Energy and Nutrients from food

Energy is the ability to do work. Virtually every bite of food gives you energy, even when it doesn’t give you nutrients. The amount of energy in food is measured in calories, the amount of heat produced when food is burned (metabolized) in your body cells. You can read all about calories in Chapter 3. But right now, all you need to know is that food is the fuel on which your body runs. Without enough food, you don’t have enough energy.

Nutrients in food
Nutrients are chemical substances your body uses to build, maintain, and repair tissues. They also empower cells to send messages back and forth to conduct essential chemical reactions, such as the ones that make it possible for you to

* Breathe
* See
* Move
* Hear
* Eliminate waste
* Smell
* Think
* Taste

. . . and do everything else natural to a living body.
Food provides two distinct groups of nutrients:

* Macronutrients (macro = big): Protein, fat, carbohydrates, and water
* Micronutrients (micro = small): Vitamins and minerals

What’s the difference between these two groups? The amount you need each day. Your daily requirements for macronutrients generally exceed 1 gram. (For comparison’s sake, 28 grams are in an ounce.) For example, a man needs about 63 grams of protein a day (slightly more than two ounces), and a woman needs 50 grams (slightly less than two ounces). Your daily requirements for micronutrients are much smaller. For example, the Recommended Dietary Allowance (RDA) for vitamin C is measured in milligrams (1⁄1,000 of a gram), while the RDAs for vitamin D, vitamin B12, and folate are even smaller and are measured in micrograms (1⁄1,000,000 of a gram).