Chapter 10: Feeding the World
10.2 Food & Nutrients
Eating a variety of healthful foods promotes good physical health and provides energy for growth and activity. Many common diseases and their symptoms can be prevented or helped with healthful eating. Knowing what your body needs can help you choose foods to meet those needs.
Nutrients, Energy, and Building Materials
Nutrients are chemical elements or compounds that the body needs for normal functioning and good health. There are six main classes of nutrients: carbohydrates, proteins, lipids, water, vitamins, and minerals. The body needs these nutrients for three basic purposes: energy, building materials, and control of body processes.
A steady supply of energy is needed by cells for all body functions. Carbohydrates, proteins, and lipids provide this energy. Chemical bonds in molecules of these nutrients contain energy. When the bonds are broken during digestion to form simpler molecules, the energy is released. Energy is measured in units called kilocalories (kcal), commonly referred to as Calories.
Molecules that make up the body are continuously broken down or used up, so they must be replaced. Some nutrients, particularly proteins, provide the building materials for this purpose. Other nutrients—including proteins, vitamins, and minerals—are needed to regulate body processes. One way is by helping to form enzymes. Enzymes are compounds that control the rate of chemical reactions in the body.
Nutrients can be classified in two groups based on how much of them the body needs:
- Macronutrients are nutrients that the body needs in relatively large amounts. They include carbohydrates, proteins, lipids, and water.
- Micronutrients are nutrients the body needs in relatively small amounts. They include vitamins and minerals.
The exact amount of a macronutrient an individual needs depends on many factors, including gender and age. Recommended daily intakes of three macronutrients for young people of both genders are shown in the table below.
| Gender And Age | Carbohydrates(grams/day) | Proteins(grams/day) | Water*(liters/day) |
| Males 9–13 years | 130 | 34 | 2.4 |
| 14–18 years | 130 | 52 | 3.3 |
| Females 9–13 years | 130 | 34 | 2.1 |
| 14–18 years | 130 | 46 | 2.3 |
- Includes water in foods as well as beverages
Carbohydrates are classified as either simple or complex, based on the number of saccharides they contain.
Simple carbohydrates contain just one or two saccharides. They are all sugars. Examples of sugars in the diet include fructose, which is found in fruit, and lactose, which is found in milk. The main function of simple carbohydrates is to provide the body with energy. One gram of carbohydrate provides four kilocalories of energy. Glucose is the sugar that is used most easily by cells for energy. It circulates in the blood, providing energy to cells throughout the body. Glucose is the only source of energy used by the brain.
Complex carbohydrates, called polysaccharides, generally contain many saccharides. They include starches and fiber. Starches are found in plant foods such as vegetables and grains. They are broken down during digestion to form sugars that provide energy. Fiber consists of indigestible starches and other materials such as cellulose. It is present in all plant foods.
Fiber may be soluble or insoluble.
- Soluble fiber dissolves in water as it passes through the large intestine. It helps form substances that keep blood levels of glucose stable and blood levels of harmful lipids low (see below).
- Insoluble fiber does not dissolve but attracts water as it passes through the large intestine. This helps keep waste moist and moving easily through the intestine.
Proteins play many vital roles in the body, including:
- Making up the majority of muscle tissue.
- Regulating many body processes.
- Forming antibodies that destroy bacteria and other “foreign invaders.”
- Regulating the salt-water and acid-base balance in body fluids.
- Transporting nutrients and other vital substances in the blood.
Dietary proteins are broken down during digestion to provide the amino acids that cells need to make proteins for the body. Twenty different amino acids are needed for this purpose. Ten of these amino acids can be synthesized by cells from simple components. The other ten cannot be synthesized and must be obtained from foods. They are called essential amino acids because they are essential in the diet.
Proteins that contain all ten essential amino acids are referred to as complete proteins. They are found in animal foods such as milk and meat. Proteins that are missing one or more essential amino acids are referred to as incomplete proteins. They are found in plant foods such as legumes and rice. By eating a variety of different plant foods containing incomplete proteins, you can include all ten essential amino acids in your diet.
If you eat more protein than needed for the synthesis of new proteins by cells, the excess is used for energy or stored as fat. One gram of protein provides four kilocalories of energy. This is the same amount of energy that one gram of carbohydrate provides.
