Carbohydrates

Structures and Function

• Composed of carbon, hydrogen, and oxygen
• Simple sugars: monosaccharides, disaccharides, oligosaccharides)
• Complex sugar: polysaccharides (starch and fiber)
• Simple carbohydrates
• Monosaccharides (glucose, fructose, and galactose – isomers of each other)
  • Glucose (also called dextrose and blood sugar) has a six carbon (hexose) ring structure
  • Fructose (also called levulose) has a six carbon ring structure
    • Found in fruit, honey, and corn syrup used in soft drink and food production
    • 8 to 10% of our energy intake
    • Metabolized into glucose in the liver
    • Converted into glycogen, lactic acid, or fat if consumption is high
  • Galactose has a six carbon ring structure
    • Not usually found in nature but exists mostly as a unit of the disaccharide lactose which is found in nature
    • Converted to glucose in the liver or stored as glycogen
  • Ribose has a five carbon ring structure and used in genetic material (why is ribose not a dietary consideration?)
    
• Disaccharides
  • Maltose (glucose + glucose) - commonly used in the production of alcohols
  • Sucrose (glucose + fructose) - table sugar and plants are the major source
  • Lactose (glucose + galactose) - primary sugar found in milk and milk products
• Oligosaccharides
  • Raffinose (trisaccharide - made up of glucose, fructose, and galactose)
    
  • Stachyose (tetrasaccharide - made up of a glucose, fructose, and two galactose)
    
  • Found in beans, cabbage, brussel sprouts, broccoli, etc...
    
  • Digestive enzymes cant break them apart
  • Bacteria in the large intestines break apart these oligosaccharides, producing gas and other byproducts
• Complex Cabohydrates (Digestible starch and glycogen and indigestible fiber)
  • Starch
    • Long chains of glucose
    • Amylose is a straight chain polymer
    • Amylopectin is a branched chain polymer
    • Food sources include potatoes, breads, pasta, and rice
    • Amylopectin raises blood sugar levels quicker because of the branched configuration which enables more digestive capabilities
  • Glycogen
    • Storage form of glucose in the human body
    • Long branched chains of glucose
    • Highly digestible because of the branched structure
  • Fiber
    • Dietary fibers chemically composed of non-starch polysaccharides:
      • Cellulose and hemicellulose – found in wheat, rye, rice, vegetables
      • Pectins, gums, and mucilage – citrus fruits, oat products, beans
    • Dietary fibers also composed of the non-carbohydrate called lignin
    • All dietary fibers come from plants and are not digested in the stomach
    • But fibers can be soluble and insoluble in water
      • Those that are soluble include pectins, gums, and mucilages and are metabolized by bacteria in the intestines

Carbohydrate Digestion and Absorption

• Begins in the mouth (salivary amylase) during mastication
• Some starches are broken down to maltose
• Acid environment of stomach inactivates salivary enzymes
• Pancreatic amylases release from pancreas into small intestine further breaks down starches into mono and disaccharides
• Brush border cells (small intestine cells) release various other enzymes (e.g., maltase, sucrase, and lactase)
• Glucose and galactose are actively absorbed in the small intestine
• Fructose is absorbed through passive facilitated diffusion

Functions of Glucose and Other Sugars

• Yields energy (4 kcal/g)
• Sparing protein from use as an energy source (sparing the body from having to undergo gluconeogenesis)
• Preventing Ketosis
• Adequate intake of carbohydrates is necessary for the complete metabolism of fats to carbon dioxide and water
  
• Low carbohydrate intake leads to incomplete breakdown of fatty acids and formation of ketone bodies
• Imparting flavor and sweetness to foods – order of sweetness (high to low): fructose, sucrose, glucose, maltose, lactose

Functions of Dietary Fiber

• Dietary fiber
  • Adds mass and water
  • Feces then are larger and softer
  • Less pressure needed to expel stool
• RDA for fiber: Men 14-50 (38 grams) and Women 14-50 (26 grams)
  
• Little or no fiber in diet can cause constipation and thus more pressure in intestine is needed
• May cause diverticula and hemorrhoids
• Diverticulosis versus diverticulitis
• Additional health benefits:
  • Aids in weight control
  • Link between fiber consumption and decrease in colon cancer
  • Higher intakes of fiber correlated to inhibition of cholesterol and bile acid absorption

