Digestible carbohydrates are simple sugars or complex starches in structure.

The simplest sugars, monosaccharides , or "single sugars," consist of hydrogen and oxygen atoms attached to a single chain of about three to seven carbon atoms.

Examples of monosaccharides include glucose and fructose ("fruit sugar"). Two monosaccharides linked together form a disaccharide, or "double sugar."

Everyday table sugar (sucrose) and the sugar in milk (lactose) are common disaccharides. During digestion, enzymes in your digestive tract split a disaccharide into two monosaccharides, which can then be absorbed.

Starches are called complex carbohydrates because they consist of many monosaccharides linked together to make a polysaccharide. During digestion, enzymes must break down these large structures into simple sugars so they can be absorbed.

Carbohydrates are your body's primary source of "calories" , or energy. Each gram of carbohydrate provides 4 calories of heat energy.

Health authorities recommend that 55-60% of your calories come from carbohydrates, especially complex carbohydrates. This means that if you eat 2,500 calories a day, carbohydrates should supply between 1,375 and 1,500 calories. (To calculate grams of carbohydrate, divide the carbohydrate calories by 4. So, you should be getting from 344 to 375 grams of carbohydrate per day.)

Carbohydrates aren't usually thought of in terms of excess or deficiency states. A diet abnormally high in carbohydrates may leave you feeling hungry, causing you to overeat in an effort to satisfy your appetite.

If you eat a diet quite low in carbohydrate, your intake will necessarily be high in protein and fat-a pattern with some definite disadvantages. When your body doesn't receive sufficient carbohydrate- its preferred fuel- protein may be burned for energy instead of being used for its primary functions. A diet very low in carbohydrate can lead to ketosis, a condition that results in weakness, dehydration, and nausea.

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Proteins are extremely essential for growth and development. These large molecules are made up of building blocks called amino acids. We require about 20 amino acids to build, repair, and maintain our many different body tissues.

Your body can't synthesize essential (indispensable) amino acids, meaning that you must get them from food. The amino acids that your body can synthesize are described as being nonessential (dispensable).

When you hear an amino acid described as conditionally essential, it means that under certain circumstances you aren't able to produce the amount of a nonessential amino acid you need and must obtain it from food.

Protein Quality

Food proteins classified as "complete proteins" provide all nine essential amino acids in sufficient quantities to meet human needs. Incomplete food proteins, on the other hand, are deficient in one or more of the essential amino acids. Most complete proteins are from animal sources -- meats, dairy products, and eggs. However, soy protein is now recognized as the only plant protein that is complete.

Over the years a number of different methods have been used to evaluate protein quality. Until fairly recently the US Food and Drug Administration (FDA) used the Protein Efficiency Ratio (PER) to calculate the percent of the US Recommended Daily Allowance (USRDA) that used to be shown on food labels. Animal proteins have a higher PER than plant proteins. However, the PER is based on the ability of a food protein to support growth in young, growing rats-not young, growing humans.

In 1993, the FDA adopted a new, more accurate method for evaluating protein quality to calculate the percent Daily Value of protein on food labels for adults and children over the age of one year. The Protein Digestibility-Corrected Amino Acid Score, or PDCAAS, takes several factors into account, including a food protein's:

Protein Needs

How much protein do you need? As a rule, most Americans get more protein than their bodies require to build, repair, and maintain tissues. When you eat more protein than your body needs, the excess is either burned for energy-providing 4 calories of energy per gram of protein-or converted to fat for storage.

The Recommended Dietary Allowance (RDA) for protein for adults is 0.8 grams per kilogram (kg), or 2.2 pounds (to calculate weight in kg, divide weight in pounds by 2.2), of body weight; the RDA for adolescents is 0.9 grams/kg. Therefore, an adult weighing 185 pounds (84 kg) should consume about 67 grams of protein per day.

As a rule, fully-grown people don't build new tissue. Even people exercising regularly typically consume enough protein to compensate for any increase in muscle mass. (Remember that consuming protein doesn't increase muscle-only exercise can do that.) Some experts in sports nutrition recommend that athletes undergoing strenuous heavy training, especially aerobic exercise, consume between 1.2 and 1.8 grams of protein/kg of body weight each day.

Protein deficiency isn't common in developed countries. When only protein is deficient, the resulting disease is called kwashiorkor. A diet low in both protein and calories-basically starvation-is called marasmus. A diet very high in protein, on the other hand, is typically low in carbohydrate. In addition to increasing the work required of the kidneys, diets very high in protein and very low in carbohydrate may cause nausea, weakness, and dehydration due to ketosis.

Fats are the third primary source of calories. When you compare the 9 calories/gram provided by fats with the 4 calories/gram for both carbohydrates and protein, it's easy to see why people who eat a high-fat diet typically get a lot of calories. Health authorities have emphasized the need to reduce dietary fat intake, although the body does need some fat. During infancy through two years of age, fat is necessary for brain development. After the age of two, the body only needs small amounts of fat. The typical American diet is much higher in fat than what the body requires. Consuming excessive amounts of fat is a major causative factor in coronary heart disease, high blood pressure (hypertension), obesity, certain types of cancers, and has been linked to a number of other disorders as well.

