Nutritional Concepts

Core Concepts

Macronutrients: the energy and building team

• Carbohydrates are the body's preferred, most efficient energy source. They break down into glucose, which fuels the brain and muscles. Complex carbohydrates (whole grains, vegetables, legumes) also provide fiber, which supports bowel function; simple carbohydrates (sugars, sweets) provide quick energy but little else.
• Proteins are the body's building blocks. Made of amino acids, they build and repair tissue, make enzymes and antibodies, and maintain muscle. Protein comes from animal sources (meat, fish, eggs, dairy) and plant sources (beans, lentils, tofu, nuts). Nine amino acids are essential: the body cannot make them, so they must come from food.
• Fats are a concentrated energy source, help the body absorb fat-soluble vitamins (A, D, E, K), form cell membranes, and cushion organs. Unsaturated fats (olive oil, nuts) are emphasized over saturated fats (butter, fatty meats) in most dietary guidance — specific recommendations vary by country and organization.

Micronutrients: small amounts, big jobs

• Vitamins are organic compounds needed in small amounts. Water-soluble vitamins (B vitamins, vitamin C) are not stored in large amounts, so regular intake matters; fat-soluble vitamins (A, D, E, K) can be stored. Each has specific roles — vitamin D in bone health, vitamin K in blood clotting, and so on.
• Minerals are inorganic elements with essential roles: calcium in bones and muscle contraction, iron in oxygen transport, potassium and sodium in nerve and muscle function, iodine in thyroid function.
• The best source of micronutrients is a varied diet; supplements are used when intake or absorption is insufficient, under the direction of a provider or dietitian.

Water and hydration

Water is the largest single component of the body and the medium for nearly every chemical reaction. It carries nutrients, removes waste, regulates temperature, and maintains blood volume. The body loses water through urine, sweat, breath, and stool, and replaces it through drinking and food — many foods are mostly water. Fluid needs change with illness, fever, activity, and kidney function, an important bridge to Chapter 20's fluid and electrolyte content.

Energy balance and metabolism

The body uses energy (calories or kilojoules) for basal metabolism — the energy needed just to stay alive at rest — plus physical activity and the work of digestion. Intake equal to expenditure keeps weight stable; intake above it stores the excess; intake below it forces the body to use stored fat and protein, causing weight loss and muscle wasting. Metabolic rate is influenced by age, body size, muscle mass, hormones, fever, and illness — which is why a sick patient's energy needs often rise just when eating becomes harder.

Digestion, absorption, and dietary guidance

Digestion begins in the mouth and continues in the stomach and small intestine, where most nutrients are absorbed. The liver processes absorbed nutrients; the large intestine absorbs water and forms stool. Anything that disrupts this chain — missing teeth, stomach surgery, intestinal inflammation, medications — can turn an adequate diet into poor nutrition. Public dietary guidance (such as the U.S. Dietary Guidelines and MyPlate tool) translates this science into practical plate proportions: roughly half vegetables and fruits, a quarter whole grains, a quarter protein, plus dairy or an alternative. Tools vary by country and update over time — teach from current, locally relevant guidance.

Common Confusions