Vol. XXIII · The Everyday Cook Plate 9 of 12nutrition, Atwater factors, calories, kcal, macronutrients, protein, fat, carbohydrate, fibre, salt, sodium, food label, energy density, dietary guidelinesTF

Nutrition Without Fads.

A tablespoon of olive oil and a medium potato carry about the same energy: 120 kcal. One weighs 14 g, the other 170.

The energy in food comes from its protein, fat and carbohydrate, and in 1900 Wilbur Atwater measured how much the body actually gets from each: about 4 kcal per gram of protein, 9 of fat and 4 of digestible carbohydrate (fibre gives roughly 2, alcohol 7). Every food label still uses these factors. Because fat carries more than twice the energy per gram and has no water, oily and dry foods are energy-dense, while vegetables, fruit, soups and lean protein are bulky for their energy — which is why the same 500 kcal can be a small handful of nuts or a large plate of stew.

A label is read per 100 g (or per portion, checking the portion's weight): energy, protein, fat (and saturated fat), carbohydrate (and sugars), fibre, and salt — often given as sodium, multiplied by 2.5 to get salt. Dietary guidelines across countries agree more than headlines suggest: plenty of vegetables, fruit, legumes and whole grains (XXI.1); fish and some lean meat or plant protein; less salt (under about 5 g a day by WHO advice), less sugar and less saturated fat; and an energy intake matched to activity. The workshop reads a food.

Below: the energy in the amount you choose of a food, calculated from its protein, fat, carbohydrate and fibre with the Atwater factors and compared with the database's own figure, the share of energy from each, and its salt (USDA FoodData Central).

What you will learn
Workshop
Atwater general factors: 4 kcal/g protein, 9 kcal/g fat, 4 kcal/g digestible carbohydrate, 2 kcal/g fibre. Compositions per 100 g raw from USDA FoodData Central (SR Legacy); "label" is the database's own energy value. Salt = sodium × 2.54. Reference: 2 000 kcal a day; WHO salt advice under 5 g a day.

Four, nine, four — and read the salt

By the Atwater factors a tablespoon of olive oil (14 g) carries about 126 kcal and a 170 g potato about 134: the same energy, a twelfth of the weight. A tablespoon of soy sauce carries about 2.1 g of salt — about half the WHO's daily limit in one spoon, which is why soy, stock cubes (XXII.4), cured meats and bread are where most salt in a diet comes from. The energy calculated from composition matches the database's figure closely for most foods, because both use the same factors.

Nutrition without fads, as cooking: build plates around vegetables, legumes and whole grains; use fat for flavour and cooking, but measure oil rather than pouring it; season with acid, herbs and spices as well as salt (XI.4, XI.9); cook more at home, where you control salt and fat; keep portions in proportion to the day's activity. Single foods neither cure nor kill; patterns of eating over years matter. For allergies and medical diets, the label's ingredient list (XII.10) and professional advice come first.

At the stove

Read the labels of three packaged foods in your kitchen: write down energy, salt and protein per 100 g, and check the energy against 4, 9 and 4.

Wilbur O. Atwater built a sealed "respiration calorimeter" at Wesleyan University in the 1890s, in which volunteers lived for days while every gram of food in and waste out was measured — the experiments that gave the 4-9-4 factors still printed on every label.

Terms

Atwater factors
4, 9 and 4 kcal per gram of protein, fat and carbohydrate.
Energy density
Kilocalories per 100 g of food.
Sodium to salt
Multiply sodium by 2.5 for salt.
Macronutrients
Protein, fat and carbohydrate.
Energy is 4, 9 and 4 kcal per gram of protein, fat and carbohydrate. Read labels per 100 g, convert sodium × 2.5 to salt, and favour vegetables, legumes and whole grains, with less salt, sugar and saturated fat. Patterns, not fads.

Sources

  1. Atwater general factors: 4 kcal/g protein, 9 kcal/g fat, 4 kcal/g carbohydrate (USDA, Energy Value of Foods, Agriculture Handbook 74, 1955/1973).
  2. USDA FoodData Central, SR Legacy (April 2018), public domain.
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.