Carbohydrates

Carbohydrates

 

 

With this chapter we begin our study of the macronutrients— carbohydrates, fats, and proteins. These three nutrients share a unique characteristic: the ability to supply energy. Carbohydrates are of prime importance in the human diet. Over the ages they have nurtured cultures throughout the world as the major source of energy for work and growth. In recent years researchers have focused attention on digestible and indigestible carbohydrates and their various effects on health. Indigestible carbohydrates, usually referred to as fiber, have specific functions in maintaining gastrointestinal health. Plant foods identified as functional foods have distinct characteristics that positively influence health. Unfortunately, added sugars, which contribute kcalories but little else, are making up a greater proportion of the typical American diet at the expense of complex carbohydrates and fiber. We will first review the different types of carbohydrate and then examine their functions.

THE NATURE OF CARBOHYDRATE

Basic Fuels: Sugars and Starch

Two forms of digestible carbohydrate occur naturally in plant foods: (1) sugars and (2) starch. Energy on planet Earth comes ultimately from the sun and its action on plants. Using their internal process of photosynthesis, plants transform the sun’s energy into the stored fuel of carbohydrate  Plants use carbon dioxide (CO2) from the air and water from the soil—with the plant pigment chlorophyll as a chemical catalyst—to manufacture sugars and starch. The carbohydrates that plants store for their own energy needs become a source of fuel for humans who eat those plants. Because our bodies can rapidly break down starch and sugars, carbohydrates are often referred to as quick energy foods. They are our primary source of energy.

CLASSIFICATION OF CARBOHYDRATES

The term carbohydrate comes from its chemical nature. Carbohydrates contain the elements carbon, hydrogen, and oxygen, with the hydrogen/oxygen ratio usually that of water (CH2O). Carbohydrates are classified according to the number of basic sugar or saccharide units that make up their structure. The monosaccharides and disaccharides are referred to as simple carbohydrates because of their relatively small size and structure. The polysaccharides, including starch and certain fibers, are called complex carbohydrates based on their larger size and more complicated structure.

·         Monosaccharides

The simplest form of carbohydrate is the monosaccharide, or single sugar. The three monosaccharides important in human nutrition are (1) glucose, (2) fructose, and (3)

·         Disaccharides

The disaccharides are double sugars made up of two monosaccharides linked together. The three disaccharides of physiologic importance are sucrose, lactose, and maltose. Their monosaccharide components are as follows: Sucrose ,Lactose, Maltose.

·         Polysaccharides

Complex carbohydrates are called polysaccharides because they are made up of many (poly) single glucose (saccharide) units. Starch is the most important digestible polysaccharide; others are glycogen and dextrin. Nondigestible polysaccharides, such as cellulose, add important bulk to the diet and are categorized as dietary fiber. Box 3-2 displays some common food sources of polysaccharides. Starch ,Glycogen, Dextrins.

IMPORTANCE OF CARBOHYDRATES

The complex carbohydrate found in vegetables, legumes, and grains should be the major dietary source of energy. On digestion, starch yields glucose, the favorite energy source of body cells. Fruit and dairy products supply carbohydrate in the form of naturally occurring sugars (fruit contains fructose, and milk contains lactose). Vegetables, legumes, fruits, and grains (especially whole grains) supply other important nutrients, including vitamins, minerals, and fiber. Dairy products are good sources of calcium, magnesium, and protein. Currently, major food sources of starch in the American diet are refined grains, such as white bread and ready-toeat cereals, white potatoes, pasta, and rice, which are often highly processed and limited in micronutrients. Whole grains include the bran covering of the grain kernel when compared with refined grains from which the bran has been removed. Refined grains are enriched with several B vitamins, iron, and folate but provide less fiber and other trace minerals. Carbohydrate foods high in added sugar supply kcalories but little else. Based on the connection between added sugar and cardiovascular risk factors, including obesity, the American Heart Association recommends a prudent intake not to exceed 100╯kcal per day for women and 150╯kcal for men.18 The Perspectives in Practice box, “Cutting Down on Sugar,” offers suggestions on how to lower added sugar intake.

