REGULATION OF ENERGY EXPENDITURE
For 2 adults of similar size/age, TDEE can vary by ~ 1500 kcal/day.
Weight gain results wen EE is inappropriately low for the caloric intake of a person.
Measures of metabolic rate:
Basal metabolic rate (BMR) – laying awake but resting (ideally right after waking up), stress free,
not digesting food, absence of thermoregulatory heat production
o Measured directly by calorimetry, and indirectly by measuring oxygen uptake
Resting metabolic rate (RMR) – similar to BMR but not measured in morning after sleep (10%
Minimal metabolic rate – metabolic rate in various conditions that cause rate to fall below BMR
o Sleep = decreases it by 10%, anesthesia, continued starvation (leptin decreases energy)
Field metabolic rate – TDEE is higher than BMR due to energy requirements of feeding, cold
exposure and muscle use.
Maximal metabolic rate – maximal steady state metabolic rate during hard exercise (20X BMR-
athlete; 12X BMR – sedentary)
Energy use in adult human body
Of total energy intake, most is used up but some is lost in urine and feces.
Of the total energy used, most is lost as heat, some used in growth, reproduction and work.
Of total energy lost, most of it is due to BMR, muscle use, food TEF, cold/diet.
o BMR – 60-70%
Mitochondrial proton leak – 20%
Protein synthesis – 25-30%
Na/K ATPase – 19-28%
Ca ATPase – 4-8%
Gluconeogenesis – 7-10%
Ureagenesis – 3%
Actomyosin ATPase – 2-8%
o Physical activity – 15-30% (>50% in highly active individuals)
Exercise – purposeful physical activity undertaken for sport. Avg ~ 100kcals/day
NEAT – occupation, leisure, sitting, talking etc. Avg 450 – 2000 kcals/day
o TEF – 8-12%
Typically stimulates metabolic rate by ~25% with peak stimulation 1-2 hrs after
a meal in humans. Frequent eating has cumulative effect.
Metabolism of ingested amino acids in liver for glucose, fat, urea, and protein
synthesis (~35-35%) = active process
Swallowing, digesting, and absorption of food, enzyme secretion (~25-30%)
SNS activation (~30-40%) o Other – 2-3%
Nucleic acid synthesis
Brain contributes very little to body mass, but contributes a lot to BMR because of the increased
number of cells present in humans, thus more activity (not in rats)
Skeletal muscle contributes to both body mass and BMR in humans and rats
Overview of the acute control of brown adipose tissue (BAT)
VMN – ventromedial hypothalamic nucleus
SNS to release norepinephrine (NE)
Beta-3 adregenic receptor on BAT and increases CAMP
cAMP activates PKA which phosphorylates and activates HSL
HSL hydrolyses triglycerides
Lipolysis releases FFA from TG
FFA is substrate for thermogenesis and activator of UCP1
o FFA gets taken up in mitochondria and undergoes beta-oxidation
TEF is not the same as adaptive thermogenesis, although both activated by SNS
o TEF always happens, but adaptive thermogenesis is the net + energy balance due to
chronic increase in intake of energy that leads to adaptations over time.
Thermogenesis is due to activation of UCP1 through lipolysis
o Experiments with BAT isolated from UCP1 ablated mice show that in the absence of
UCP1, no thermogenesis can be induced in BAT by NE.
NE leads to response in WT, but no response in UCP1-KO
o Stimulation of lipolysis stimulates thermogenesis, and the thermogenic process in BAT
can be mimicked by the addition of fatty acids (also UCP1 dependent)
Lipolysis is NE induced in brown adipocytes
All manipulations that induce lipolysis in BAT also induce thermogenesis.
HSL- hormone sensitive lipase
o Contained in BAT.
o NE induces phosphorylation of this enzyme in order to activate lipolysis
o Artificial TG emulsions are not endowed with perilipin – protein that normally covers the
TG droplets within the cell
Protects the TG against HSL activity.
