Before You Read Any Further, Know These Terms
The handful of concepts everything else in this guide builds on
If you have never studied nutrition formally, start here. If you already know what a calorie is and what macronutrients are, skip ahead to Section 1.
What a calorie actually is
A calorie is a unit of energy. The "Calories" on a food label are actually kilocalories (kcal). Food contains chemical energy your body extracts and spends on everything from typing a sentence to growing muscle — "burning calories" just means using that stored energy.
The three macronutrients
Almost everything you eat is some combination of three macronutrients, each supplying a different amount of energy per gram:
| Macronutrient | Energy per gram | Primary role |
|---|---|---|
| Protein | 4 kcal/g | Building and repairing tissue (muscle, skin, enzymes, hormones) |
| Carbohydrate | 4 kcal/g | Primary fuel for the brain and high-intensity exercise |
| Fat | 9 kcal/g | Long-term energy storage, hormone production, vitamin absorption |
| (Alcohol) | 7 kcal/g | No essential nutritional role; covered in Section 10 |
Micronutrients — vitamins and minerals — are needed in much smaller amounts and supply no energy, but they are essential for the chemical reactions that keep you alive. Section 6 covers these in detail.
Energy balance, in one sentence
Eat more than you expend and you gain weight; eat less and you lose it; match the two and weight stays stable. This is basic physics, not a theory up for debate[1]. What's genuinely debated — and what most of this guide is actually about — is what determines how much you eat, how much you expend, and how sustainable any approach is.
A note on certainty
Nutrition science is harder to do well than exercise science. Most evidence comes from observational studies that can show association but not proof of cause, because controlling exactly what free-living humans eat for years is rarely practical or ethical.