Metabolism, Thermoregulation, and Exercise Physiology

When is body weight stable?

when there is equal energy intake and output around a homeostatic set point

What is body weight determined by?

• a combination of environmental and hereditary factors

  • 30-50% of variation between individuals is due to heredity

  • the rest is due to eating and exercise habits

What is a calorie?

the amount of heat required to raise temperature of 1g of water 1 degree Celsius

How many kcal/g do fats contain?

9 kcal/g

How many kcal/g do carbohydrates contain?

4kcal/g

How many kcal/g do proteins contain?

4kcal/g

What are considered “empty” calories?

• sugar and alcohol

  • few nutrients but can still produce energy

  • can lead to malnutrition over time

  • displaces foods that contain essential nutrients

Definition of nutrients

ingested chemical used for growth, repair, or maintenance

Macronutrients

• consumed in large amounts

  • ex: proteins, fats, and carbohydrates

Micronutrients

• needed in small amounts

  • ex: vitamins/minerals

Recommended daily allowances (RDA)

• safe estimate of daily intake for standard needs

• essential nutrients cannot be synthesized

  • minerals, vitamins, 9 amino acids, and 2- fatty acids must be consumed in the diet (based on a 2000 calorie diet)

Functions of Minerals: calcium and phosphorus

bones and teeth

Functions of Minerals: Phosphorus

phospholipids, ATP, CP, buffers, nucleic acids

Functions of Minerals: calcium, magnesium, copper, and manganese

cofactors for enzymes

Functions of Minerals: Iron

essential for hemoglobin and myoglobin

Functions of Minerals: Chlorine

component of stomach acid (HCl)

Functions of Minerals: Mineral Salts

• electrolytes

  • govern function of nerve and muscle cells

    • regulate distribution of body water

What are the dietary sources of minerals?

• found naturally in soil, water, and food

  • vegetables, legumes, milk, eggs, fish, and shellfish

• animal tissues contain large amounts of salt

  • carnivores rarely lack salt in their diets

What is the daily recommended sodium intake vs. typical American daily intake?

Recommended = 1.1g/day

American diet = 4.5g/day

Definition of vitamins

organic micronutrients that the body needs in small amounts to regulate metabolism

Which vitamins does the body synthesize from precursors?

• niacin, vitamin A, vitamin D

• vitamin K, pantothenic acid, biotin, folic acid

  • produced by intestinal bacteria

Which vitamins are water-soluble?

• Vitamins C and B

• absorbed with water in the small intestine

Which vitamins are fat-soluble?

• Vitamins A, D, E, and K

• absorbed with dietary lipids; stored

Which 3 places in the body are carbohydrates found?

1. Muscle

2. Liver glycogen

3. Blood glucose

What do most carbohydrates serve as?

• fuel

• neurons and RBCs depend on glucose

What do sugars serve as?

• structural components

  • examples:

    • nucleic acids

    • glycoproteins

    • glycolipids

    • ATP

What is blood glucose carefully regulated by?

insulin and glucagon

Which nutrient has the greatest RDA scoring?

• carbohydrates (175g/day)

• rapidly oxidized

What are dietary sources of carbohydrates?

1. Monosaccharides

   • glucose, galactose, and fructose

     • liver converts galactose and fructose to glucose

2. Disaccharides

   • table sugar (sucrose), maltose, and lactose

3. Polysaccharides

   • starch, glycogen, and cellulose

Outside of the hepatic portal system, what is the only form of glucose?

blood sugar

What is the normal range for blood sugar concentration?

70-110mg/DL

Where do nearly all dietary carbohydrates come from?

plants

What is dietary fiber?

• fibrous material that resists digestion

• important to the diet

• includes pectin and cellulose

What is the RDA for fiber?

30g/day

What does excessive fiber intake result in?

Interference with mineral absorption-such as iron

What is pectin?

• water-soluble fiber

  • dissolves in water to form a viscous, gel-like substance—slows down rate

  • decreases blood cholesterol and LDL levels by absorbing it in the intestines

What is cellulose?

• water-insoluble fiber

  • absorbs water in intestines

  • softens stool

  • gives stool bulk

  • speeds transit time of stool

What % of fat is the average adult male and female?