FUNCTIONS OF CARBOHYDRATES

1.      Energy

The primary function of starch and sugars is to supply energy to cells, especially brain cells that depend on glucose. When carbohydrate is lacking, fats can be used as an energy source by most organ systems; however, body tissues require a constant supply of glucose to function most efficiently. Body stores of carbohydrate are relatively small but still serve as an important energy reserve. An adult man has about 300 to 350╯g of carbohydrate stored in his liver and muscle in the form of glycogen, and another 10╯g of glucose circulates in his blood (Table 3-4). Together, this glycogen and glucose will supply the energy for only a half day of moderate activity. To meet the body’s constant demand, carbohydrate foods must be eaten regularly and at reasonably frequent intervals.

2.      Special Functions

Carbohydrates have other specialized roles in overall body metabolism.

·         Glycogen–Carbohydrate Storage

Liver and muscle glycogen are in constant interchange with the body’s overall energy system. These energy reserves protect cells, especially brain cells, from depressed metabolic function and injury and support urgent muscle responses.

·         Protein-Sparing Action

Carbohydrates help regulate protein metabolism. An adequate supply of carbohydrate to satisfy ongoing energy demands prevents the channeling of protein for energy. This protein-sparing action of carbohydrate allows protein to be reserved for tissue building and repair.

·         Heart Action

Heart action is a life-sustaining muscle activity. Although fatty acids are the preferred fuel for the heart, the glycogen stored in cardiac muscle is an important emergency source of contractile energy.

·         Central Nervous System

The brain and central nervous system (CNS) depend on carbohydrate for energy but have very low carbohydrate reserves—enough to last only 10 to 15 minutes. This makes them especially dependent on a minute-to-minute supply of glucose from the blood. Sustained hypoglycemic shock causes irreversible brain damage. Providing an adequate morning supply of glucose for brain function may help to explain why individuals who eat breakfast do better in school than those who skip breakfast.19 Glucose increases the synthesis of acetylcholine, a neurotransmitter that acts on areas of the brain responsible for memory and cognitive function.

Do Nonnutritive Sweeteners Help You Lose Weight?

Identify the Problem: In an effort to avoid or lose unwanted weight gain, consumers have looked for ways to decrease their kcalorie intake while continuing to enjoy their favorite foods, especially sweet foods. Nonnutritive sweeteners (NNSs) that satisfy the palate and add few if any kcalories would appear to be a welcome alternative, and their use in commercial food processing and home food preparation continues to rise. Review the Evidence: Although it might be expected that use of NNSs would promote weight loss or at least arrest weight gain, research evidence has not supported this conclusion. Taste experts quoting animal studies suggest that intense sweeteners, such as aspartame or sucralose, create neural responses that increase appetite and food intake;1 however, evidence from human studies reviewed by the Academy of Nutrition and Dietetics indicated that none of the NNSs used in the United States have this effect in adults.2 Findings among children are uncertain. The 2010 Dietary Guidelines Advisory Committee evaluated existing evidence as to the effect of NNSs on body weight. Although those who gain weight or have a higher body mass index are more likely to use NNSs, this does not mean that these sweeteners led to their weight gain. Overweight persons often choose NNSs in an effort to address their weight problem. NNSs will influence energy intake and assist in weight loss or weight management only if they replace higher-kcalorie foods and beverages.3 Implement the Findings: If an individual consumes two 12-oz cans of sugar-sweetened soda each day at a cost of 300╯kcal (150kcal/can), substitution of a calorie-free beverage could over time lead to a modest loss of body weight. However, kcalories “saved” by use of reduced-calorie or calorie-free items cannot be used to add other foods to the diet or enjoy larger portions. Overcompensation for kcalories eliminated with use of NNSs can bring about additional weight gain.4 Use food labels to evaluate the content of added sugars, naturally occurring sugars, and NNSs in foods and beverages.