Chapter 14 Digestive – Metabolism & Development

Nutrients – used for growth, maintenance & repair Carbohydrates - Most from plants; Exceptions: lactose from milk and some glycogens from meats Lipids - Saturated fats from animals, Unsaturated fats from plants, Cholesterol from egg yolk, meats, and milk products Proteins - Complete proteins –all essential amino acids, Most from animals, Legumes and beans also have proteins, but are incomplete Vitamins - used as cofactors and act with enzymes, in all major food groups Minerals - many roles in the body, mineral-rich foods are vegetables, legumes, milk, and some meats Water

Metabolism Chemical reactions necessary to maintain life  Catabolism – substances are broken down to simpler substances  Anabolism – larger molecules are built from smaller ones  Energy is released during catabolism

Carbohydrate Metabolism  The body’s preferred source to produce cellular energy (ATP)  Glucose (blood sugar) is the major breakdown product and fuel to make ATP

Carbohydrates Dietary sources Starch (complex carbohydrates) in grains and vegetables Sugars in fruits, sugarcane, sugar beets, honey and milk Insoluble fiber: cellulose in vegetables; provides roughage Soluble fiber: pectin in apples and citrus fruits; reduces blood cholesterol levels

Carbohydrates Uses Glucose is the fuel used by cells to make ATP Neurons and RBCs rely almost entirely upon glucose Excess glucose is converted to glycogen or fat and stored

Fat Metabolism  Handled mostly by the liver -Use some fats to make ATP -Synthesize lipoproteins, thromboplastin, and cholesterol -Release breakdown products to the blood  Body cells remove fat and cholesterol to build membranes and steroid hormones

Lipids Dietary sources Triglycerides Saturated fats in meat, dairy foods, and tropical oils Unsaturated fats in seeds, nuts, olive oil, and most vegetable oils Cholesterol in egg yolk, meats, organ meats, shellfish, and milk products

Lipids Essential uses of lipids in the body Help absorb fat-soluble vitamins Major fuel of hepatocytes and skeletal muscle Phospholipids are essential in myelin sheaths and all cell membranes

Lipids Functions of fatty deposits (adipose tissue) Protective cushions around body organs Insulating layer beneath the skin Concentrated source of energy

Protein Metabolism  Proteins are conserved by body cells because they are used for most cellular structures  Ingested proteins are broken down to amino acids  Cells remove amino acids to build proteins  Synthesized proteins are actively transported across cell membranes  Amino acids are used to make ATP only when proteins are overabundant or there is a shortage of other sources

Proteins Dietary sources Eggs, milk, fish, and most meats contain complete proteins Legumes, nuts, and cereals contain incomplete proteins (lack some essential amino acids) Legumes and cereals together contain all essential amino acids

Proteins Uses Structural materials: keratin, collagen, elastin, muscle proteins Most functional molecules: enzymes, some hormones

Role of the Liver in Metabolism  Several roles in digestion  Detoxifies drugs and alcohol  Degrades hormones  Produce cholesterol, blood proteins (albumin and clotting proteins)  Plays a central role in metabolism  Converts ammonia to urea which is eliminated in urine

Metabolic Functions of the Liver

Glycogenesis and Glycogenolysis Glycogenesis Glycogen formation when glucose supplies exceed need for ATP synthesis Mostly in liver and skeletal muscle Glycogenolysis Glycogen beakdown in response to low blood glucose

Gluconeogenesis Glucose formation from noncarbohydrate (glycerol and amino acid) molecules Mainly in the liver Protects against damaging effects of hypoglycemia

Metabolic Functions of the Liver  Fats and fatty acids are picked up by the liver -Some are oxidized to provide energy for liver cells - The rest are broken down into simpler compounds and released into the blood

Metabolic Role of the Liver Hepatocytes Process nearly every class of nutrient Play a major role in regulating plasma cholesterol levels Store vitamins and minerals Metabolize alcohol, drugs, hormones, and bilirubin

Cholesterol Metabolism  Functions of cholesterol -Structural basis of steroid hormones & vit D -Major building block of plasma membranes  Most cholesterol is produced in the liver and is not from diet

Cholesterol Transport  Cholesterol and fatty acids cannot freely circulate in the bloodstream  They are transported by lipoproteins (lipid-protein complexes) -Low-density lipoproteins (LDLs) transport to body cells -High-density lilpoproteins (HDLs) transport from body cells to the liver

Lipoproteins High levels of HDL are thought to protect against heart attack High levels of LDL, especially lipoprotein (a) increase the risk of heart attack

Body Energy Balance Energy intake = total energy output = (heat + work + energy storage)  Energy intake is liberated during food oxidation  Energy output -Heat is usually about 60% -Energy to do work -Storage energy is fat or glycogen

Regulation of Food Intake  Body weight is usually relatively stable -Energy intake and output remain about equal  Mechanisms that may regulate food intake -Levels of blood nutrients -Hormones -Body temperature -Psychological factors

Metabolic Rate & Body Heat Production Basic metabolic rate (BMR) – amount of heat produced by the body per unit of time at rest Influencing factors:  Surface area – small body higher BMR  Gender – males higher BMR  Age – children & adolescents higher BMR  Thyroxine - most important control factor

Total Metabolic Rate (TMR)  Total amount of kilocalories the body must consume to fuel ongoing activities  TMR increases with an increase in body activity  TMR must equal calories consumed to maintain homeostasis and maintain a constant weight

Body Temperature Regulation  Most energy is released as foods are oxidized - Protein – 4 Kcal/g - Carbs – 4 Kcal/g - Fats – 9 Kcal/g  Most energy leaves as heat  body temp 35.6° °C (96° to 100° F)  thermostat is hypothalamus - Initiates heat-loss or heat- promoting mechanisms

Heat Mechanisms Heat Promoting Mechanisms  Vasoconstriction of blood vessels  Blood rerouted to deeper, more vital body organs  Shivering – contraction of muscles produces heat Heat Loss Mechanisms  Heat loss from the skin via radiation and evaporation  Skin blood vessels and capillaries are flushed with warm blood  Evaporation of perspiration cools the skin

Figure Basal metabolism Muscular activity (shivering) Thyroxine and epinephrine (stimulating effects on metabolic rate) Temperature effect on cells Radiation Conduction/ convection Evaporation Heat productionHeat loss

Homeostatic Imbalance Hyperthermia Elevated body temperature depresses the hypothalamus Positive-feedback mechanism (heat stroke) begins at core temperature of 41  C Can be fatal if not corrected

Homeostatic Imbalance Heat exhaustion Heat-associated collapse after vigorous exercise Due to dehydration and low blood pressure Heat-loss mechanisms are still functional May progress to heat stroke

Homeostatic Imbalance Hypothermia Low body temperature where vital signs decrease Shivering stops at core temperature of  C Can progress to coma a death by cardiac arrest at ~ 21  C

Fever Controlled hyperthermia Due to infection (also cancer, allergies, or CNS injuries) Macrophages release interleukins (“pyrogens”) that cause the release of prostaglandins from the hypothalamus

Developmental Aspects – fetal/baby  Continuous tube by the 5 th week  Digestive glands bud from the mucosa of the alimentary tube  fetus receives nutrients through placenta  In newborns, feeding frequent, peristalsis is inefficient, and vomiting is common

Developmental Aspects  Teething ~ age six mos  Metabolism decreases with age  Middle age -Ulcers -Gall bladder problems Old age - Fewer digestive juices - Peristalsis slows - Diverticulosis & cancer more common