Nutrition

Macronutrients are the three classes of nutrients -- carbohydrates, proteins, and fats -- that the body requires in large quantities to provide energy and support structural and functional processes. They are measured in grams and collectively account for all caloric intake in the diet. Each macronutrient has a distinct caloric density: carbohydrates and proteins yield approximately 4 kilocalories per gram, while fats yield about 9 kilocalories per gram.

Micronutrients are vitamins and minerals needed in small amounts -- typically milligrams or micrograms -- that are essential for normal physiological function, including enzyme activity, immune defense, and cellular signaling. Unlike macronutrients, they do not provide energy directly but are indispensable for the metabolic pathways that extract energy from food. Deficiencies in micronutrients can cause specific clinical diseases even when caloric intake is adequate.

Metabolism refers to the complete set of chemical reactions occurring within living cells that convert nutrients into energy and building blocks for growth and repair. It is divided into catabolism, which breaks down molecules to release energy, and anabolism, which uses energy to build complex molecules like proteins and glycogen. The rate of metabolism varies among individuals based on factors such as age, body composition, hormonal status, and physical activity level.

Basal metabolic rate is the number of kilocalories the body expends at complete rest to maintain vital functions such as breathing, circulation, cell production, and thermoregulation. It typically accounts for 60 to 75 percent of total daily energy expenditure and is influenced by lean body mass, age, sex, and genetics. BMR is commonly estimated using equations such as the Harris-Benedict or Mifflin-St Jeor formulas.

The glycemic index (GI) is a ranking system that measures how quickly a carbohydrate-containing food raises blood glucose levels compared to a reference food, usually pure glucose assigned a value of 100. Foods with a high GI (70 or above) cause rapid spikes in blood sugar, while low-GI foods (55 or below) produce a slower, more gradual rise. Understanding GI helps individuals manage blood sugar levels, which is especially important for people with diabetes or insulin resistance.

Dietary fiber consists of indigestible plant polysaccharides and lignin that pass through the human digestive tract largely intact, providing bulk and promoting healthy bowel function. It is classified as soluble fiber, which dissolves in water and can lower blood cholesterol and glucose levels, and insoluble fiber, which adds bulk to stool and aids in regular bowel movements. Adequate fiber intake is associated with reduced risk of cardiovascular disease, type 2 diabetes, and colorectal cancer.

Essential amino acids are the nine amino acids that the human body cannot synthesize and must therefore be obtained from dietary protein sources. They include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids serve as building blocks for proteins involved in muscle repair, enzyme production, neurotransmitter synthesis, and immune function.

Vitamins are organic compounds required in small quantities for diverse biochemical functions including energy metabolism, antioxidant defense, blood clotting, and gene expression. They are classified as either water-soluble (B vitamins and vitamin C) or fat-soluble (vitamins A, D, E, and K), with each category having different absorption, transport, and storage characteristics. Most vitamins cannot be synthesized by the body in sufficient amounts, making dietary intake or supplementation necessary.

Minerals are inorganic elements obtained from food and water that serve structural, regulatory, and catalytic roles in the body. Major minerals such as calcium, phosphorus, magnesium, sodium, and potassium are needed in quantities greater than 100 milligrams per day, while trace minerals like iron, zinc, copper, and selenium are required in smaller amounts. They contribute to bone formation, fluid balance, nerve impulse transmission, and oxygen transport.

Caloric balance is the relationship between the number of kilocalories consumed through food and beverages and the number of kilocalories expended through basal metabolism, physical activity, and the thermic effect of food. When caloric intake equals expenditure, body weight remains stable; a sustained caloric surplus leads to weight gain, while a sustained deficit results in weight loss. This principle underpins all evidence-based approaches to weight management.