Carbohydrates – The Good and Bad Points
THE GOOD AND BAD POINTS
By Robert G. Black
Carbohydrates are a primary source of energy for birds, animals, and humans. In quantity consumed and energy value extracted, the carbohydrates usually rank first in the diets of most species of birds commonly maintained in aviculture. The carbohydrates function primarily in the diet as a source of energy in the avian body for heat and muscle work, but can also be stored to a certain extent.
Most carbohydrates are plant products, formed by photosynthesis, using sunlight in the presence of chlorophyll. Technically, the plants turn light energy into chemical energy by combining carbon, hydrogen and oxygen in a complex series of reactions. The resulting sugars and starches are stored in the structure and seeds of the plant. When birds eat this material, it yields carbon dioxide and water in the body as by-products. The dry matter of most plants ranges from 60% to 90% carbohydrates.
As surprising as it may seem, there is no definite, known nutritional requirement for carbohydrates in the diet. Fats or protein can substitute, if perhaps less efficiently, for the known functions of carbohydrates in avian metabolism. Nevertheless, it is fortunate that most species can digest and utilize carbohydrates easily, or the available food supply would be drastically reduced.
Carbohydrates are classed under several headings, depending upon their chemical structure. They range from very simple in their structure to incredibly complex. This structure determines whether the carbohydrate is digestible, absorbable, and capable of being utilized by the avian body. The characteristics and properties of the carbohydrates are dictated by their molecular structure.
The food carbohydrates are classified as sugars and starches, and the sugars are the simplest of the carbohydrates in chemical structure. The sugars are subdivided into several categories, depending upon their structure, while the starches are made up of groupings of the various sugar molecules. The numbers of different starches possible through the joining of the various sugar molecules in different combinations is truly astronomical.
The simplest of these sugars are called monosaccharides. These are the simplest carbohydrates. Monosaccharides contain from three to seven carbon atoms. By far the most important in nutrition is glucose, which I will cover in more detail later. Glucose is found in some fruits and honey in pure form. This is why diluted honey can be such an instant pick-me-up for a sick bird after the cause of the sickness has been removed. Glucose needs no digestion; it is absorbed as is, circulates to the cells that need the energy, and is used almost instantly for energy without any chemical change being needed along the way.
Another important monosaccharide is fructose, and it is also called fruit sugar. As the name implies, this sugar is found primarily in fruits. Fructose is also known as levulose, and it is also found in vegetables and honey.
Galactose is one of the two simple sugars that make up each molecule of lactose, the compound sugar found in milk. The other simple sugar in lactose is glucose. Most birds cannot digest the lactose in milk products, because their bodies cannot make the enzyme that is necessary to break lactose down into the two simple sugars.
Glucose, fructose, and galactose are the major forms of carbohydrate circulating in the bloodstreams of avian species. Though more than twenty monosaccharides have been identified by science, these three are the most important in the study of nutrition. All three are known in three separate forms, one with a straight carbon chain, one with a left facing molecule, and one with a right facing molecule.
One other monosaccharide is quite important in nutrition, and that is ribose. Ribose is found in corn and beets naturally and ribose alone forms the carbohydrate portion of both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Ribose is also a constituent of the vitamin riboflavin, also known as vitamin B2.
The term disaccharide means two monosaccharides linked together to form a more complex sugar. The disaccharides are the simplest of the multiple sugars. Three of these disaccharides are important in the nutrition of birds, but the digestive system cannot absorb them until the required enzymes have broken down the disaccharides into their component monosaccharides.
Sucrose is the most important of these disaccharides. It is formed by the combination of one glucose molecule and one fructose molecule. Sucrose is our common white sugar, extracted from sugar cane or sugar beets. In the process of digestion, an enzyme called sucrase or invertase splits these two joined molecules and releases the free monosaccharides glucose and fructose.
Maltose is the second important disaccharide. It is composed of two glucose molecules linked together. The enzyme maltase breaks these apart in the small intestine and thereby releases two molecules of glucose for each molecule of maltose.
Lactose is the third important disaccharide. Lactose is of great importance in human nutrition, since it is the sugar found in milk. One glucose molecule and one galactose molecule join to make one molecule of lactose. Lactose is the only common sugar that is of animal origin. The digestion of lactose requires that the enzyme lactase be present to break the lactose molecule into its glucose and galactose constituents. Lactose is much less soluble than the other sugars, and it therefore is digested much more slowly in the intestinal tract.
Most birds, however, lack the enzyme lactase to break the lactose down. Before giving any of your birds milk or milk products, test them for lactose tolerance. This means that you should give them a little of the product and then watch closely for any sign of diarrhea or other distress. Most avian species have not been tested for lactose tolerance, since no one has yet been willing to pay for the extensive testing that would be required for the over 9,000 living species of birds. Since milk is not a natural food for avian species, however, it is a safe bet that most species will not be able to digest milk products effectively because of their lactose content. Even within the same species, however, there may be strains in captivity that can tolerate lactose with no problem whatsoever, and other strains that will show symptoms of indigestion and diarrhea from any lactose-containing material that they consume.
Once past the infant stage, over half of the human race can no longer tolerate lactose in the diet. For the Caucasian peoples, over 90% can eat milk or milk products with no problem. This is why milk products can be so important in the nutrition of Americans and Europeans. With the native African peoples, this tolerance drops to about 50% of the population. Even half of black Americans cannot tolerate lactose, and milk products will upset their digestive systems and cause diarrhea, as the body tries to rid itself of this indigestible substance. Of the Asian peoples, 90% cannot tolerate lactose, because of the digestive upset it causes. This is why you find absolutely no milk products ever used in the justly famous and delicious Chinese cooking.
