Hlth Basic Human Nutrition - Unit 3 (Ch. 4)

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Carbohydrates

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2 carbohydrates - glucose andits storage form glycogen - provide about half of all the energy muscles use.

The other half of the body’s energy comes mostly from fat. Glucose (also called blood sugar or dextrose) provides nearly all of the energy the human brain uses

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Monosaccharides:

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single sugars.

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Disaccharides:

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sugars composed of pairs of monosaccharides (1 half of a pair MUST be a glucose)

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Polysaccharides:

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large molecules composed of chains of monosaccharides

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The three monosaccharides most important in nutrition (they all have the same numbers and kinds of atoms)

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glucose (mildly sweet), fructose (intensely sweet), galactose (not too sweet) -

difference in taste = difference in structure

each contains 6 carbon atoms,12 hydrogens, and 6 oxygens (written in shorthand as C6H12O6

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Condensation

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To make a disaccharide, a hydroxyl (OH) groupfrom one monosaccharide and a hydrogen atom (H) from the other combine tocreate a molecule of water (H2O). The two monosaccharideslink together with a single oxygen (O).

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Hydrolysis

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T o break a disaccharide in two, a molecule of water (H2O) splits to provide the H andOH needed to complete the resulting monosaccharides.

Commonly occurs during digestion.

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Maltose

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disaccharide: two glucose units

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Sucrose

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disaccharide: glucose & fructose

= the sweetest disaccharide (because it has fructose)

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Lactose

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disaccharide: glucose & galactose

= the principal carbohydrate of milk, it contributes half of the energy provided by fat-free milk. .

= "milk sugar",

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Three types of polysaccharides most important in nutrition:

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glycogen, starches, and fibers.

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Glycogen

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Food = not a significant source of glycogen.

Glycogen (polysaccharide) stores glucose for future use. It is made of many glucose molecules linked together in highly branchedchains. To get energy, enzymes attack the many branches simultaneously making a surge of glucose available.

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Starches

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Plant cells store glucoseas starches, not glycogen. --> All starchy foods come from plants.

= Long, branched or unbranched chains of hundreds or thousands ofglucose molecules linked together (polysaccharide)

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Fibers

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Dietary fibers are the structural parts of plants and thus are found in allplant-derived foods (vegetables, fruits, whole grains, and legumes). Most dietaryfibers are polysaccharides. Fibers differ from starches in that the bonds between their monosaccharides cannot be broken down by digestive enzymes.

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soluble fibers

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dissolve in water

Soluble fibers (gums, pectins and some hemicelluloses) delay GI transit, providing bulk and satiety, delays glucose absorption, lowers blood cholesterol levels.

INsoluble fibers (Cellulose, many hemicelluloses and lignins) promote bowel movements, alleviate constipation,and prevent diverticular disease. They accelerates GI transit and delay glucose absorption.

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viscous fibers

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form gels

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fermentable fibers

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easily digested by bacteria in the colon

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1) dietary fibers

2) functional fibers

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1) occur naturally in plants. Known as resistant starches,these starches escape digestion and absorption in the small intestine.

2) fibers that havebeen extracted from plants or are manufactured and then added to foods or supplements

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Carbohydrate Digestion: In the Mouth

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The salivary enzyme amylase starts to work, hydrolyzing starch to shorter polysaccharides & maltose.

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Carbohydrate Digestion: In the Stomach

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The activity ofsalivary amylase diminishes as the stomach’s acid and protein-digesting enzymesinactivate the enzyme.

The stomach’s digestive juices contain no enzymes tobreak down carbohydrates.

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Carbohydrate Digestion: In the Small Intestine

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The enzyme, "pancreatic amylase",enters the intestine and continues breaking down the polysaccharides to shorter glucose chains and maltose.

Final step = onthe outer membranes of the intestinal cells: specific enzymes break downspecific disaccharides:

"Maltase" breaks maltose --> 2 glucose

"Sucrase" breaks sucrose --> 1 glucose, 1 fructose

"Lactase" breaks lactose --> 1 glucose, 1 galactose

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Carbohydrate Digestion: In the Large Intestine

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Within 1 to 4 hours after a meal, all the sugars and mostof the starches have been digested. Only fibers remain in the digestive tract.

They attract water, which softens the stools for passage.

Also, bacteria in the GI tract ferment some fibers. This processgenerates water, gas, and short-chain fatty acids (used for energy)

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How does

1) glucose, 2) galactose and 3) fructose enter the cells liningthe small intestine (get absorbed)?

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1) active transport

2) active transport

3) facilitated diffusion.

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lactase deficiency:

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A lack of the enzyme required to digestthe disaccharide lactose into its component monosaccharides(glucose and galactose)

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Lactose Intolerance

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Normally, the intestinal cells produce enough of the enzyme lactase to ensure that the disaccharide lactose gets digested efficiently. When more lactose is consumed than the available lactase can handle, lactose molecules remain in the intestine undigested, attracting water and causing bloating, abdominal discomfort, and diarrhea. Theundigested lactose becomes food for intestinal bacteria, --> producing gas and making it worse.

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Lactose Intolerance - Strategies

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To increase intake of milk products gradually, consume them with other foods in meals, and spread their intakethroughout the day. In addition, yogurt containing live bacteria seems to improvelactose intolerance.

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Why does glucose become available to supply energy to the brain and other tissues regardless of whether the person has eaten recently?