• Male: 15%

• Female: 25%

What is considered the body’s stored energy?

• lipids

• contains 2X energy/g

• compact storage

What vitamins are fat-soluble?

• A, D, E, K

• absorbed with dietary fat

What amount of fat-soluble vitamin ingestion would result in risk of deficiency?

less than 20g/day

What are the functions of lipids?

1. Structural

   • phospholipids and cholesterol are components of plasma membranes and myelin

2. Chemical Precursors

   • cholesterol

   • fatty acids

What is cholesterol a precursor of?

steroids, bile salts, and vitamin D

What are fatty acids precursors of?

prostaglandins and other eicosanoids

What is the recommended fat intake vs. the typical American’s fat intake?

Recommended = 30% of daily calorie intake

American diet = 40-50% of daily calorie intake

What are most fatty acids synthesized by?

• the body

  • essential fatty acids must be consumed

    • ex) omega 3,6

What are saturated fats?

• fats originating from animals

  • ex: meat, egg yolks, and dairy products

• result in arteriole sclerosis if consumed in excess

What are unsaturated fats?

• fats found in nuts, seeds, and most vegetable oils

• help clean out your system

What foods include cholesterol?

egg yolks, cream, shellfish, organ meats, and other meats

What are serum lipoproteins?

• Lipids transported in the blood as lipoproteins

  • protein and phospholipid coat around a hydrophobic cholesterol and triglyceride core

  • soluble in plasma; bind to cells for absorption

Serum lipoproteins: categorized into 4 groups by density

more protein=higher density

1. Chylomicrons

2. Very low-density (VLDLs)

3. Low-density (LDLs)

4. High-density (HDLs)

Describe how chylomicrons form

1. capillary endothelium has lipoprotein lipase to hydrolyze monoglycerides

2. resulting free fatty acids and glycerol enter fat cells to be resynthesized into triglycerides for storage

3. chylomicrons form in absorptive cells of the small intestine

4. enter the lymphatic system, then the blood

5. remnants of chylomicrons are degraded by the liver

What is VLDL?

• Very low density lipoproteins

• produced by the liver to transport lipids to adipose tissue for storage

• when triglycerides are removed by lipoprotein lipase, VLDL become LDLs (mostly cholesterol)

What is LDL?

• low density lipoproteins

• absorbed by cells in need of cholesterol for membrane repair or steroid synthesis

What is HDL + its production and function?

• High density lipoprotein

  1. liver produces an empty protein shell

  2. travels through the blood and picks up plaque (cholesterol)

  3. delivers cholesterol to the liver for elimination in the bile

What is the desirable total cholesterol concentration?

< 200 mg/dL

*measurement derived from HDL levels with everything else extrapolated

What are some ways to lower blood cholesterol levels?

• most cholesterol is endogenous

• dietary restrictions can lower blood cholesterol levels

  • restriction of dietary cholesterol: lowers levels by 5%

  • restriction of certain saturated fats: lowers levels by 15-20%

• Vigorous exercise lowers blood cholesterol

What are the desirable lipoprotein levels?

• HDL: high levels

  • indicates cholesterol is being removed from the arteries

• LDL: low levels

  • high LDL correlates with cholesterol deposition in arteries

• Recommendations:

  • Ratio 2 LDL : 1 HDL

  • Norms 2-5: 1

    • exercise regularly

    • avoid smoking, saturated fats, coffee, and stress

Describe the 3 pathways for lipoprotein processing

• Chylomicron pathway

  1. Lymph absorbs chylomicrons from small intestine

  2. lymph drains into the bloodstream

  3. lipoprotein lipase removes lipids from chylomicrons

  4. lipids are stored in adipocytes or used by other cells OR liver disposes of chylomicron remnants

• VLDL/LDL pathway

  1. liver produces VLDLs

     • 2. triglycerides are removed and stored in adipocytes

     • OR

     • 2. VLDLs become LDLs containing mainly cholesterol

       • 3. cells absorb LDLs by receptor-mediated endocytosis

• HDL pathway

  1. Liver produces empty HDL shells

  2. HDL shells pick up cholesterol and phospholipids from tissues

  3. Filled HDLs return to the liver

  4. Liver excretes excess cholesterol and bile acids

What percentage of body mass do proteins account for?

