“Percent Daily Values are based on a 2,000 calorie diet.” We recognize this — along with serving sizes and the percentages of fat and carbohydrates — from nearly everything we eat. Sometimes we make decisions about what to eat and by based on these numbers.
CALORIES: One Calorie (with a capital “C” and what nutritionists actually call a “kilocalorie”) is the amount of energy it takes to raise 1 kilogram of water 1 degree Celsius at sea level.
How calories are produced and used in the body — our metabolism — varies from person to person. It’s mostly affected by exercise, amount of fat and muscle in the body, and the rate at which the body uses energy while at rest (called the basal metabolic rate, which is responsible for a surprising 70% of the calories we use).
Daily calorie needs range from 1,200 to 2,800. A moderately active female age 31 to 50 for example, needs 2,000. As you know, when we consume more calories we need, they’re stored, primarily as fat.
The calorie content of food was first determined in the late 1800s when chemist Wilbur O. Atwater built a 4-by-8-foot device called a respiration calorimeter (pictured). When a person stepped in, the device measured the amount of heat they released, the amount of oxygen they consumed, and the carbon dioxide they gave off after eating foods. Using this device, Atwater measured the precise amount of energy in specific food items:
- Carbohydrates and proteins were worth 4 calories per gram.
- Fats are about 9 calories per gram.
- This 4-9-4 rule remains embodied in the Nutrition Facts Labels we have today.
PROTEINS: Adults should consume a minimum of 0.36 grams of protein for every pound of body weight per day.
The Kjeldahl method is used to determine the amount of protein in food. A sample of food is heated in boiling sulfuric acid, and then converted into ammonia gas with sodium hydroxide. The ammonia gas ends up in a flask with boric acid; the amount of acid needed corresponds to the amount of ammonia present.
That tells you the amount of nitrogen initially in the sample. Since nitrogen in food is contained mostly in proteins, that tells you the amount of protein.
FATS: 30% of our daily calories should come from fat.
For the past century, fat content was measured using the Soxhlet extraction. Food is ground up and continuously washed with an organic solvent, dissolving only the fat.
A new method uses nuclear magnetic resonance. The sample is placed in a strong magnetic field, bombarded with a pulse of radio frequency, and the amount of fat can be determined.
CARBOHYDRATES: About half of the calories that we consume should come from carbohydrates.
The amount of total carbohydrates in food has traditionally been calculated — rather than measured. The other components of food — like protein, fat, and water — are measured and added together. When this sum is subtracted from the total, the difference is assumed to be the amount of total carbohydrates.
However, that doesn’t distinguish between carbs used by our bodies to produce energy (such as sugars) and carbs we can’t digest (such as fiber), which are then excreted.
Image: American Chemical Society