Nutrition is the science of how food and its components affect the body, encompassing ingestion, digestion, absorption, and utilization of nutrients for growth, maintenance, and repair. At its core are macronutrients (carbohydrates, proteins, and fats), needed in large amounts to provide energy and structural materials, and micronutrients (vitamins and minerals), needed in smaller quantities to regulate metabolism and prevent deficiency diseases. Water, often overlooked, comprises about 60% of body weight and supports digestion, nutrient transport, temperature regulation, and waste elimination; daily needs average 2-3 liters from fluids and food, though excess water in a short period can dangerously dilute blood sodium in a condition called hyponatremia.
The energy content of food is measured in dietary Calories (kilocalories), with carbohydrates and proteins providing 4 kcal/g, fats 9 kcal/g, alcohol 7 kcal/g, and fiber approximately 2 kcal/g. Recommended intakes are guided by the Recommended Dietary Allowance (RDA), which covers the needs of 97-98% of healthy individuals, the Adequate Intake (AI), used when evidence is insufficient for an RDA, and the Tolerable Upper Intake Level (UL), beyond which toxicity risks rise, especially for fat-soluble vitamins and minerals like iron. To interpret body weight status, Body Mass Index (BMI, kg/m²) classifies individuals as underweight (below 18.5), normal (18.5-24.9), overweight (25-29.9), or obese (30 or higher), though it remains only an estimate of body fat. Nutrition labels translate these concepts into practice: serving size, calories, and percent Daily Value (%DV) reveal nutrient density, with under 5% DV indicating low and over 20% DV indicating high content, and ingredients listed by weight from most to least.
The NOVA classification further helps consumers by grouping foods into unprocessed or minimally processed (apples, milk), processed culinary ingredients (oils, salt), processed foods (cheese, bread), and ultra-processed products (sodas, chips). Ultra-processed foods now supply roughly half of calories in many Western diets and are linked to obesity and chronic disease, so limiting them to under 20% of intake is widely recommended. It is also important to distinguish food allergies, which are IgE-mediated immune reactions (such as those to peanuts and shellfish) that can cause anaphylaxis even from trace amounts, from food intolerances, which are non-immune and dose-dependent, as in lactose intolerance caused by lactase deficiency. Together, these frameworks translate nutrition science into everyday food choices.