BLG 111 - Nutrition, Metabolism, and Energy Balance

nutrients

substance in food needed for growth, maintenance, repair

macronutrients

• three major nutrients that make up bulk of ingested food; carbohydrates, lipids, and proteins

• two other nutrients required but in small amounts; vitamins and minerals

• water

essential nutrients

nutrients that must be eaten because body cannot make them from other nutrients; 40-50 nutrients are considered this

nonessential nutrients

are also vital to life, but if not enough is available, liver can usually convert another nutrient into one needed

carbohydrates dietary sources

• primarily from plants, such as starch (complex carbohydrates) in grains and vegetables

• sugars (mono- and disaccharides) in fruits, sugarcane, sugar beets, honey, and milk

• insoluble fiber: cellulose in vegetables

• soluble fiber: pectin in apples and citrus fruits

• small amount in milk sugar, glycogen in meats

how does the body use carbohydrates?

• glucose fuels is most used by cells to make ATP

• some cells use fat for energy

• neurons and RBCs rely entirely on glucose; neurons die quickly without glucose

• excess glucose is converted to glycogen or fat, then stored

• fructose and galactose are converted to glucose by liver before entering circulation

what are dietary requirements for carbohydrates?

• recommended daily intake is 45-65% of total calories

• should consists mostly of complex carbs (whole grains and vegetables); simple carbs should be limited

lipids dietary sources

• triglycerides (neutral fats): most abundant form

→ found in saturated fats in meat, dairy foods, tropical oils, or hydrogenated oils

→ unsaturated fats found in seeds, nuts, olive oil, and most vegetable oils

• cholesterol found in egg yolk, meats, organ meats, shellfish, milk products; liver makes ~85% cholesterol

how does the body use lipids?

• adipose tissue offers protection, insulation, fuel storage

• phospholipids essential in myelin sheaths and all cell membranes

• cholesterol stabilizes membranes

• prostaglandins → smooth muscle contraction, BP control, inflammation

• major fuel of hepatocytes and skeletal muscle

• help absorb fat-soluble vitamins

what are dietary requirements for lipids?

• recommended daily intake is 20-35% of daily calories

• saturated fats should be limited to 10% or less

• cholesterol is not required in diet and should be kept low

proteins dietary sources

• animal products (eggs, milk, fish, most meats) are considered complete proteins → contain all essential amino acids that must be obtained from diet

• legumes, nuts, and cereals contain incomplete proteins → lack some essential amino acids

• legumes and cereal grains together contain all essential amino acids

how does the body use proteins?

• structural materials (keratin-skin, elastin-connective tissue)

• functional molecules (enzymes and some hormones)

three factors that help determine whether amino acids are used to synthesize proteins or burned as fuel

1. all-or-none rule: all amino acids must be present for protein synthesis; if not then they are burned for energy

2. adequacy of caloric intake: protein is used as fuel if not enough carbs or lipids are available

3. hormonal controls: anabolic hormones accelerate protein synthesis, stress hormones promote protein breakdown

nitrogen balance and proteins

• state where rate of protein synthesis equals rate of breakdown and loss

• positive nitrogen balance: synthesis exceeds breakdown

• negative nitrogen balance: breakdown exceeds synthesis

metabolism

sum of all biochemical reactions inside a cell involving nutrients

anabolism

synthesis of large molecules from small ones (example: synthesis of proteins from amino acids)

catabolism

hydrolysis of complex structures to simpler ones (example: breakdown of proteins into amino acids)

cellular respiration

catabolic breakdown of food fuels whereby energy from food is captured to form ATP in cells

phosphorylation

enzymes shift high-energy phosphate groups of ATP to other molecules; these molecules become activated to perform cellular function

what happens in stage 1 of processing nutrients?

digestion, absorption, and transport to tissues

what happens in stage 2 of processing nutrients?

cellular processing in cytoplasm

• synthesis of lipids, proteins, and glycogen, or

• catabolism (glycolysis) into pyruvic acid and acetyl CoA

what happens in stage 3 of processing nutrients?

