Nutrition looks like a list of foods to eat and avoid. It is really three questions asked over and over: what does the body need, how does it get it out of food, and how do you tell whether a claim about either is true. This guide walks them in order — the nutrient classes and energy, the three fuels, the gut that takes them apart, the reference numbers everything is measured against, the vitamins and minerals, the label, the diseases diet moves, the life stages that change the answer, and the safety and misinformation problems on top. Every chapter ends with one rule you can carry.
Each chapter opens with the short version. Tap one to read the detail.
Six nutrient classes, one energy ledger
~2 min
The body needs proteins, carbohydrates, fats, vitamins, minerals and water; the first three carry energy, and fat carries twice as much per gram. Weight moves when calories in and calories used diverge — but the forces pushing that ledger are environmental and inherited as much as chosen, which is why "just eat less" fails as advice.
Sugars, starches and the carbohydrate you cannot digest
~2 min
All digestible carbohydrate ends up as glucose, the main fuel for cells, used now or stored in liver and muscle. Fibre is the carbohydrate the body mostly cannot break down, and that is why it helps: fullness, regular digestion, lower cholesterol and blood sugar. Add it slowly.
Protein you must eat daily, fat you must eat carefully
~2 min
Protein is in every cell and, unlike fat and carbohydrate, is not stored, so it has to arrive every day; animal proteins are complete, most plant proteins are not, so vary them. Some fat is required — for energy and for absorbing vitamins — but saturated and trans fats are the ones to replace with oils.
Taking food apart, then burning it
~2 min
The gut is a tube from mouth to anus with the liver, pancreas and gallbladder feeding juices into it; motion plus acid, bile and enzymes reduce protein to amino acids, fat to fatty acids and glycerol, and carbohydrate to simple sugars. Once absorbed, fat oxidation and glycolysis run as one system, and fasting shifts the liver from stored glycogen to new glucose.
The reference numbers, and the guidelines built on them
~2 min
Four Dietary Reference Intake values answer four questions: the Estimated Average Requirement covers half of a group, the Recommended Dietary Allowance nearly all of it, the Adequate Intake stands in when evidence is thin, and the Tolerable Upper Intake Level is the ceiling. The Dietary Guidelines and the DASH plan translate those numbers into food.
Vitamins that wash out and vitamins that build up
~2 min
The B vitamins and vitamin C are barely stored, so intake has to be steady and deficiency arrives fast — but "water-soluble" does not mean harmless at any dose. Vitamins A, D, E and K need bile and micelles to be absorbed, are stored in liver and fat, and can accumulate to toxicity.
Minerals, electrolytes and the iron ledger
~2 min
Macrominerals are needed in larger amounts and trace minerals in small ones, but the rules match: a job, a store, a way in and a way out. Electrolytes are the charged minerals that manage water, pH, nerves and heartbeat. Iron is the worked example — two dietary forms, a regulated door in, almost no door out, and interactions a supplement can trip.
Reading the label, and reading the claims on it
~2 min
The Nutrition Facts label is built on a 2,000-calorie reference; 5 percent Daily Value is low, 20 percent is high, and comparisons only work at the same serving size. Ingredients run in descending order by weight, claim words are legally defined, the nine major allergens must be named, and a supplement's structure claim carries a disclaimer because nobody reviewed it.
Weight, blood sugar and the diseases diet moves
~2 min
Excess weight raises the risk of type 2 diabetes, heart disease, kidney disease and some cancers, and the forces behind it are environmental and inherited as well as chosen. Diabetes is blood glucose that stays too high; a fifth of adults who have it do not know. Diet, alcohol and obesity sit on the cancer institute's list of risk factors.
Pregnancy, age and training change the answer
~2 min
Needs are not constant. Pregnancy adds prenatal care and a folic acid requirement for anyone who could become pregnant; older adults need fewer calories but the same nutrients and sometimes more protein, against real barriers; endurance athletes run on carbohydrate, and the supplements with evidence are a short list.
Four safety steps, and how to judge what you are told
~2 min
Clean, separate, cook, chill — four steps prevent most foodborne illness at home. On information: evidence for supplements varies from a lot to almost none, a bottle can differ from what was tested, labels can lie, and the rules are looser than for drugs, so the source of a claim matters more than its confidence.
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