Lipids, or fatty acids, are organic compounds that consist of repeating units of carbon, hydrogen, and oxygen. They provide the body with energy. The heart and skeletal muscles rely mainly on lipids for fuel. One gram of lipids provides nine kilocalories of energy, more than twice the amount provided by carbohydrates or proteins. Lipids have several other functions as well. Lipids form an insulating sheath around nerve cells that helps nerve messages travel more quickly. Lipids also help form substances that regulate blood pressure, blood clotting, and blood lipid levels. In addition, lipids make up the membranes that surround cells.
The term fat is often used interchangeably with the term lipid, but fats are actually a particular type of lipid, called triglycerides, in which three fatty acids are bound to a compound called glycerol. Fats are important in the body. They are the main form in which the body stores energy. Stored body fat is called adipose tissue. Stored fat not only provides an energy reserve but also cushions and protects internal organs. In addition, stored fat insulates the body and helps prevent heat loss in cold weather.
Although lipids and fats are necessary for life, they may be harmful if they are present in the blood at high levels. Both triglycerides and the lipid called cholesterol are known to damage blood vessels if their concentrations in the blood are too high. By damaging blood vessels, triglycerides and cholesterol also increase the risk of heart disease.
Lipids are classified as either saturated fatty acids or unsaturated fatty acids. This classification is based on the number of chemical bonds between carbon atoms in lipid molecules.
- Saturated fatty acids have only single bonds between carbon atoms. This gives them properties that make them unhealthful. Their amount in the diet should be kept as low as possible. If consumed in excess, they contribute to high blood levels of cholesterol and triglycerides. Saturated fatty acids are found in animal foods, such as meat, whole milk, and eggs.
- Unsaturated fatty acids have at least one double bond between carbon atoms. This gives them properties that make them more healthful. Eaten in appropriate amounts, they may help lower blood levels of cholesterol and triglycerides and decrease the risk of cardiovascular disease. They are found mainly in plant foods.
The human body can synthesize all but two of the fatty acids it needs: omega-3 fatty acids and omega-6 fatty acids. Both are unsaturated fatty acids. They are called essential fatty acids because they must be present in the diet. They are found in salmon, vegetable oil, flaxseed, eggs, and whole grains. Small amounts of these two fatty acids may help lower blood pressure as well as blood levels of harmful lipids.
Unsaturated fatty acids known as trans fatty acids (or trans fats), are manufactured from plant oils and do not occur naturally. They are added to foods to extend their shelf life. Trans fats have properties like saturated fats and may increase risk of cardiovascular disease. They should be avoided in balanced eating. In September of 2018, Canada banned the main source of artificial trans fats – Partially hydrogenated oils (PHOs) – which were previously often found in store-bought cookies, margarine and shortening. The ban is being implemented with a two-year phase-in period, meaning that food manufactured after Sept. 17, 2018 will be PHO free, but products manufactured before that will be on shelves for the next two years, until the stock is exhausted. The ban applies to both domestic manufactured and foreign imported foods.
Water
You may not think of water as a food, but it is a nutrient. Water is essential to life because it is the substance within which all the chemical reactions of life take place. An adult can survive only a few days without water.
Water is lost from the body in exhaled air, sweat, and urine. Dehydration occurs when a person does not take in enough water to replace the water that is lost. Symptoms of dehydration include headaches, low blood pressure, and dizziness. If dehydration continues, it can quickly lead to unconsciousness and even death. When you are very active, particularly in the heat, you can lose a great deal of water in sweat. To avoid dehydration, you should drink extra fluids before, during, and after exercise.
Taking in too much water—especially without consuming extra salts—can lead to a condition called hyponatremia. In this condition, the brain swells with water, causing symptoms such as nausea, vomiting, headache, and coma. Hyponatremia can be fatal, so it requires emergency medical care.
Balanced Eating
Balanced eating is a way of eating that promotes good health. It includes eating several medium-sized meals regularly throughout the day. It also includes eating the right balance of different foods to provide the body with all the nutrients it needs. How much of these foods should you eat to get the right balance of nutrients?
Harvard University recently developed a healthy eating pyramid, which is shown in the figure below. It places emphasis on exercise and a focus on eating fruits, vegetables, and healthy plant oils. It places red meats and starchy, low-nutrient foods, such as white bread and white rice, to the category of foods to eat in very limited amounts. Some experts think that the Harvard pyramid represents a healthier way of eating (as compared to some of the food pyramids created by national governments where the political influence of the meat and dairy industries has been shown to influence dietary recommendations).
