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CarbohydratesCarbohydratesCarbohydratesCarbohydrates

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Carbohydrates

Complex

Simple

Fiber

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Simple Sugars• Monosaccharides:

– glucose– fructose– galactose

• Disaccharides – digestion splits to mono– Sucrose: glucose / fructose– Maltose: glucose / glucose – Lactose: glucose / galactose

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Complex CHO (Polysaccharides)

• CHO consist > 2 single sugar units

• Starch: long chains of glucose– plant sources

• Glycogen: branched chains of glucose

• Cellulose: long chain of glucose– “dietary fiber”– Plant sources

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Polysaccharides

Glucose

multiple

Glucose

Moieties

Linked together

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Dietary Fiber - only in plants

Indigestible by human enzymes

Solubleoat branlegumes fruit pectin

InsolubleFruit skinvegetablesWhole grains Seeds

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DIGESTION OF CARBOHYDRATESDIGESTION OF CARBOHYDRATES

Glycogen, starch and disaccharides (sucrose, lactose and maltose) are hydrolyzed to monosaccharide units in the gastrointestinal tract.The process of digestion starts in the mouth by the salivary enzyme –amilase.

The time for digestion in mouth is limited.

Salivary -amilase is inhibited in stomach due to the action of hydrochloric acid.

Another -amilase is produced in pancreas and is available in the intestine.

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-amilase hydrolyzes the -1-4-glycosidic bonds randomly to produce smaller subunits like maltose, dextrines and unbranched oligosaccharides.

-amilase

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The intestinal juice contains enzymes hydrolyzing disaccharides into monosaccharides (they are produced in the intestinal wall)

Sucrase hydrolyses sucrose into glucose and fructose

Sucrose

sucrase

Fructose

Glucose

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Lactose

lactase

Maltase hydrolyses maltose into two glucose molecules

Lactase hydrolyses lactose into glucose and galactose

Maltose

maltase

Galactose Glucose

GlucoseGlucose

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ABSORPTION OF ABSORPTION OF CARBOHYDRATESCARBOHYDRATESOnly monosaccharides are absorbed

The rate of absorption: galactose > glucose > fructoseGlucose and galactose from the intestine into

endothelial cells are absorbed by secondary active transport

Na+ GlucoseProtein

Protein

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Digestion & Absorption of CHO

Digestible CHO Functions

• Insulin stimulates cells to absorb glucose

• Energy: 4 cal / gram

• Glucose in cell is “metabolized” to:

H2O + CO2 + ATP + Heat

• Sugar alcohols : sorbitol, xylitol energy

Fate of Glucose from Food

• Some used for immediate energy (ATP)

• Some stored as glycogen in liver & muscle– Glucose in glycogen is readily available for

ATP

• Excess changed to fat– for storage/reserve of energy

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Glycogen

CHO storage in muscle and liver for quick energy

Glycogen glucose

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Sugar and Health

• Empty calories• Dental caries

• NOT diabetes• NOT hyperactivity

Lactose Intolerance • Caused by insufficient enzyme lactase;

Undigested and unabsorbed lactose is acted upon by bacteria

• Symptoms: bloating, flatulence, abdominal pain, diarrhea

• No need to omit milk, milk products- Lactaid may be added to milk or use

lacatase treated milks- use fermented milk products such as yogurt,

cheese where most of lactose is already broken down

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Fiber and Health

Insoluble fiber prevents constipation and hemorrhoids

Soluble fiber Blood cholesterol (helps prevent

heart disease)Helps control blood glucose levels

Excess fiber (>50 g/d) mineral absorptionGI discomfort

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• Current recommendations suggest that adults consume 21-38 grams of dietary fiber per day, depending on age and gender. Children ages 1 and up should consume at least 19 grams of fiber per day. Yet the average American eats only 15 grams of dietary fiber a day. (See the table below for specific fiber recommendations based on age and gender.)