Most of the fat you eat-and most of the fat stored in your body-is in the form of triglycerides .
Another fat that has become familiar is cholesterol . Cholesterol is present only in animal foods-meat, poultry, fish, dairy products, egg yolk-and is also produced in the human body. Elevated levels of cholesterol and/or triglyceride in the blood increase your risk of heart disease .

The fatty acids in triglycerides have three different types of structures. Fatty acids can be saturated or unsaturated-either monounsaturated or polyunsaturated .

When you hear food fat referred to as a saturated fat it means there are more saturated fatty acids present than the other two types. A high intake of saturated fat and cholesterol can raise blood cholesterol levels, thereby increasing your heart disease risk.

Hydrogenation is a process in which unsaturated fatty acids are chemically changed to make them saturated. Another result of hydrogenation is the production of trans fatty acids , which, although still unsaturated, increase blood cholesterol levels much the same as saturated fatty acids.

Hydrogenated products, including margarine , shortening, and, to a lesser extent, vegetable oils, typically contain trans fatty acids. Some margarines are now processed using methods that prevent trans fatty acid formation.

Keep in mind that you also get a lot of saturated fat when you eat fatty meats and whole-fat dairy products, such as whole milk, cheese, and ice cream.

You need some fat in your diet. Fats have important functions, such as supplying essential fatty acids and helping absorb the fat-soluble vitamins A, D, E, and K. In addition to adding aroma and flavor to food, fats have a high satiety value, meaning they satisfy your appetite.
In some people, diets very low in fat tend to increase blood levels of triglycerides and reduce high-density lipoprotein-cholesterol . (HDL-cholesterol), the so-called "good cholesterol" that helps protect against heart disease.

Health authorities typically recommend limiting your total fat intake to 30% or less of your calories. For heart health, it's even more important to limit your intake of saturated fat to less than 10% of your calories. If you typically consume 2,500 calories, you should eat a maximum of 83 grams of fat (2,500 x 30% = 750 calories from fat divided by 9 calories/gram = 83 grams).

To calculate the recommended maximum intake of saturated fat, multiply 2,500 x 10% to get 250 calories from saturated fat. Then, divide 250 by 9 to get a maximum of about 28 grams of saturated fat.

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Vitamins are essential organic nutrients that don't provide calories. Most vitamins were discovered when scientists identified substances in foods that cured specific deficiency diseases.

Vitamins act as agents to control cell metabolism and are components of body tissues. They are classified as being soluble in fat or in water. The fat-soluble vitamins are A, D, E, and K. The water-soluble vitamins are vitamin C and the eight B-complex vitamins- thiamin, riboflavin, niacin, Vitamin B6, pantothenic acid, biotin, folate, and Vitamin B12.

Minerals are single inorganic elements essential to human life. Those minerals needed in larger amounts-more than 100 milligrams (mg) per day- are called major minerals. Minerals needed in smaller amounts-less than 100 mg/day- are called trace elements. Other trace elements thought to be important to health are under study. One important issue when looking at minerals is their bioavailability , or the degree to which the mineral is available for absorption in the body.

RDA, EAR, DRI, AI, UL - What Are They?

You're probably acquainted with the Recommended Dietary Allowances, or RDAs, which have been around since the early 1940s. Newcomers to this "alphabet soup" of recommended levels of vitamins and minerals include the DRI, EAR, AI, and UL.

Dietary Reference Intakes (DRIs) are reference values used to estimate nutrient intakes to be used for planning and assessing diets for healthy people. The DRI is a type of umbrella term that includes EARs, RDAs, AIs, and ULs.

An Estimated Average Requirement (EAR) is the level of a nutrient estimated to meet the requirement of half the healthy people in a life stage and gender group. Of course, at this intake level 50% of the group wouldn't meet its nutrient needs. The EAR is used to evaluate the adequacy of nutrient intake in groups and to develop RDAs.

A nutrient's RDA is the average daily intake that is sufficient to meet the nutrition requirement of nearly all (97-98%) individuals in a life stage and gender group. The RDA is based on the EAR plus an added amount.

Adequate Intake (AI) is used instead of the RDA if sufficient scientific evidence isn't available to calculate an EAR. The AI is based on estimates of average nutrient intake in one or more groups of healthy people.

The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects to almost all people in the general population. The higher the intake of a nutrient at levels above the UL, the greater the risk of adverse side effects. Even though there may not be enough evidence to establish a UL for every nutrient, an excessive intake of that nutrient may still cause negative side effects.

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Water is the single largest component of the human body, making up about 60% of your total body weight. You get water by drinking it and from various foods. Your body loses water in urine, bowel movements, perspiration, and exhaled air.

