5.3_Human_nutrition_II - Today, we will be looking more...

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Today, we will be looking more closely at how humans use carbohydrates which typically constitute the majority of calories in many people’s diets. 1
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We’ll start with a broad overview of nutrition, review the chemistry of carbohydrates, and then dive into human carbohydrate metabolism. Finally, we will relate this to the Western diet typical of most Americans. 2
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Animal nutritional requirements can be distilled down to three categories: (1) molecules that can be used in cellular respiration to re-energize ATP and thus provide for energy needs; (2) molecules such as amino acids and fatty acids that we can used to build news cells and tissue; and (3) molecules such as essential amino acids, essential fatty acids, minerals, vitamins, etc. that we need to function but can not synthesize ourselves. In this presentation, we will look at the first category primarily, but first, let’s review some basic biochemistry of carbohydrates. 3
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Carbohydrates are made up of carbon, hydrogen, and oxygen in a typical ratio of two hydrogen atoms and one oxygen atom for every carbon atom. We generally categorize carbohydrates as simple or complex; however, there is actually a span of complexity from single saccharides such as glucose to gigantic complexes of sugars such as cellulose. 4
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5 Glucose is a very common monosaccharide that has the greatest importance to living organisms. It is used as a primary energy source by cells and also is the basic building block for complex carbohydrates used for structural support in plants and energy storage in plants and animals. When in solution, it forms a ringed structure as shown on right. There are several forms (called isomers) of glucose depending on the position of the hydroxide molecules attached to the carbon chain. The one shown here is sometimes called dextrose which is sometimes listed on food ingredient labels.
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6 Another simple carbohydrate type is called a disaccharide which involves two monosaccharides bonded together. We are most familiar with sucrose or common sugar which is fructose and glucose bonded together with the same dehydration reaction as shown here. This disaccharide is maltose which is what forms in our mouth when we chew starchy foods and the amylase in our saliva breaks up the starch into maltose molecules. This will be further broken down to glucose molecules in our small intestine.
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human diets, we see that starch is made up of simple chains of glucose with only modest branching and is quite digestible by animals; glycogen involves highly branched chains of glucose and is digestible; but cellulose forms more rigid and complex sheets of glucose and is not digestible. Some cultures eat insects and thus consume another form of complex carbohydrate called chitin which makes up the exoskeletons of arthropods such as insects and crustaceans; however, it is essentially indigestible for humans like cellulose. As we will see later, eating something that is
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5.3_Human_nutrition_II - Today, we will be looking more...

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