Carbohydrates, fats, and proteins provide energy for all metabolic processes.
- Deficiencies can lead to the usage of structural molecules (such as muscle proteins), which may result in death.
- Excess energy intake can lead to obesity, which has detrimental effects on health.
Energy Values of Foods
The potential energy of foods is measured in Calories.
Definitions:
- 1 calorie: The amount of heat needed to raise 1000 grams of water from 14.5°C to 15.5°C, and is equivalent to 4.184 joules.
Cellular Oxidation Yields:
- 4.1 calories from 1 g of carbohydrates.
- 4.1 calories from 1 g of protein.
- 9.5 calories from 1 g of fat.
Basal Metabolic Rate (BMR)
The rate at which the body expends energy under basal conditions.
- It reflects the energy needed to support the activities of the organs.
- The BMR varies with factors like gender, body size, body temperature, and thyroid hormone activity.
- Maintaining BMR necessitates the greatest energy expenditure in the body.
- Energy is needed to:
- Maintain BMR
- Support muscular activity
- Maintain body temperature
- Foster growth in children and during pregnancy.
Energy balance exists when caloric intake from food equals caloric output from BMR and muscular activities.
Maintaining energy balance results in constant body weight.
Positive Energy Balance:
- Occurs when caloric intake exceeds caloric output, leading to weight gain.
Desirable Weight
Body Mass Index (BMI):
- Used to assess weight, taking height into account.
- BMI classifications:
- Underweight: BMI < 18.5
- Normal weight: BMI 18.5 to 24.9
- Overweight: Exceeding desirable weight by 10% to 20% or a BMI of 25 to 30
- Obese: Exceeding desirable weight by >20% or a BMI over 30.
- It is more accurately defined in terms of excess adipose tissue.