Science of Nutrition

Definition of Nutrition

Nutrition is the scientific study of nutrients and other substances in food, their actions, interactions, and balance in relation to health and disease. It also includes the processes by which the body ingests, digests, absorbs, transports, utilizes, and excretes food substances.

• Nutrients are chemical substances in food that are essential for growth, energy, and maintenance of body functions.

• Nutrition examines how food affects health, disease risk, and overall well-being.

• Example: Studying how vitamin C intake affects immune function.

Classes of Nutrients

Nutrients are classified based on their chemical properties and physiological roles.

• Macronutrients: Needed in large amounts; provide energy. Includes carbohydrates, proteins, and fats.

• Micronutrients: Needed in small amounts; do not provide energy. Includes vitamins and minerals.

• Water: Essential for life; involved in many body processes.

• Example: Iron (a mineral) is a micronutrient, while fat is a macronutrient.

Organic vs. Inorganic Nutrients

In chemistry, "organic" refers to compounds containing carbon-hydrogen bonds.

• Organic nutrients: Carbohydrates, proteins, fats (lipids), and vitamins.

• Inorganic nutrients: Minerals and water.

• Example: Vitamin C is organic; calcium is inorganic.

Energy Content of Nutrients

Different nutrients provide different amounts of energy, measured in kilocalories (kcal).

• Carbohydrates: 4 kcal/gram

• Proteins: 4 kcal/gram

• Fats (Lipids): 9 kcal/gram

• Alcohol: 7 kcal/gram (not a nutrient, but provides energy)

Calculating Total Calories

To determine total energy intake from food, multiply the grams of each macronutrient by its respective kcal value and sum the results.

• Formula:

• Example: A food with 10g protein, 20g carbohydrate, 5g fat: kcal

The Human Body & Digestion

Order of Organs in Digestion

Food passes through the digestive tract in a specific order:

1. Mouth

2. Esophagus

3. Stomach

4. Small intestine

5. Large intestine (colon)

6. Rectum

7. Anus

Digestive Processes in Each Organ

Each organ in the digestive system has specialized functions:

• Mouth: Mechanical breakdown (chewing); salivary amylase begins carbohydrate digestion.

• Stomach: Secretes gastric juices; protein digestion begins; churns food into chyme.

• Small Intestine: Main site of digestion and absorption; enzymes from pancreas and bile from liver aid digestion.

• Large Intestine: Absorbs water and electrolytes; forms feces.

• Most absorption occurs in the small intestine.

Digestive Enzymes

Enzymes are biological catalysts that speed up chemical reactions in digestion.

• Salivary amylase: Produced in the salivary glands; breaks down starches in the mouth.

• Gastric lipase: Produced in the stomach; begins fat digestion.

• Other enzymes: Pancreatic amylase, proteases, lactase, sucrase, maltase, etc.

Hormones in Blood Sugar Regulation

Hormones help maintain blood glucose levels:

• Insulin: Lowers blood glucose by promoting uptake into cells; secreted by the pancreas.

• Glucagon: Raises blood glucose by stimulating breakdown of glycogen; secreted by the pancreas.

Nutrient Transport Mechanisms

Nutrients cross cell membranes by several mechanisms:

• Passive diffusion: Movement from high to low concentration without energy.

• Facilitated diffusion: Uses a carrier protein; no energy required.

• Active transport: Requires energy (ATP) and a carrier protein; moves substances against a concentration gradient.

• Endocytosis: Cell engulfs particles or fluids.

Tools for Healthful Eating

Food Labels

Food labels provide information to help consumers make informed choices.

• Serving size: Standardized amount for comparison.

• Calories: Energy per serving.

• Macronutrient content: Grams of fat, carbohydrate, protein.

• Percent Daily Value (%DV): Indicates how much a nutrient in a serving contributes to a daily diet.

• Ingredients list: Listed in descending order by weight.

Dietary Guidelines

The Dietary Guidelines for Americans provide science-based advice for healthy eating patterns.

• Emphasize fruits, vegetables, whole grains, lean proteins, and low-fat dairy.

• Limit saturated fat, added sugars, and sodium.

• Encourage physical activity.

USDA Food Patterns

The USDA Food Patterns offer practical examples of healthy eating plans.

• Includes MyPlate, which divides the plate into fruits, vegetables, grains, protein, and dairy.

• Helps individuals meet nutrient needs within calorie limits.

AMDR: Acceptable Macronutrient Distribution Ranges

AMDRs specify the percentage of total calories that should come from each macronutrient.

• Carbohydrates: 45–65% of total calories

• Proteins: 10–35% of total calories

• Fats: 20–35% of total calories

Dietary Guidelines for Sodium

Excess sodium intake is linked to high blood pressure and cardiovascular disease.

• General recommendation: Less than 2,300 mg per day for most adults.

Carbohydrates, Proteins, and Fats

Food Sources

• Carbohydrates: Grains, fruits, vegetables, legumes, dairy.

• Proteins: Meat, poultry, fish, eggs, dairy, legumes, nuts, seeds.

• Fats: Oils, butter, nuts, seeds, fatty fish, avocados.

Water-Soluble and Fat-Soluble Vitamins

• Water-soluble vitamins: B vitamins and vitamin C; not stored in large amounts, excess excreted in urine.

• Fat-soluble vitamins: Vitamins A, D, E, K; stored in body fat, can accumulate to toxic levels.

Principles of a Healthful Diet

Characteristics of a Healthful Diet

• Adequacy: Provides sufficient energy and nutrients.

• Moderation: Not too much or too little of any nutrient or food.

• Balance: Appropriate proportions of nutrients and food groups.

• Variety: Includes many different foods from all food groups.

Nutrient Density vs. Energy Density

• Nutrient dense: Foods high in nutrients but low in calories (e.g., vegetables, fruits).

• Energy dense: Foods high in calories relative to weight (e.g., chips, desserts).

Appetite vs. Hunger

• Hunger: Physiological need for food.

• Appetite: Psychological desire to eat, often influenced by sensory factors.

Diseases Associated with Diet

• Chronic diseases such as heart disease, diabetes, obesity, and some cancers are linked to dietary patterns.

• Deficiency diseases (e.g., scurvy, rickets) result from inadequate intake of specific nutrients.

Summary Table: Nutrient Classes and Key Properties

Additional info:

• For exam preparation, focus on understanding concepts, not just memorizing facts.

• Practice reading food labels and calculating energy content from sample nutrition facts panels.

• Review the USDA MyPlate and Dietary Guidelines for Americans for practical applications.