Carbohydrates nutrition

   ›      ›   Carbohydrates in human nutrition.

What are carbohydrates?

Carbohydrates are integral part of human nutrition. Carbohydrates are the primary source of energy for most of the biological functions.
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Carbohydrates are compounds organized as ring structures and are always composed of the elements carbon, hydrogen and oxygen. They are known as organic compounds, because they are made up of a chain of carbon atoms. Other organic compounds include lipids, proteins, and nucleic acids. One gram of carbohydrate (starch or sugars) provides 3.75kcal (16kJ) energy.

Type of carbohydrates

Carbohydrates, also known as saccharides, are classified into four chemical groups. They are monosaccharides, disaccharides, oligosaccharides and polysaccharides. The monosaccharides and disaccharides, having lower molecular weight, are generally known as sugars. The oligosaccharides and polysaccharides are complex carbohydrate molecules with greater molecular weight.
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Monosaccharides

Monosaccharides are the basic units of carbohydrates and they include, glucose, fructose and galactose. These carbohydrate forms are directly absorbed by the intestines. The glucose form is used for energy production for biological activities in the human body. Monosaccharides are present in several fruits and vegetables.

Disaccharides

Disaccharides are formed by a condensation reaction of two monosaccharides. Common examples of these carbohydrates are sucrose, lactose and maltose. They are broken down by digestive enzymes into glucose for absorption in the intestine. These carbohydrates are water soluble and are naturally found in fruits, sugarcane and beet.

Oligosaccharides

Oligosaccharides are formed by a chain of three to nine monosaccharide units. Oligosaccharides in the form of fructo-oligosaccharides are found in several fruits and vegetables. They consist of short chains of fructose molecules. Galacto-oligosaccharides are a type of oligosaccharides present in several natural diets. They are considered as dietary fibers. They are prebiotics and are non-digestible. These carbohydrates are helpful in stimulating the activity of beneficial bacteria in the colon and provide bulkage for stimulating peristalsis.

Polysaccharides (Complex Carbohydrates)

Polysaccharides have, from linear to highly branched structure and are composed of long chains (more than ten) of monosaccharide units. Some of the examples are starch, glycogen, cellulose and chitin. In human nutrition, starch is broken down to monosaccharide units by the intestinal amylases enzymes. Glycogen forms an energy reserve and is primarily made by the liver and the muscles.

Cellulose is the most abundant carbohydrate in nature and is of no direct nutritional value to humans as we lack enzymes to digest it. Chitin and pectins are the other natural polysaccharides, being indigestible, have no direct value in human nutrition. However the colon bacteria can digest a part of these carbohydrates and an energy value of 2kcal/g (8.4kJ) may be contributed by these dietary polysaccharides.

Dietary fibers in nutrition

All the carbohydrates in the diet that are not directly digestible by humans are grouped under dietary fibers. However gut bacteria may digest and convert part of it to absorbable forms. Soluble fibers are soluble in water. Some of the dietary fibers are arabinoxylans, cellulose, inulin, lignin, waxes, chitins, pectins, beta-glucans and oligosaccharides. These carbohydrates function as prebiotics and also provide bulkage for stimulating peristalsis. They are particularly helpful in lowering the glycemic index of the ingested food. Dietary fibers slowing down the digestion and absorption of carbohydrates in the intestines.

Carbohydrate nutritional status

Excess of carbohydrates in food can lead to obesity and cardiovascular diseases. Balancing the need of energy source as well as the risk of heart disease and obesity certain nutritional recommendations were made. According to the Dietary Guidelines for Americans recommendation, carbohydrates should account for 45 to 65 percent of an individual's daily calorie consumption. It means, if an individual get 2,000 calories a day, between 900 and 1,300 calories (approximately 225-325 grams) should be from carbohydrate sources. The USDA Dietary Guidelines for Americans also recommends consuming 14 grams of fiber for every 1000 calories.

Persistent carbohydrate deprivation in diet will lead to depletion of the blood glucose, as well as the stored glycogen in body. The next step by the body is to utilize fats and amino acids for energy production. However this route of energy production results in acidosis, ketosis and loss of cellular proteins. Those trying weight loss without doctor/dietician guidance usually end up upsetting their metabolic balance. Such rapid weight losses are unlikely to be maintained for long span of time. Carbohydrate cravings may takeover and the individual may end up gaining weight.

Healthy carbohydrates

Most of the food we consume has carbohydrates. Sugars are broken down faster and the released glucose is absorbed into the blood leading to increase in blood glucose. Recurrent sudden spikes in the rise of blood glucose can lead to several health problems including diabetes. The rapidity and magnitude of carbohydrate's effect on the increase in blood glucose levels is the glycemic index of the food. Apart from that, excess glucose in nutrition is stored as glycogen to certain extent and further excesses get converted into fats. Hence, the carbohydrate in our nutrition should not be in excesses and also should not be in easily absorbable sugar forms.

Diet containing greater amounts of polysaccharides and dietary fibers is considered healthier than the diet having greater amounts of sugars. Whole grains, vegetables, fruits, nuts, seeds, legumes and dairy products are good sources of complex carbohydrates and dietary fibers. Though some of these carbohydrates may also have sugars, the presence of dietary fibers lowers their glycemic index as their absorption is prolonged.
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References:
1. Mozaffarian D, Hao T, Rimm EB, Willett WC, Hu FB. Changes in diet and lifestyle and long-term weight gain in women and men. N Engl J Med. 2011;364:2392-404.
2.Sacks FM, Bray GA, Carey VJ, et al. Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates. N Engl J Med. 2009;360:859–73.
3.Eric C Westman, Richard D Feinman, John C Mavropoulos, Mary C Vernon, Jeff S Volek, James A Wortman, William S Yancy, Stephen D Phinney. Low-carbohydrate nutrition and metabolism. Am J Clin Nutr August 2007 vol. 86 no. 2 276-284.
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Current topic in nutrition, deficiency & diseases: Carbohydrates nutrition.