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Food for Life - Why we Need Food

logo DK PublishingDK Publishing 02/07/2014 DKBooks
Photo: Nutritious snack - A simple tortilla wrap, filled with lettuce, tomato, and slices of chicken breast, is a healthy snack, containing many of the nutrients that your body needs. © Provided by DKBooks Nutritious snack - A simple tortilla wrap, filled with lettuce, tomato, and slices of chicken breast, is a healthy snack, containing many of the nutrients that your body needs.

Nutritious snack - A simple tortilla wrap, filled with lettuce, tomato, and slices of chicken breast, is a healthy snack, containing many of the nutrients that your body needs.

Photo: High-energy activity - Physical activity accounts for 15–30 percent of total energy expenditure. An athlete expends more energy than someone who sits at a desk. © Provided by DKBooks High-energy activity - Physical activity accounts for 15–30 percent of total energy expenditure. An athlete expends more energy than someone who sits at a desk.

Digestive system - This is made up of the digestive tract—a long, muscular tube that extends from mouth to anus and includes the esophagus, stomach, intestines, and rectum. Also part of this system are various organs such as the liver, pancreas, and gallbladder.

High-energy activity - Physical activity accounts for 15–30 percent of total energy expenditure. An athlete expends more energy than someone who sits at a desk.

Why we Need Food

The foods we eat are the essential building blocks of life.

All foodstuffs, from apples to whole-wheat bread or ice cream, contain two main categories of nutrients: macro- and micronutrients. Macronutrients are required in large amounts for healthy growth and development; they form the basis of every diet and provide energy for all the body’s everyday functions and activities. Macronutrients are usually further categorized as being either primarily fats, proteins, carbohydrates, or fiber—though most foods contain all of these in varying proportions .

Vitamins and minerals, which make up the micronutrients, are chemical compounds found in tiny amounts in foods . Unlike macronutrients, vitamins and minerals do not provide energy and are needed in only minute amounts, but they do play a critical role in the normal functioning of the body and in digestive processes.

How we get nutrients

Take a look at what you eat in an average day: the chances are that your diet includes a wide variety of foods from all the basic food groups, and that it provides a range of essential nutrients. Your breakfast, for example, may be rich in carbohydrates and fiber from cereal or toast; you may have a mixed salad for lunch and grilled fish and vegetables for dinner, providing protein and a variety of vitamins and minerals. Whatever you eat at individual meals, your diet is made up of foods from the five basic food groups.

The energy yield of food

In addition to supplying nutrients, food provides your body with energy. About half to two-thirds of the energy that we obtain from food goes to support the body’s basic, involuntary functions—these are the activities that are performed without any conscious control, such as maintaining breathing, heart rate, and body temperature. The minimum energy needed to carry out these functions is determined by your basal metabolic rate (BMR), which is your baseline rate of metabolism, measured when the body is at rest.

You also expend energy through conscious, voluntary activities, which range from the sedentary to the strenuous. All your body’s energy needs are met from the foods that you eat or from your body’s energy stores.

Nutrition and health

In the following sections of this guide, we examine in detail the various elements of nutrition—proteins, fats, carbohydrates, fiber, and micronutrients such as vitamins and minerals—and how your body utilizes them. For example, you need protein for growth and repair, carbohydrates for energy, and fiber for effective digestion. We shall also suggest ways of improving your general health and reducing your risks of developing certain diseases by making healthier food choices.

Calories and energy

The energy you obtain from food is measured in calories. However, since one calorie represents a tiny amount of energy, kilocalorie units are used in nutritional analysis. 1 kilocalorie (kcal) equals 1,000 calories, and this is the amount of energy required to raise the temperature of 1 kilogram of water 1° Celsius. However, the term “calorie” has come to be used as a shorthand reference to kilocalorie, and we have used this convention (1cal represents 1kcal) in this guide. Each type of nutrient generates a specific amount of energy:

31/2oz (100g) protein: 400cal

31/2oz (100g) carbohydrate: 400cal

31/2oz (100g) fat: 900cal


Energy is sometimes measured in kilojoules (kJ), and you may find this information on food labels alongside the caloric value. 1cal (1kcal) equals 4.184kJ.

