NUSCTX 10: Carbohydrate notes

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University of California - Berkeley
Nutritional Sciences And Toxicology
Robert Ryan

Lecture 6: Carbohydrates A. Learning Outcomes » Identify major types of carbohydrates and give examples of food sources for each » List alternative sweeteners that can be used to reduce sugar intake » Describe recommendations for carbohydrate intake and health risks caused by low or excessive intakes » List the functions of carbohydrates in the body B. Carbohydrates » Composed of carbon, hydrogen and oxygen » Produced by plants via photosynthesis » Simple carbohydrates – Monosaccharides – Disaccharides » Polysaccharides (more complex) – Starch – Glycogen – Fiber C. Monosaccharides » Monosaccharide – Glucose ◊ Blood sugar ◊ We do everything we can to take glucose to harness its energy ◊ Central monosaccharide in our diet ◊ But we rarely eat glucose! Because it is rarely found in foods, and is a component of other carbohydrates ◊ We have to liberate the glucose from other food sources in order to get its energy – Fructose ◊ High-fructose corn syrup – Galactose ◊ Part of lactose (present in milk) – Sugar alcohols (monosaccharide derivatives) ◊ Xylitol, mannitol and sorbitol ◊ Are related structures, but which are not exactly like the monosaccharides – they are harder to convert into sugar, but they have a sweet taste. ◊ Used in sugarless gum, or other products of consumption that leaves a sweet taste but with reduced calories – Pentoses (5 carbon) ◊ Ribose and deoxyribose ◊ Makes nucleic acid in DNA and RNA » Different structures between glucose, fructose and galactose, but when all is said and done the body turns the fructose and galactose into glucose (happens in the liver) » It is the way in which the atoms join together that determine whether or not we can access the energy contained in them » Metabolism of sugar is to yield energy D. Disaccharides » Two monosaccharides linked by a condensation reaction – Alpha or beta C-O-C bonds connect them » Maltose – Glucose-glucose; alpha bond » Sucrose – Glucose-Fructose; alpha bond » Lactose – Galactose-Glucose; beta bond E. Complex Carbohydrates » Oligosaccharides – Contain 3-10 sugar units – Raffinose and Stachyose – Indigestible; produces gas due to bacterial fermentation » Polysaccharides – Contain many glucose molecules – Able to be digested efficiently – Alpha or beta bond determines digestibility F. Digestible Polysaccharides » Starch (plants) – Amylose: straight chain – Amylopectin: branched » Glycogen (animals) – Storage form of glucose in human body; glucose can be metabolized into ATP (energy for the body) – Liver glycogen (90g): converted to blood sugar – Muscle glycogen (300g): glucose for muscle use – We do this so we have an immediate energy source – When we need it, we can subject the glycogen to the oxidation process that yields energy – When we have too much glycogen in the body, we convert the sugar into fat G. Indigestible Polysaccharides » Total fiber – Dietary fiber + functional fiber » Soluble fibers – Pectin, gum, mucilages, and some hemicelluloses » Insoluble fibers – Cellulose, hemicellulose, and lignin » Health benefits of fibers – We can’t get energy from this in the form of glucose because we don’t have the enzyme to break it down – A way to help decrease our uptake of other less healthy foods – Soluble fibers help to absorb water in intestine – Insoluble fibers provide substance and helps faeces have more form, making it easier to release from the body H. Carbohydrates in Foods » Starch – Digestible polysaccharides – Able to digest and liberate glucose » Fiber – Indigestible polysaccharides – Provides benefit, but not as a direct nutritional benefit » Nutritive sweeteners – Mono and disaccharides – High fructose corn syrup – Sugar alcohols I. Alternative (Non-Nutritive) Sweeteners » Yield no energy » Acceptable Daily Intake (ADI) » Saccharin – Oldest alternative sweetener – Develop bitter taste with cooking » Aspartame – Cannot be used in cooking – Contains phenylalanine ◊ An amino acid that’s usually converted into tyrosine (another amino acid) › Tyrosine usually goes to melanin, proteins and dopamine ≈ Melanin is skin pigment ◊ The reason they put it in the label is because some people are intolerant to phenylalanine (Phenylketonuria) – no enzyme to convert phenylalanine to tyrosine and converts it into phenylpyruvic acid instead J. Recommended Intake of Carbohydrates » RDA – 130g/day » 45-65% of total energy needs » Limit added sugars and caloric sweeteners » Fiber: 14g/1000kcal is adequate intake – 25g/d for women under 50 (21g/d after 51) – 38g/d for men under 50 (30g/d after 51) K. Our Carbohydrate Intake » 50% of total energy needs » Added sugars 16% of kcal – Recommendations (need to decrease): ◊ 6% of kcal (Dietary Guidelines) ◊ 10% of kcal (WHO) ◊ 25% of kcal (DRIs) » Dietary fiber (need to increase) – 25-50% less than recommended L. Functions of Digestible Carbohydrates in the body » Provides energy: 4 kcal/g – We can combust them, forming carbon dioxide and ATP » Spare protein – Consuming sufficient amounts of carbs spares protein – Carbohydrate breaks down into glucose and it stor
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