Functions
Water is essential to most processes that take place in the human body. For example, water:

• Dissolves minerals, vitamins, amino acids, glucose, and many other molecules, making them available for use in the body
• Takes part in chemical reactions
• Is essential to digestion, absorption, and elimination
• Transports nutrients to and removes waste from the cells of the body
• Maintains the structure of large molecules, such as protein
• Acts as a cushion and lubricant around joints and inside the eyes, spinal cord, and amniotic sac, which surrounds the fetus during pregnancy
• Helps regulate body temperature
• Maintains blood volume

How much do you need?
In most people, thirst is a good guide to your body's need for water. Exceptions include infants, athletes exercising heavily, people who are sick, and the elderly. Keep in mind that thirst may lag behind the body's actual need. Adults typically need about 8-12 cups of water per day. Your body may need more water when:

• You are exposed to extreme temperatures, including both very hot and cold weather conditions,
• With strenuous work or exercise,
• When exposed to heated or recirculated air (i.e. air plane),
• If you are pregnant or breastfeeding,
• If you are sick and have a fever, diarrhea, or vomiting,
• If you eat a high-fiber diet, your body needs extra fluid to process the additional roughage and prevent constipation.

Toxicity
There's a rare condition, called water intoxication, in which the body's water content is too high. It can occur when someone drinks excessive amounts of water or when kidney disorders reduce urine output. Symptoms include confusion, convulsions, and, in extreme cases, even death.

Common food sources
Just as your body is mostly water- but doesn't really look it-foods contain varying amounts of water. For example, watermelon, strawberries, celery, and broccoli are from 90-99% water, while legumes, pasta, and a chicken breast are from 60-69% water.

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Many substances in food affect your health even though they aren't classified as nutrients.

Fiber :

Fibers are indigestible complex carbohydrates that come from the structural parts of plants. Although digestive juices don't digest fiber, some fibers are broken down by bacteria naturally occurring in the gastrointestinal (GI) tract. Fiber isn't technically a nutrient; however, since there is a recommended intake for fiber, it's typically treated much like other nutrients. Fiber is often classified as being soluble or insoluble in water.

Functions
Soluble fibers slow down the passage of food in the GI tract, delay the absorption of glucose, and lower blood cholesterol levels. Insoluble fibers speed up the passage of food in the GI tract, promote regular bowel movements, slow down the breakdown of starch, and delay the absorption of glucose.

How much do you need?
Unlike many vitamins and minerals, there is no Recommended Dietary Allowance (RDA) for total fiber intake. Health experts recommend eating approximately 20 to 35 grams of fiber daily.

Deficiency
There isn't a fiber deficiency, as such. Consuming too little fiber can contribute to constipation and the development of diverticula , may contribute to higher blood cholesterol levels, and is associated with colorectal cancer.

Toxicity
Fiber isn't toxic. However, eating a diet extremely high in fiber may result in insufficient calories being consumed and may cause intestinal discomfort and excessive gas formation. The presence of too much fiber in the digestive tract may also interfere with mineral absorption.

Common food sources
The major food sources of soluble fiber are fruits, oats, barley, and legumes. Insoluble fiber is found in wheat or corn bran, whole-grain breads and cereals, and vegetables.

Choline :

Choline is synthesized (produced) in the body from the amino acid methionine. However, most people also need to get some choline from food, making it a conditionally essential nutrient.

Functions
The body uses choline to make the neurotransmitter acetylcholine and lecithin , which is a phospholipid (fat containing phosphorus).

How much do you need?
Even though choline isn't technically a nutrient, Adequate Intakes and Tolerable Upper Intake Levels have now been established for it.

Deficiency
Choline deficiencies are rare in humans.

Toxicity
High doses of choline have been associated with a "fishy" body odor, sweating, salivating, low blood pressure, and mild liver damage in humans. The UL was established based on the level causing low blood pressure.

Common food sources
Food sources of choline include milk, liver, eggs, peanuts, soybeans, beef, cauliflower, kale, and cabbage.

Phytochemicals :

It's only in recent years that phytochemicals have been included in many discussions of nutrition. The word phytochemical simply means "plant chemical," and refers to thousands of substances that occur naturally in plants. Phytochemicals aren't classified as nutrients because they aren't known to be essential for life and growth. Although their role in health and nutrition is not yet fully understood and much of the research is only preliminary, many phytochemicals appear to play important roles in reducing the risk of disease and maintaining health.

One type of phytochemical currently under study are the isoflavones found almost exclusively in soy- genistein, daidzein, and glycitein. Isoflavones are sometimes classified as phytoestrongen . This means that their chemical structures resemble that of the hormone estrogen .

Researchers are investigating potential roles of soy protein with naturally occurring health components including the isoflavones in helping to reduce the risk of heart disease, promote bone health, alleviate menopausal symptoms, and research is underway concerning their potential role in cancer management and prevention.

Scientists continue to unravel the secrets of individual phytochemicals, including whether they are less effective or even ineffective when they are removed from the foods in which they occur. The wealth of different phytochemicals present in about all plant foods provides just one more important reason for including generous amounts of these healthful foods in your daily diet.

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