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After a meal, blood glucose rises, and liver cells linkexcess glucose molecules (by condensation reactions) into long, branching chains ofglycogen.

When blood glucose falls, the liver cells breakdown glycogen (by hydrolysis reactions) into single molecules of glucose and releasethem into the bloodstream.

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What does the body use for short term and long term energy?

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Glycogen holds water and, therefore, is rather bulky. The body can store onlyenough glycogen to provide energy for relatively short periods of time—less thana day during rest and a few hours at most during exercise.

For its long-term energyreserves, for use over days or weeks of food deprivation, the body uses fat.

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Making Glucose (from protein)

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Fat cannot make glucose to any significant extent. The amino acids of proteincan be used to make glucose to some extent. The conversion of protein to glucose is called gluconeogenesis.

Only adequate dietary carbohydrate can prevent this useof protein for energy, and this role of carbohydrate is known as its protein-sparingaction.

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Making Ketone Bodies from Fat Fragments

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With an inadequate supply of carbohydrate, fat takes an alternative route: instead of entering the main energy pathway, fatfragments combine with one another, forming ketone bodies.

Ketonebodies = alternate fuel source during starvation, but whentheir production exceeds their use, they accumulate in the blood, causing ketosis.

Ketosis disturbsthe body’s normal acid-base balance, because most ketone bodies are acidic.

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Using Glucose to Make Fat

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A wayto handle any extra glucose:

When glucose is abundant, energy metabolism shifts to use more glucose instead of fat.

If that isn’t enough, the liver breaks glucose into smaller molecules and puts them together into the more permanent energy-storagecompound- fat.

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Blood glucose homeostasis

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= regulated by2 hormones:

"insulin" (moves glucose from the blood into the cells), and "glucagon" (brings glucose out of storage)

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Insulin

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After a meal, as blood glucose rises, insulin is secreted into the blood (from pancreas). Receptors respond by ushering glucose from the blood into the cells. Elevated bloodglucose returns to normal levels as excess glucose is stored as glycogen and fat.

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Glucagon

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When blood glucose falls (as occurs between meals), glucagon is secreted into the blood (from pancreas). Glucagon raises bloodglucose by signaling the liver to break down its glycogen stores and release glucoseinto the blood for use by all the other body cells.

Another hormone that signals liver to release glucose = the “fight-or-flight” hormone, epinephrine.

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Diabetes

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blood glucose rises after a meal and remains above normal levels, because insulin is either inadequate or ineffective.

type 1 diabetes (less common): pancreas produces little or no insulin.

type 2 diabetes (more common): cells fail to respond to insulin (often precipitated by obesity).

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Hypoglycemia

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When blood glucose drop below normal.

Symptoms: weakness, rapid heartbeat, sweating, anxiety, hunger, and trembling.

Most commonly, it is result of of poorly managed diabetes.

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The Glycemic Response

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= how quickly glucose isabsorbed after a person eats, how high blood glucose rises, and how quickly itreturns to normal.

Slow absorption, a modest rise in blood glucose, and a smoothreturn to normal are desirable (a low glycemic response) (legumes, milk).

Fast absorption, a surgein blood glucose, and an overreaction that plunges glucose below normal are lessdesirable (a high glycemic response) (processed foods).

practical utility = limited

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Dental caries

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Bacteriain the mouth ferment SUGARS and, in the process, produce an acid that erodestooth enamel, causing dental caries, or tooth decay. But it depends on how long foods stayin the mouth and how often people eat sugar (limit total time of exposure, and drink/eat sugars quickly). Also on the bacteria in dental plaque, the saliva, the minerals that form theteeth, and the foods that remain after swallowing.

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Recommended Intakes of Sugars

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TheDRI suggests that added sugars should account for no more than 25 percent ofthe day’s total energy intake.

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artificial sweeteners:

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sugar substitutes that providenegligible, if any, energy; sometimes called nonnutritivesweeteners (= sweeteners that yield no energy(or insignificant energy in the case of aspartame)).

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sugar alcohols:

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sugarlike compounds that can be derivedfrom fruits or commercially produced from dextrose. They are absorbed more slowly thanother sugars and metabolized differently in the human body.

They are added for bulk, texture, a cooling effect, sweet taste, to inhibit browning from heat,and to retain moisture. They yield energy so = nutritive sweetener.

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Cellulose (type of fiber)

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= the most abundant form of fibre. It is present in plant cell walls in vegetables, fruit and legumes.

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Hemicellulose (type of fiber)

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present in cereal grains. Some hemicelluloses are soluble while others are insoluble.

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Pectin (type of fiber)

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has a gel-like consistency to it, is used in industry to thicken and provide texture to different food products. It is present naturally in vegetables and fruits (such as apples and lemons).

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Gums and Mucilages:

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= secreted from plants as a result of a cut. It is also used in industry as a stabilizer (maintain uniformity of the product).

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Lignin

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a non-polysaccharide (not composed of carbohydrate units). Its structure consists of polymers of phenol (organic alcohol). It has a ‘woody’ texture and it is present in the seeds of strawberries, asparagus, broccoli stems etc.

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Celiac Disease

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= condition in which the absorptive surface of the small intestine is damaged by a protein called "gluten". Results in an inability of the body to absorb nutrients.

There is no cure, but celiac disease is readily treated with a gluten-free diet.

Gluten = protein found in wheat, rye, triticale, barley. Used to prevent crumbling in bread etc.