Calculating energy requirements

Your energy requirements depend on various factors, including age, gender, physical activity, muscle mass, body temperature, and whether you are still growing. Pregnancy, breast-feeding, menstruation, illness, infection, how much you eat or sleep, and hormone levels are additional factors.

Basal metabolic rate (BMR) is a measure of how much energy you need for essential functions such as breathing and heart rate. BMR is highest in the young and decreases after the age of ten. Because of their greater muscle mass, men generally have a higher BMR (and therefore require more calories) than women. Due to declining muscle mass, older adults generally have a lower BMR and require fewer calories.

Examples of the maintenance calorie requirements for different activities in adults are listed here.

Sedentary or bedbound people: 11.5cal per lb (0.45kg) of body weight per day.

People who only do light or routine activities: 13.5cal per lb (0.45kg) of body weight per day.

People doing moderate activities and a regular exercise program: 16cal per lb (0.45kg) of body weight per day.

Those doing vigorous exercises, such as athletes, manual laborers, or patients recovering from serious trauma: 18cal per lb (0.45kg) of body weight per day.

Jargon buster


A collective term for all the chemical processes constantly occurring within the body, including those in which the nutrients from food are converted into substances that the body uses or excretes as waste.

Guidelines for nutritional requirements

In addition to identifying the types of nutrients that we must include in our diets on a daily basis, we also need to know how much of each element is required for optimum health. Official guidelines have been established that provide us with this information.

Dietary reference intakes

Until fairly recently, the dietary standards in North America were published as the Recommended Dietary Allowances (RDA). Since 1997, however, this advice has been extended by the introduction of the Dietary Reference Intakes (DRI ). These intakes are considered to better address the changing nutritional needs of North Americans.

DRIs not only provide a range of safe and appropriate intakes for nutrients, but they also include advice on Tolerable Upper Intake Levels, which are based on current research. The change in guidelines for nutritional needs reflects a shift in focus toward the prevention of long-term, or chronic, disease.

The nutrition labels on food products show a breakdown of the product into its component parts and displays the percentage of protein, carbohydrate, and fat in a particular food, as well as the amounts of certain vitamins and minerals it contains. Each percentage refers to the Dietary Reference Intake. In addition, the nutrition label indicates the number of calories the product provides .

Dietary Reference Intake (DRI) values

There are four nutrient-based dietary reference values for every life stage and gender group:

In the Vitamin and Mineral Directories that follow , we provide the DRI for healthy men and women, wherever this is available. If the DRI has not yet been established, the RDA is given.

Recommended Dietary Allowance (RDA) is the intake level that meets the daily requirements of 97–98 percent of the people in a specific life-stage and gender group.

Estimated Average Requirement (EAR) is the intake estimated to meet the needs of 50 percent of people in a defined group.

Adequate Intake (AI) is used when no EAR has been established.

Tolerable Upper Intake Level (UL) is the maximum intake that poses little risk of adverse effects for most people in a defined group .

How do we process food?

Before the nutrients in food can be used, they must be broken down into components that the body can absorb. This process, which starts in the mouth and ends with the expulsion of waste products, can take between one and three days. Food is subjected to chemical changes, as digestive juices break it down into its smallest components. Proteins are broken into amino acids, fats into fatty acids and glycerol, and carbohydrates into simple sugars, such as glucose. The vitamins and minerals consist of tiny molecules that the body can absorb without breaking them down first. In the small intestine, bile produced by the liver helps digest fats, while pancreatic secretions break down carbohydrates and continue the digestion of proteins and fats. Nutrients are absorbed into the bloodstream through the intestinal walls. The food that is not digested and absorbed is passed out of the anus.

Macronutrients and micronutrients

Most food contains both macro- and micronutrients. When we try to plan a balanced diet, we tend to think of the macronutrient groups first: carbohydrates, proteins, and fats, and the quantities required for good health and weight management. These foods provide the fuel the body needs for all its key functions.

Food labels show the amount of each of these key macronutrients, to help you make the best choice for your diet.

All foods also contain a variety of micronutrients, otherwise known as vitamins and minerals. Each food contains a different selection of micronutrients, in different quantities. Micronutrients play an important role in many processes in the body including:

Driving metabolic processes in the body, such as enzyme reactions and manufacture of red blood cells.

Proper functioning of the heart and nervous system.

Helping to manufacture the antibodies that fight infection.

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