oxidative breakdown of intermediates into CO2, water, and ATP

• occurs in mitochontria

oxidation reactions

involve the gain of oxygen or loss of hydrogen atoms (and their electrons)

reduction reactions

reactions where a substance gains electrons and energy

redox reactions

paired reactions where one substance loses electrons (oxidation) and another gains electrons (reduction)

substrate-level phosphorylation

ATP is made directly when a phosphate group is transferred from s substrate to ADP

oxidative phosphorylation

a complex process that produces most ATP using the electron transport chain and proton gradient in mitochondria (chemiosmosis)

chemiosmosis

the movement of H+ ions across a membrane, which provides energy to make ATP

ATP synthase

a membrane protein channel that uses the flow of H+ ions to convert ADP into ATP

glycolysis

a 10-step pathway in the cytosol where glucose is broken down into two pyruvic acid molecules; three major phases

sugar activation phase of glycolysis (phase 1)

glucose is phosphorylated using 2 ATP to form fructose-1, 6-biphosphate

sugar cleavage phase of glycolysis (phase 2)

fructose-1, 6-biphosphate splits into two 3-carbon molecules

sugar oxidation phase of glycolysis (phase 3)

the fragments are oxidized, producing 2 pyruvic acid, NADH, and 4 ATP

citric acid cycle (krebs cycle)

a metabolic cycle in the mitochondrial matrix that breaks down acetyl CoA to CO2 and produces NADH and FADH2

decarboxylation (step 1 of citric acid cycle)

1 carbon from pyruvic acid is removed, producing CO2 gas, which diffuses into blood to be expelled by lungs

oxidation (step 2 of citric acid cycle)

remaining 2-C fragment is oxidized to acetic acid by removal of H atoms, which are picked up by NAD+

formation of acetyl CoA (step 3 of citric acid cycle)

acetic acid combines with coenzyme A to form acetyl CoA

two phases of oxidative phosphorylation

1. electron transport chain

2. chemiosmosis (uses energy of proton gradient to synthesize ATP

electrons transport chain

• a chain of carrier proteins in the inner mitochondrial membrane that transfer electrons to create a proton gradient

• only pathway that uses oxygen

flavins

proteins derived from riboflavin

cytochromes

proteins with iron-containing pigment

glycogenesis

glycogen can be formed with excess glucose

• catalyzed by glycogen synthase

• glucose is converted to glucose-6-phosphate, then converted to isomer glucose-1-phosphate

mostly occurs in liver and skeletal muscle cells

glycogenolysis

breakdown of glycogen via glycogen phosphorylase in response to low blood glucose

• enzyme splits and phosphorylates terminal glucose on glycogen

• forms glucose-1-phosphate, which is converted to glucose-6-phosphate, which then can enter glycolysis in that cell

• liver cells, as well as some kidney and intestinal cells, also contain enzyme glucose-6-phosphates that removes terminal phosphate, producing free glycogen; glucose can enter bloodstream to be used by other cells

gluconeogenesis

• process of forming new glucose from noncarbohydrate sources

• occurs in the liver

• glucose can be formed from glycerol and amino acids when blood glucose levels drops

• protects against damaging effects of low blood glucose levels

→ especially important for nervous system

summary of carbohydrate reactions

• glycolysis: converts glucose to pyruvic acid

• glycogenesis: polymerizes glucose to form glycogen

• glycogenolysis: hydrolyzes glycogen to glucose membrane

• glyconeogenesis: forms glucose from noncarbohydrate precursors

beta oxidation

occurs during fatty acid breakdown

• fatty acid chains are broken into two carbon acetic acid fragments and coenzymes are reduced in process

• acetic acid fragment fuses with CoA to form acetyl CoA, which enter acid cycle

• referred to as “beta” oxidation because two carbons are broken off fatty acid chain, allowing third-position carbon to be oxidized

lipogenesis

triglyceride synthesis that occurs when cellular ATP and glucose levels are high

• dietary glycerol and fatty acids not needed for energy are stored as triglycerides

• glucose is easily converted to fat because acetyl CoA is an intermediate in glucose catabolism and the starting point for fatty acid synthesis

lipolysis

breakdown of stored fats into glycerol and fatty acids; reverse of lipogenesis

• fatty acids are actually preferred by liver, cardiac muscle, resting skeletal muscle for fuel