Healthy eating pyramid.
Food Labels
Packaged foods are required by law to carry a nutrition facts label, like the one in the figure below, showing the nutrient content and ingredients in the food.
Nutrition facts label.
Reading nutrition facts labels can help you choose foods that are high in nutrients such as protein and low in nutrients such as fat. Nutrition facts labels can also help you choose foods that are nutrient dense. Nutrient density is the ratio of nutrient content, measured in grams, to total energy content in kilocalories.
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In terms of protein, Food B is more nutrient dense than Food A, because it provides more protein per kilocalorie. Eating nutrient-dense foods helps you to get enough of each nutrient without taking in too many kilocalories.
Reading the ingredients list on food labels can also help you choose healthful foods for balanced eating. At the top of the list, look for ingredients such as whole grains, vegetables, and fruits. These are foods you need the most of in a balanced diet. Avoid foods that contain processed ingredients, such as white flour or white rice. Processing removes nutrients. As a result, processed foods generally supply fewer nutrients than whole foods, even when they have been enriched or fortified with added nutrients.
Vitamins and Minerals
Unlike the major macronutrients, micronutrients—including vitamins and minerals—do not provide energy. Nonetheless, adequate amounts of micronutrients are essential for good health. The needed amounts generally can be met with balanced eating. However, many people do not eat enough of the right foods to meet their requirements. They may need vitamin or mineral supplements to increase their intake of micronutrients.
Vitamins
Vitamins are organic compounds that are needed by the body to function properly. There are 13 vitamins that humans need. They are described in the table below, which also includes recommended daily vitamin intakes for teens.
Vitamins play many roles in good health, ranging from helping maintain vision to helping form red blood cells. Many vitamins are components of enzymes. For example, vitamin K is a component of enzymes involved in blood clotting. Several vitamins, including vitamins C and E, act as antioxidants. An antioxidant is a compound that neutralizes chemicals called free radicals. Free radicals are produced naturally during cellular activities and may cause some types of cancer. Neutralizing free radicals makes them harmless.
Some vitamins, including vitamin B6, are produced by bacteria that normally live in the intestines, where they help digest food. Vitamin D is synthesized in the skin when it is exposed to UV radiation in sunlight. Most other vitamins must be obtained from foods because the body is unable to synthesize them. Good food sources of vitamins are listed in the table below. They include whole grains, vegetables, fruits, milk, and nuts.
Consuming inadequate amounts of vitamins can cause deficiency diseases. For example, consuming inadequate amounts of vitamin D causes soft bones. In children this is called rickets. It can cause permanent bone deformities. Consuming inadequate amounts of vitamin A may lead to blindness and visual impairment.
Consuming too much of some vitamins can also be dangerous. Overdoses of vitamins can cause problems ranging from diarrhea to birth defects and even death.
Vitamins are either fat-soluble or water-soluble. This determines whether they can accumulate in the body and lead to overdoses.
- Vitamins A, D, E, and K are fat soluble. Excess intakes of these vitamins are stored in fatty tissues of the body. Because they are stored in the body, they can build up to toxic levels, especially if they are taken improperly in supplements.
- Vitamin C and all the B vitamins are water soluble. Excess amounts of these vitamins are excreted in the urine, so they are unlikely to reach toxic levels in the body.
- Recommended daily intakes not established; figures given are adequate daily intakes.