Once milk has been converted to other foods, such as cheese or yogurt, birds may be able to tolerate it. The bacteria in milk change the lactose into lactic acid, and this is what causes milk to go sour. The bacteria that transform milk into cheese and yogurt also often feed on the lactose in the milk, and leave these foods safe for the birds. However, the expense of such milk products alone is enough to limit their use as a food for birds when in the care of most aviculturists.
Many old books recommend milk sop, bread soaked in milk, as a food for cage birds. I definitely do not recommend milk products of any kind for birds, for the reasons already noted. Feeding this milk product can create far more problems with indigestion than it can solve through its nutritional value. Also, white bread is an extremely poor food for birds or people – Most of the vitamins and minerals have been removed in making the white flour and most bread has a zero value of complete protein. Bread is a carbohydrate food only, and most cage birds are already forced to eat more carbohydrates than they need. Whole wheat bread is marginally better, but I still do not recommend any kind of bread as a food for birds.
The polysaccharides are those carbohydrates that contain three or more monosaccharide molecules joined together. Starch is the only polysaccharide that can be used efficiently in the metabolism of birds. Nutritionally, the starches are the most important group of carbohydrates. There can be more than a thousand simple monosaccharides, such as glucose, in a single polysaccharide carbohydrate chain. The resulting carbon chain formation can be straight or branched. Rice, wheat, corn, and millet contain about 70% starch. Even as much as 40% of the content of beans and other seeds is starch. Cereal grains are the most important source of starch in avian nutrition, but tubers, such as potatoes, are also important sources of the carbohydrates for human consumption. Agar and pectin are examples of common products that are polysaccharides.
In the digestive process, the digestive system splits complex carbohydrates into monosaccharides through the action of the specific enzymes needed to detach each type of simple sugar from the carbohydrate chain. This splitting process is called hydrolysis. Hydrolysis occurs quite rapidly under the influence of enzymes. The enzymes act as catalysts only. They speed up the reaction, but are not altered in structure by so doing. The digestive tract cannot absorb starches, and absorbs only the resulting monosaccharides after the starches are broken down.
Glucose is the most common monosaccharide, and it is by far the most important in the nutrition of birds and animals. Glucose is the blood sugar for all birds and animals, and it is controlled in this function by the body within very narrow limits. It is the basic source of energy in all birds and animals. Glucose occurs in three structural forms, called the open-chain form and the ring forms. All three forms are equal in every respect, both nutritionally and metabolically. Glucose is highly soluble in water, is neither acidic nor alkaline, and the tissues utilize it directly. The body can convert glucose to other monosaccharides as needed in a reversible process. At the cell level, energy becomes available when glucose is broken down in the tissues.
The body can store carbohydrates within the body in the form of glycogen. Glycogen is a polysaccharide, similar to starch. For this reason, glycogen is sometimes called ‘animal starch’. Glycogen is stored in the liver and in the muscles.
Cellulose is the primary carbohydrate that plants use to form their stems, branches and trunks. Though cellulose is a carbohydrate and an energy source, it furnishes only bulk in the avian diet, since birds do not possess the enzyme cellulase in the digestive tract for breaking it down. Cellulose is completely unabsorbable.
The best sources of carbohydrate content in the diet of seed-eating birds will be the cereal grains: wheat, millet, oats, canary seed, etc. For softbills, lories and similar feeders, fruit will be the best source. In preparing mixed foods from which birds cannot separate protein from carbohydrate, it is unwise in the extreme to dilute a high protein food with additional carbohydrate. Leave carbohydrates in their natural form or feed them separately. For example, I strongly advise against mixing bread crumbs or corn meal with hard boiled egg, as a nestling food. Such a practice forces the birds to consume excess carbohydrate in order to get the protein content they crave, particularly while feeding young ones. This can and will cause obesity, as the birds are forced to eat an excess of carbohydrates in order to get the protein that they must have.
Mashed, hard-boiled eggs are one of the best and cheapest high-protein foods you can feed. Because of their high water content, add a spoon of powdered vitamin-mineral supplement to each boiled egg and mix it thoroughly. If the egg mix still seems too wet and sticky, add a couple of spoons of soy protein and mix it again. The soy protein greatly increases the protein content of the mix, and makes it even better for the birds nutritionally. NEVER add corn meal, flour, or cereals to the egg mix, as these items are predominantly carbohydrates – they will do no good and will do positive harm to the nutritional value of an egg mix. Also, never add anything oily or wet to the egg mix, as these items will cause the egg to spoil very rapidly.
I have been able to maintain lories in perfect health with this mix and other soft foods, without the need for messy nectars or complicated fruit mixes. Softbills love the egg mix and will take it in preference to every other food except chopped raisins! I have successfully hand raised such unusual babies as Least Terns and a Purple Martin using the egg as their primary nestling food.
Most breeders, unfortunately, feed a huge excess of carbohydrates in the diets of their birds. The endless repetition of seeds, grain products, fruits, cereals and vegetables adds primarily sugar and starch to the diet, rather than the proteins, oils and fats that are required for the birds’ health.
As I previously mentioned, no bird will ever suffer from a carbohydrate deficiency. Carbohydrates in any form are simply not necessary in the nutrition of birds, though you can hardly keep from feeding some carbohydrate foods to your birds in any normal diet. Overloading cage birds with high carbohydrate foods, however, is a sure road to illness and death.
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