12-15%

Which type of muscles are proteins primarily found in?

skeletal

What are the functions of proteins?

• muscle contraction

• movement of the body, cells, and cell structures

  • cell membranes (receptors, cell identity, pumps)

  • fibrous proteins (collagen, keratin)

• structural

  • globular proteins

• functional

  • plasma proteins: blood osmolarity and viscosity

What are some examples of globular proteins?

antibodies, myoglobin, enzymes

What is the RDA requirement for protein?

44-60g/day

differs for activity levels, breastfeeding, pregnancy

What does the nutritional value of proteins depend on?

• proportions of amino acids

  • 9 essential amino acids cannot be synthesized

  • complete proteins (dietary) require a supply of all amino acids in the right amount in order to synthesize protein

What are the 9 essential amino acids?

1. isoleucine

2. leucine

3. lysine

4. methionine

5. phenylalanine

6. threonine

7. tryptophan

8. valine

9. histidine

Do cells store surplus protein?

• no, they break them down

• protein → amino acids -→ deamination → ammonia (NH3) → urea → excreted in urine

What are the dietary sources of protein?

• Animal proteins (meat, eggs, and dairy) are complete proteins

  • closely match human proteins in amino acid composition

• Plant sources must be combined in the right proportions

  • beans and rice are a complementary choice

What is the nitrogen balance?

• rate of nitrogen ingestion = rate of excretion

  • proteins are chief dietary source of nitrogen

  • excretion chiefly as nitrogenous wastes

What is a positive nitrogen balance?

• when you ingest more than you excrete

  • found in children

  • goal with athletes

  • promoted by growth and sex hormones

What is a negative nitrogen balance?

• body proteins are broken down for fuel (muscle atrophy)

• glucocorticoids promote protein catabolism in states of stress

Describe the process of glucose storage and use

The Cori Cycle:

• Liver:

  • Fatty acids → CO2 + H2O + ATP

  • Lactate + ATP → Glucose (through gluconeogenesis)

    • Glucose can be produced by/ produce glycogen outside of liver

• Muscle

  • glucose → lactate + APT (through glycolysis)

    • lactate will be brought to the liver

Where are triglycerides stored?

• in adipocytes

  • constant turnover of molecules every 3 weeks

  • released into the blood, transported and either oxidized or redeposited in other fat cells

What is lipogenesis?

• synthesizing fat from other sources

  • amino acids and sugars are used to make fatty acids and glycerol

What is lipolysis?

• breaking down fat for fuel

  • glycerol is converted to PGAL — enters glycolysis

  • fatty acids are broken down to 2 carbons at a time to produce acetyl-CoA (beta oxidation)

Describe the lipogenesis and lipolysis pathways

What is ketogenesis

• When fatty acids are catabolized into acetyl groups (by beta-oxidation in mitochondrial matrix)

  • may enter citric acid cycle as acetyl-CoA and undergo ketogenesis

    • metabolized by liver to produce ketone bodies

What are examples of ketone bodies?

• acetoacetic acid

• B-hydroxybutyric acid

• acetone

What could a rapid/incomplete oxidation of fats result in?

• raises blood ketone levels (ketosis)

• may lead to pH imbalance (ketoacidosis)

What is the amino acid pool?

• dietary amino acids + 100g of tissue protein broken down each day into free amino acids

• may be used to synthesize new proteins

  • as fuel: must be deaminated (through removal of NH2)

    • remains are converted to pyruvic acid, acetyl-coA, or part of the CAC

What happens during a shortage of amino acids?

• NH2 becomes ammonia (NH3), which is toxic

  • the liver will convert the ammonia to urea, which is excreted in the urine

Pathway of amino acid metabolism

What happens in urea synthesis?

Liver converts ammonia (NH3) to urea, which is removed from the blood by the kidneys

What is the absorptive state and its effects on carbohydrates, fats, and amino acids?