|
Vitamin (Chemical Name) |
Functions in the Body |
Good Food Sources |
|
(Retinoids) Vitamin A |
Needed for good vision, reproduction, and fetal development |
Carrots, spinach, milk, eggs |
|
(Thiamine) Vitamin B1 |
Helps break down macronutrients; essential for proper functioning of nerves |
Whole wheat, peas, beans, fish, peanuts, meats |
|
(Riboflavin) Vitamin B2 |
Helps the body process amino acids and fats; acts as antioxidant |
Milk, liver, green leafy vegetables, almonds, soybeans |
|
(Niacin) Vitamin B3 |
Helps release energy from macronutrients; needed for healthy skin and nerves |
Beets, beef liver, pork, turkey, fish, sunflower seeds, peanuts |
|
(Pantothenic Acid) Vitamin B5 |
Helps form critical enzymes for synthesis of macronutrients |
Whole grains, legumes, eggs, meat |
|
(Pyridoxine) Vitamin B6 |
Forms enzymes needed for amino acid synthesis and energy storage |
Cereals, yeast, liver, fish, avocados, nuts, green beans |
|
(Biotin) Vitamin B7 |
Enables synthesis of fatty acids; helps store energy; keeps level of blood sugar stable |
None |
|
(Folate) Vitamin B9 |
Needed to make red blood cells |
Liver, green leafy vegetables, dried beans and peas |
|
(Cyanocobalamin) Vitamin B12 |
Needed for normal functioning of nervous system and formation of blood |
Meat, liver, milk, shellfish, eggs |
|
(Ascorbic Acid) Vitamin C |
Needed to make many biological chemicals; acts as antioxidant |
Citrus fruits such as oranges, red peppers, broccoli, kiwi |
|
(Ergocalciferol and Cholecalciferol) Vitamin D |
Helps maintain blood levels of calcium; needed for healthy bones and teeth |
Salmon, tuna, eggs, mushrooms |
|
(Tocopherol) Vitamin E |
Acts as antioxidant; protects cell membranes from LDL cholesterol damage |
Vegetable oils, nuts, green leafy vegetables, whole grains, fish |
|
(Naphthoquinone) Vitamin K |
Helps transport calcium; helps blood clot |
Kale, spinach, Brussels sprouts, milk, eggs, soy products |
Minerals
Dietary minerals are chemical elements that are essential for body processes. Minerals are inorganic, meaning they do not contain carbon. Minerals needed by humans in relatively large amounts (greater than 200 mg/day) are listed in the table below. Minerals not listed in the table are called trace minerals because they are needed in very small amounts. Trace minerals include chromium, iodine, iron, molybdenum, selenium, and zinc.
|
Mineral Name (Symbol) |
Functions in the Body |
Good Food Sources |
|
(Ca) Calcium |
Needed for nerve and muscle action; builds bone and teeth; helps blood clot |
Milk, soy milk, green leafy vegetables, sardines |
|
(Cl) Chloride |
Helps maintain water and pH balance; helps form stomach acid |
Table salt, most processed foods |
|
(Mg) Magnesium |
Needed to form several enzymes |
Whole grains, green leafy vegetables, nuts, seeds |
|
(P) Phosphorus |
Component of bones, teeth, lipids, and other important molecules in the body |
Meat, poultry, whole grains |
|
(K) Potassium |
Needed for muscle and nerve function; helps maintain salt-water balance in body fluids |
Meats, grains, orange juice, potatoes, bananas |
|
(Na) Sodium |
Needed for muscle and nerve function; helps maintain salt-water balance in body fluids |
Table salt, most processed foods |
|
(S) Sulfur |
Necessary component of many proteins |
Whole grains, meats, seafood, eggs |
Minerals play many important roles in the body. Most are found in the blood and cytoplasm of cells, where they control basic functions. For example, calcium and potassium regulate nerve and muscle activity. Several minerals, including zinc, are components of enzymes. Other minerals, including calcium, form the bulk of teeth and bones. Recommended daily intakes not established; figures given are adequate daily intakes.
Minerals cannot be synthesized by the body. Good food sources of minerals include dairy products, green leafy vegetables, and legumes. Mineral deficiencies are uncommon, but inadequate intakes of a few minerals may lead to health problems. For example, an inadequate intake of calcium may contribute to osteoporosis, a disease in which bones become brittle and break easily. A deficiency in iodine (necessary for the thyroid hormones that regulate growth) may lead to goiter. Iron deficiency is the primary cause of anemia.
Some minerals may be toxic in excess, but overdoses of most minerals are uncommon. Overdoses are more likely when mineral supplements are taken. Salt (sodium chloride) is added to many foods, so the intake of sodium may be too high in many people. Too much sodium in the diet can cause high blood pressure in some individuals.
Other Micronutrients
Recently, new micronutrients called phytochemicals have been found in plants. They occur primarily in colorful fruits and vegetables, like those shown in the figure below.
Good sources of phytochemicals.
Thousands of phytochemicals have been discovered, and some have already been shown to lower the risk of certain diseases. For example, the phytochemical lutein helps reduce the risk of macular degeneration, an eye disease that leads to blindness. Lutein is found in many yellow and orange fruits and vegetables. Several phytochemicals, including some found in berries and cherries (Figure bellow), have proven to be powerful antioxidants.
Attribution
Essentials of Environmental Science by Kamala Doršner is licensed under CC BY 4.0. Modified from the original.