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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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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
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
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
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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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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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