• lipolysis is accelerated when carbohydrate intake is inadequate

• accumulated acetyl CoA can be converted by ketogenesis in liver to ketone bodies (ketones)

summary of lipid reactions

• beta oxidation: converts fatty acids to acetyl CoA

• lipolysis: breaks down lipids to fatty acids and glycerol

• lipogenesis: forms lipids from acetyl CoA and glyceraldehyde 3-phosphate

nutrient pools

the total supply of nutrients in the body that are ready to be used or converted into other molecules.

amino acid pool

body’s total supply of free amino acids

• proteins are lost in urine, hair, and skin cells

• replaced typically by diet

• pool is the source for:

→ resynthesizing body proteins

→ forming amino acid derivatives

→ gluconeogenesis

carbohydrate and fat pools

are easily interconverted through key intermediates

• fats and carbohydrates are oxidized directly to produce energy while amino acids must first be converted to citric acid keto acid

• excess carbohydrates and fat can be stored as such while amino acids cannot be stored as proteins, they are converted

absorptive state

• lasts ~ 4 hours after eating, when absorption of nutrients is occurring

• anabolism exceeds catabolism

• excess nutrients are stored as fats if not used

absorptive state of carbohydrates

• glucose is major cellular energy fuel

• can be converted in liver to glycogen or fat

• glycogen remains in liver, but fat joins protein to form very low density lipoproteins released to blood for storage by adipose tissue

• blood borne glucose enters cells; excess stored as glycogen in muscles or fat in adipose cells

absorptive state of triglycerides

• chylomicrons from lymph are hydrolyzed to fatty acids and glycerol before passing through capillary wall

• lipoprotein lipase is enzyme that catalyzes hydrolysis of lipids

• triglycerides are used by adipose tissue, liver, and skeletal and cardiac muscle as primary energy source

• most glycerol and fatty acids are converted back to triglycerides for storage

absorptive state of amino acids

• some absorbed amino acids are deaminated in liver to keto acids that can be used in citric acid cycle

• excess amino acids can be stored as fat in liver

• most amino acids are taken up by cells and used for protein synthesis

hormonal control of the absorptive state

• absorptive state is controlled primarily by insulin

• insulin secretion by beta cells of pancreas is stimulated by:

→ elevated blood levels of glucose and amino acids

→ intestinal GIP and parasympathetic simulation

• when insulin binds to membrane receptors, it facilitates diffusion of glucose into muscle and adipose cells (brain and liver take up glucose without insulin)

what type of hormone is insulin?

a hypoglycemic hormone that enhances:

• glucose oxidation for energy

• glycogen and triglyceride formation

• active transport of amino acids into tissue cells

• protein synthesis

it also inhibits glucose release from liver and gluconeogenesis

sources of blood glucose

1. glycogenolysis in liver: first reserve used

2. glycogenolysis in skeletal muscle

3. lipolysis in adipose tissues and liver

4. catabolism of cellular respiration: major source during prolonged fasting

glucose sparing

during prolonged periods of fasting, body used more noncarbohydrate sources to conserve glucose

→ more fat and protein are converted to intermediate that can enter citric acid cycle

• brain uses bulk of glucose while other body cells switch to fatty acids as fuel source

hormonal and neural controls of the postabsorptive state

sympathetic nervous system interacts with several hormones to control events of post absorptive states

→ more complex than absorptive state, which utilizes hormone, insulin

post absorptive state is triggered by reduced insulin release as blood glucose levels drop

glucagon: hyperglycaemic hormone

release is stimulated by:

• declining blood glucose levels

• rising amino acid levels

• glucagon promotes:

→ glycogenolysis and gluconeogenesis in the liver

→ lipolysis i adipose tissue, causing fatty acids and glycerol to be released

sympathetic nervous system and post absorptive state

adipose tissue is innervated by sympathetic nervous system

• it can quickly supply glucose if blood levels are low

low plasma glucose, fight or flight response, or exercise can trigger fat mobilization and glycogenolysis

• initiated by sympathetic nervous system and epinephrine from adrenal medulla

other hormones in metabolism and nutrient flow

growth hormone, thyroxine, sex hormones, and corticosteroids can have effect on metabolism

→ most have indirect effect