Recommended Carbohydrate Intakes

• NET CARBS? (define)
• 130 grams of carbohydrates daily
• Problems with high intakes:
  • May not contribute the right proportion of energy to diet
  • High fiber diet can produce phytobezors
  • May lower nutritional value of a diet by contributing to the lack of consuming other energy-yielding nutrients
  • May cause dental caries
  • Diets may contain too many carbohydrates with high glycemic indexes
    • Glycemic index - the blood glucose response of a given food, compared to a standard (glucose or white bread); Influenced by starch structure, fiber content, food processing, physical structure, and macronutrients in the meal, such as fat
  • Lactose intolerance may occur

Food Sweeteners (Nutritive and alternative sweeteners)

• Nutritive Sweeteners
  • Sugars (mono and disaccharides)
    • 50+ pounds are consumed per year per person
    • Fructose in the form of fructose corn syrup is predominately used
    • Others: brown sugar, turbinado sugar, honey, and maple syrup
  • Stevia (Truvia)?
    
  • Sugar alcohols (sorbitol, mannitol, xylitol)
    • Contribute energy (1.5 to 3 kcal/gram)
    • Broken down more slowly than simple sugars
    • Sorbitol and xylitol are used in sugarless gum, breath mints, and candy – these sugar alcohols aren’t readily metabolized by bacteria so prevent the development of dental caries
• Non-Nutritive or Alternative Sweeteners (saccharin, aspartame, acesulfame potassium, sucralose)
  • Saccharine
    • Discovered in 1879
    • 300 times sweeter than sugar
    • May cause bladder cancer (?)
  • Aspartame
    • Discovered in 1969, and in 1981 was approved by the FDA for use in foods
    • Produced from two amino acids—aspartic acid and phenylalanine
    • Trade name “Nutrasweet” and “Equal”
    • Health controversy
    • Yields 4 kcal/gram and is 180-200 times sweeter than sucrose
    • Because it is so sweet so little is used in foods thereby lowering energy content
  • Sucralose (Splenda)
    • Discovered in 1976
      
    • 600 times sweeter than sugar
    • Approved in 1998 for use in baked goods, nonalcoholic beverages, chewing gum, frozen dairy desserts, fruit juices, and gelatins
    • Today approved as general purpose sweetener for foods
  • Acesulfame Potassium (Sunett)
    • 200 times sweeter than sugar and calorie free
    • Discovered in 1967 in Germany - approved in 1988 by the FDA as a tabletop sweetener
  • Cyclamate (Sucaryl)
    • Cyclamate is 30 times sweeter than sucrose
    • Discovered in 1937
      
    • Had been used in foods since the 1950s
    • Used in Sugar Twin
    • It was removed from food products in the USA and Canada by the 1970s because several animal studies suggested it posed an increased cancer risk
    • It is completely banned in the USA, BUT it is available today in Canada as a tabletop sweetener.
    • The U.S. scientific community is reviewing more current data that may support cyclamate approval again.
    • The maximum daily limit suggested is 1.5 grams per day.

Are carbohydrates addictive?

Scientific studies have revealed that people who have higher BMI's have decreased "pleasure" receptors in the brain and require more stimuli such as sugar (and alcohol, certain ilicit drugs, etc...) to cause dopamine/seratonin release and produce pleasure. Furtermore, most carbohydrates have only been in the diet in the last couple hundred years and absent in the diet more than 10,000 years ago. Therefore, increased exposure to carbohydrates have shifted our production and consumption of foods as well as increased incidence of certain disorders such as obesity.

Blood Glucose levels

• Blood glucose is regulated by two principle hormones: insulin and glucagon
  
• Low blood glucose initiates the pancreas to produce and secrete insulin which causes glycogen (long chains of glucose) to release glucose
• High blod glucose initiates the pancreas to produce and secrete glucagon which causes muscle and liver to store glucose in the form of glycogen
  
• Hyperglycemia (high blood glucose) versus hypoglycemia (low blood glucose)
  
• Diabetes Mellitus: (Type 1, insulin dependent) versus (Type 2, non-insulin dependent)