Energy Expenditure, Exercise And Weight Loss

2.3.07

• Basal metabolic rate (BMR): basic level of metabolic activity required to stay alive.
• At cellular level:
-Pumping ions across membranes.
-Turnover of proteins etc.
• At organ level:
-Pumping blood.
-Respiration.
-Muscle tone.
• BMR closely regulated to amount of non-fat tissue in body.
• Measured after overnight fast, at comfortable temperature, with subject awake and resting.

Schofield equations - different for different age groups and gender

Total energy expenditure (TEE)
• During sleep, EE lower than BMR.
• All other time, EE>BMR.
• EE increased by:
-Performance of external work.
-Heat generated by work.
-Ingestion of meals (dietary-induced thermogenesis - DIT).
• TEE = BMR + physical activity + DIT.
• BMR usually largest component of TEE.
• To estimate TEE due to physical activity, multiply energy of each by time spent of each activity.
• PAL (physical activity level) = Ratio of overall daily TEE to BMR.

Mobilisation of fat stores
• Mismatch of energy supply to energy demand.
• Therefore, energy stores must be mobilised.
• Adequate carbohydrate intake in diet will partially maintain glycogen stores.
• Smaller meals mean absorptive period shorter:
-Insulin concentration decreased and glucagon concentration increased.
-Leads to activation of hormone-sensitive lipase.

Triglyceride NEFA (non-esterified fatty acids)
CO2/ketone bodies

Liver

Glycerol Triglyceride

Muscle
Co2
Kidney

Liver Glucose

• Extent to which this continues (and protein breakdown spared) depends upon:
-Size of energy gap.
-Duration of decreased intake.
-Maintenance of physical activity/energy expenditure.

Integrated view of metabolism: metabolic diary
1) Waking up: post-absorptive state
• Plasma glucose and insulin at lowest in 24-hour cycle.
• Plasma NEFA at highest.
• Glucose enters blood from liver glycogenolysis and hepatic gluconeogenesis.
• Blood glucose used by brain and red blood cells.
• Skeletal muscle mainly uses NEFA for energy.
• Some breakdown of protein in muscle due to low insulin.
• Some amino acids oxidised and amino group transferred to pyruvate → alanine → liver → gluconeogenesis.

A) On holiday - plenty of food, not much exercise
2) Breakfast - carbohydrate, protein and plenty of fat
• Glucose and amino acids increase in blood in 15-30 minutes.
• Plasma glucose will remain somewhat elevated for 3-4 hours.
• Plasma insulin concentration rises.
• Glucagon secretion falls.
• Glycogen metabolism switches to synthesis.
• Increased insulin, decreased lipase activity and decreased release of NEFA.
• Increased muscle uptake of glucose and amino acids → glucose oxidation, protein and glycogen synthesis.
• Arrival of chylomicron triacyl glycerol.
• Insulin stimulates lipoprotein lipase → fatty acid uptake → triacyl glycerol storage.
• 4 hours post-meal, fat storage at maximum.

3) Lunch.
• Insulin-stimulated processes primed.
• Reinforces pattern of storage.
• Plasma NEFAs remain suppressed.
• Glycogen synthesis continues.
• Storage of TAG in adipose tissue continuous.

4) Dinner.
• Virtually no break in storage of nutrients.
• At end of day, replete +++.

B) Energetic day and healthy diet
2) Breakfast - mainly carbohydrate with little fat
• Sharper rise in glucose and insulin.
• Release of NEFAs suppressed.
• Preservation of adipose tissue TAG stores.

3) Some gentle exercise (walk/cycle to work)
• Some glycogen breakdown.
• Increased heart rate and concentration.
• Blood flow to muscle increases, delivering more substrate.
• Skeletal muscles require more substrate.
• More glucose uptake from plasma.
• Increased sympathetic activity and adrenaline → fat mobilisation.
• Plasma glucose decreased → insulin decrease→ less conservation of fat.
• Less suppression of glucagon by glucose.
• Change from high insulin: glucagon ratio with intense substrate storage, to lower ratio with less storage and diversion of substrate to muscle.

4) Lunch (low-fat)
• Metabolic system less primed for storage.
• Afternoon walk will divert more substrate into oxidisation.
• At end of day, less substrate will have been stored, and more oxidised.

Summary
• Storage of energy regulated.
• Body's immediate needs met.
• Surplus stored in integrated way.
• Integration for utilisation - energy available for activity as required.