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Fiber and heart disease • In the United States, coronary heart disease is a

leading cause of death for both men and women. This disease is characterized by a buildup of cholesterol-filled plaque in the coronary arteries - the arteries that feed the heart. This causes them to become hard and narrow, a process referred to as atherosclerosis. Total blockage of a coronary artery produces a heart attack.

• High intake of dietary fiber has been linked to a lower risk of heart disease in a number of large studies that followed people for many years. In a Harvard study of over 40,000 male health professionals, researchers found that a high total dietary fiber intake was linked to a 40 percent lower risk of coronary heart disease, compared to a low fiber intake. Cereal fiber, the fiber found in grains, seemed particularly beneficial. A related Harvard study of female nurses produced quite similar findings.

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Fiber and colon cancer • For years, Americans have been told to consume

a high-fiber diet to lower the risk of colon cancer - mainly on the basis of results from relatively small studies. Larger and better-designed studies have failed to show a link between fiber and colon cancer. One of these - a Harvard study that followed over 80,000 female nurses for 16 years - found that dietary fiber was not strongly associated with a reduced risk for either colon cancer or polyps (a precursor to colon cancer).

• But just because fiber plays little role in preventing colon cancer doesn't mean you should abandon a high-fiber diet. As explained below, fiber provides many other benefits.

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Diabetes• Type 1: Insulin-dependent diabetes mellitus

(IDDM); – lack of insulin; destruction of pancreas

• Type 2: Non insulin-dependent diabetes mellitus (NIDDM); – insufficient production of insulin; insulin

resistant; – Western diets and physical inactivity are

conducive

• Gestational diabetes: 3-6% pregnant women; disappears after delivery

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Carb Sources• Starch from grain products & veggies• Sugars from fruits & refined sweets• Milk sugars from dairy • NOT from eggs, meats or oils• 55-60% of calories should come from carbs

Artificial Sweeteners

• Saccahrin• Aspartame• Sucralose• Acesulfame potassium

Contain zero calorie

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Alcohol

• NOT a “nutrient”

• Has 7 calories per gram

• Absorption is rapid from stomach

• Metabolism (detoxification) by enzymes in liver takes time

Alcohol Facts Benefits (moderate drink,1-2 drinks) • Increases HDL-cholesterol• Protects against heart diseaseAdverse effects (excessive drink, > 5 drinks)• Hypertension, stroke, liver cirrhosis, cancers, nerve

system disorders• Associated with: Death from homicide, drowning, traffic accidents, suicide

Legal limit: 0.08-0.1% blood level

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Typical “drink” has about same amount of alcohol

regardless of source

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RDA for Carbohydrates• The RDA for carbohydrates is set as the

minimum amount needed to fuel the brain but not an amount adequate to satisfy typical energy needs

• Most experts recommend that 45% to 65% of total calories come from carbohydrates

• Added sugars limited to 6-10% of calories• 25 - 38 g of fiber are recommended daily

for adult women and men respectively

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• Blood glucose is the energy source to all cells

• Ideally, levels between 70-120 mg/dL (3,3 – 5,5 mmol/l) are needed for homeostasis

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• Glycogen: Carbohydrate energy stored in –1. The liver–2. The muscles

•Glycogenesis: The process of converting glucose to glycogen

•Glycogenolysis: Glycogen back to glucose

•Gluconeogensis: Glucose from fats & protein

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Glucose-6-phosphate is the central metabolite in the synthesis and decomposition of glycogen.

In the well-fed state glucose is converted to glucose-6-phosphate, which is the precursor for the glycogen synthesis.

The glucose-6-phosphate derived from the breakdown of glycogen has three fates: (1) glycolysis; (2) pentose-phosphate pathway; (3) convertion to free glucose for transport to another organs.

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ConversionsGLUCOSE GLYCOGEN

FAT

GLUCOSE GLYCOGEN

FAT

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Conversions

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Conversions