Digestive Enzymes 34.3 &

Nutrition 34.4

Digestive Enzymes

• Break down the major components of food: carbohydrates, proteins, nucleic acids and fats.

• Found in the saliva secreted by the salivary glands, in the stomach secreted by cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the intestinal (small and large) secretions, or as part of the lining of the gastrointestinal tract.

Digestion of Starch

• Begins in the mouth with saliva which has a neutral pH

• Salivary Amylase: the first enzyme that acts on starch. Breaking it down to maltose.

Salivary Amylase

Starch + H20 Maltose

Digestion of Starch

• Starch is also broken down in the small intestine

• Pancreatic juice, which has a basic pH because it contains sodium bicarbonate (NaHCO3), enters the duodenum.

• Pancreatic Amylase digests starch Pancreatic Amylase

Starch + H20 Maltose

Completion of Starch Digestion

• Within the small intestine starch is finally broken down to glucose. – Glucose - small molecule able that cross into

the cells of the villi and enter the blood.

• Maltose, a disaccharide that results from digestion of starch, is digested to glucose by maltase.

Maltase

Maltose + H20 Glucose + Glucose

Digestion of Proteins

• Begins in stomach

• Gastric juice, low pH (about 2) because it contains hydrochloric acid (HCL).

• Pepsinogen is converted to pepsin when exposed to HCL.

• Pepsin breaks down proteins Pepsin

Protein + H20 Peptides

Digestion of Protein

• Pancreatic juice containing trypsinogen is converted to Trypsin in the duodenum which also breaks down proteins.

Trypsin

Protein + H20 Peptides

Digestion of Peptides

• Peptides are too large to be absorbed by the intestinal lining.

• They are further broken down in the small intestine by peptidases, to amino acids– which are small enough to cross into the villi

and enter blood.

Peptidases

Peptides + H20 Amino Acids

Digestion of Fat

• Lipase, a third pancreatic enzyme, digests fat molecules after they have been emulsified by bile salts.

bile salts

Fat Fat Droplets

Lipase

Fat Droplets + H20Glycerol + 3 Fatty Acids

Digestion of Fat Cont’d

• End products of lipase are monoglycerides and fatty acids.

• These enter the cells of villi, which are rejoined and packaged as lipoprotein droplets, called chylomicrons. Which enter the lacteals.

Nutrition

• Carbohydrates: are present in food in the form of sugars, starch and fiber

• Fruits, vegetables, milk and honey are natural sources of sugar.

• All forms of sugars are converted to glucose which for transport in the blood and use by cells

• Glucose it the preferred direct energy souce

Carbohydrates Cont’d

• Good sources of starch include beans, peas, cereal grains and potatoes.

• Starch is digested to glucose in the digestive tract and excess glucose is stored as glycogen.

Fiber

• Fiber includes various indigestible carbohydrates derived from plants.

• Beans, peas, nuts, whole grains fruits and vegetables are rich in fibers.– Refined grains have little fiber

• Fiber is not necessarily a nutrient for humans but it adds to fecal matter which stimulates movement within the intestines.

• High fiber diets help protect against heart diease• The average American only gets 15g of fiber a

day. The recommended value is 25g for women and 38g for males

Misconceptions of Carbohydrates

• “Carbs” are often seen as something to be avoided.– Atkins or South Beach diet cut out carbs

• Carbohydrates are necessary for energy needs

• Americans need to focus on eating the right kinds of carbohydrates. And not eat refined and processed carbs.

Lipids

• Triglycerides (fats and oils) supply energy for cells. But fat is stored for long term in the body.

• Unsaturated fatty acid (plant oils and fish) are essential within some ones diet.

• Saturated fatty acids (butter, milk, meat and cheeses) are not recommended .

• Trans-fatty acids (when unsaturated oils are hydrogenated to produce a solid fat) are more harmful than saturated because they reduce the function of the plasma membrane receptors that clear cholesterol from the blood stream.

Lipids Cont’d

• Cholesterol, a lipid, is synthesized by the body and used to make various compounds such as bile, steroid hormones and vitamin D.

• Only found in animal foods. Cheese, egg yolk, liver and shellfish are rich with cholesterol.

• Elevated cholesterol levels increases the risk of cardiovascular disease, number one killer of Americans.

Proteins

• Proteins provide essential amino acids.– 9 of the 20 are essential

• Eggs, meat, poultry and milk contain the 9 essential amino acids

• A balanced diet with grains, beans and nuts can supply the essential amino acids without meat products.

Proteins

• American generally eats twice the amount of recommended protein

• Often high-protein foods contain high amounts of fat.

• So high protein diets are not necessarily recommended

Diet and Obesity

• Over-eating is the general issue regarding obesity (obesity is defined as weighing 30% more than the ideal weight for your height)

• Also refined and processed foods also contribute to this fat epidemic.

Type 2 Diabetes

• Type 2 diabetes is generally linked to obesity.

• It is when one display an impaired insulin production or insulin resistence.

• In a person with insulin resistance, they fail to take up glucose, and therefore the blood glucose level exceeds the normal level.

• Weight loss improves type 2 diabetes.

Cardiovascular Disease

• Leading cause of death in America• Is often due to the arteries being blocked by

plaque which contains saturated fats and cholesterol.

• Low-density lipoproteins are considered “bad” because they carry cholesterol from the liver to the arterial walls.

• High-density lipoproteins are considered “good” because they dispose of cholesterol.

Cardiovascular Diseases Cont’d

• Consuming saturated fats and trans fats (beef, dairy, stick margarine) raises LDL cholesterol levels

• While consuming unsaturated fats (olive oil, canola oil, nuts, cold-water fish) lowers LDL cholesterol levels.

• The American Heart Associations recommends limiting total cholesterol intake to 300 mg per day.

Vitamins and Minerals

• Vitamins are organic compounds that regulate various metabolic activities and must be present within one’s diet.

• Vitamins are especially abundant in fruits and vegetables. Suggested to eat about 4 ½ cups per day.

• Mineral are used for various physiological functions, including regulation and biochemical reactions, maintenance of fluid balance, and incorporation into certain structures and compounds.

Balanced Diet