• The “fed” state

• lasts about 4 hours during and after a meal

  • time of nutrient absorption and use for energy needs

• Carbohydrates:

  • blood glucose is available to all cells for ATP synthesis

  • excess is converted by liver to glycogen or fat

• Fats

  • taken up by fat cells from chylomicrons in the blood

  • primary energy substrate for liver, fat, and muscle cells

• Amino acids

  • most pass through the liver and go onto other cells in liver cells

    • may be used for protein synthesis, fuel for ATP synthesis, or fatty acid synthesis

Absorptive state of carbohydrates

Main pathway: glycogenesis (glucose → glycogen)

Storage form/use: glycogen

Location: liver, muscle

Absorptive state of lipids

Main pathway: Lipogenesis (Fatty acids + glycerol → triglycerides)

Storage form/use: fat storage

Location: adipose tissue

Absorptive state of proteins

Main pathway: protein synthesis (amino acids → protein synthesis)

Storage form/use: new proteins

Location: all tissues

What regulates the absorptive state?

insulin

What is insulin secreted in response to?

• elevated blood glucose

• amino acid levels

• hormones

  • gastrin

  • secretin

  • cholecystokinin

What does insulin cause?

• increases the cellular uptake of glucose by 20-fold

• stimulates glucose oxidation, glycogenesis, and lipogenesis

  • inhibits gluconeogenesis

• stimulates active transport of amino acids into cells and promotes protein synthesis

  • high protein, low carbohydrate meals, stimulate release of insulin + glucagon, preventing hypoglycemia

What is the postabsorptive state and its affect on carbohydrates, fat, and protein metabolism?

• The fasting state

• Homeostasis of blood glucose is critical to brain

  • when stomach and small intestine are empty, the stored fuels are used

• Carbohydrates

  • glucose is drawn from glycogen reserves for up to 4 hours and then synthesized from other compounds

• Fat

  • adipocytes and liver cells convert glycerol to glucose

  • FFAs are oxidized by the liver to form ketone bodies

    • other cells use for energy, leaving glucose for the brain

• Protein metabolism

  • used as fuel when glycogen and fat reserves depleted

  • wasting away occurs with cancer and other diseases from loss of appetite and altered metabolism

What regulates the postabsorptive state?

• Sympathetic nervous system

  • as blood glucose drops 4-6 hours after meal OR during stress/exercise/anger

    • glucagon secreted

    • glycogenolysis and gluconeogenesis raise glucose levels

    • lipolysis raises FFA levels

• Adrenal Gland

  • enhance these effects through hormones

    • medulla: epinephrine and norepinephrine

    • cortex: cortisol

• Anterior Pituitary

  • growth hormone: glucose sparer (save for brain)

    • increases lipolysis -→ releases fatty acids for energy

What is metabolic rate?

• amount of energy used in the body in a given period of time (kcal/ hr or kcal/day)

  • measured directly in calorimeter (water bath)

  • measured indirectly by oxygen consumption

What is basal metabolic rate (BMR)?

• the amount of energy used with relaxed, awake, fasting, room comfortable temperature

  • adult male BMR is 2000 kcal/day (slightly less for females)

  • strongly influenced by thyroid hormone

What are factors that affect total metabolic rate?

pregnancy, anxiety, fever, eating, thyroid hormones, and depression

What are the symptoms of metabolic syndrome?

• 3 or more of the following 5 conditions (or taking medication for that condition)

  1. Large waistline (abdominal fat)

     • men > 40 inches (102cm)

     • women > 35 inches (88cm)

  2. High Triglycerides

     • > 150 mg/dL

  3. Low HDL Cholesterol

     • men < 40 mg/dL

     • women < 50 mg/dL

  4. High Blood Pressure

     • > 130/85 mmHg

  5. High Fasting Blood Glucose

     • > 100 mg/dL

What does homeostasis require?

heat loss to match heat gain

What is hypothermia?

• excessively low body temperature

• can slow metabolic activity and cause death

What is hyperthermia?

• excessively high body temperature

• can disrupt enzymatic activity and metabolic activity and cause death

What is thermoregulation?

the ability to balance heat production and heat loss