PKA phosphorylates perilipin and dissociates it from the TG droplet, making it
freely exposed to attack by HSL
o Perilipin-deficient mice = high basal lipolysis that can’t be further activated by
adrenergic stimulation, and increased BMR
o Perilipin deficient animals = BAT appears very lipid depleted.
o KO perilipin = lean but don’t become obese, increased EE Thermogenesis in BAT at a given moment is determined by the degree of activation at that
moment (y-axis) and can alter within seconds, but the capacity for thermogenesis is determined
by the degree of recruitment of the tissue (x-axis) which needs days or weeks to be significantly
o When you wake up and not eaten (BMR), its inactive
o Once you eat or get cold, you increase activation
o Chronic overfeeding leads to the recruitment of more BAT and some white adipose
tissue also gets activated. Therefore BAT hypertrophy.
o BAT even at BMR burn more energy compared to WAT.
Sites of FDG uptake corresponding to BAT in adult humans
FDG PET- fluorodeoxyglucose positron emission tomography – used to look for tumors
o Tumors are very glycosidic and take up a lot of glucose.
o Test – get people to fast, then inject them with FDG, and the cells that rely on glucose
will take it up from the blood = brain, bladder, heart
CT – computer tomography – visualizes density and composition of tissues
o Found that the dense mass was not muscle but adipose tissue, humans have BAT
o Is the staining different in warm vs cold temp?
Cold-induced BAT activation in adult humans.
Warm temp = no need to activate SNS to adapt for thermogenesis
Cold temp = tissue becomes activated and takes up more glucose
Sympathetic control of BAT activity in adult humans
Same experiment carried out under cold/normal conditions but now
with the use of propranol (beta blocker)
o Completely blocks glucose uptake and UCP1 is not activated
= humans have stores of BAT
Mice overexpressing human UCP3 in skeletal muscle = hyperphagic (eat more) and lean
o Complete prevention of diet-induced diabetes
Skeletal muscle respiratory uncoupling (overexpressed UCP1 – found only in BAT)= prevents
diet-induced obesity and insulin resistance in mice
Conclusion: Promoting inefficient metabolism in muscle represents a potential treatment for obesity
and its complications = Increasing mitochondrial uncoupling proteins promotes inefficient
metabolism in muscle.
Mice lacking mitochondrial uncoupling proteins = cold sensitive but not obese (room temp)
Genetic ablation of BAT in transgenic mice = development of obesity
Conclusion: UCP1 is not the only system involved in thermogenesis even within BAT so there may be
other energy consuming processes that can be targeted for promoting inefficient metabolism. Prof’s research – looking for another non-UCP1 site for inefficient metabolism
What contribution do Ca pumps have on total EE?
o At rest, cytosolic Ca levels must remain low. Pumps need energy to pump Ca against
concentration gradient. 1ATP used to pump 2Ca into SR = efficient
o Inefficieny = less Ca being pumped for every ATP used.
Experiment: EDL stimulated to contract normally
o Measured the amount of force consumed and ATP used in contraction.
o Repeated experiment using drug that inhibits myosin-ATPase to tease out how much
energy was used by myosin and which aren’t (Ca-ATPase)
o Ca was still being released even when muscle was not contracting
o Study assumed 5% of total skeletal muscle ATP consumption
o ATP consumption in resting mouse muscle measured at 40-45%
o Skeletal muscle accounts for 20-30% of whole body RMR
Ca pump activity could account for 18% of TDEE
o Studies show that Ca pumps contribute 30-40% to the overall muscle ATP use.
Myosin ATPase – 60%, Ca ATPase – 30%, Na ATPase – 10%
Assessing contribution of SERCA activity to RMR
Experiment: As the muscle sits at rest and uses up ATP for its basal metabolic processes, the
muscle is consuming oxygen. Therefore, oxygen content of the bath decreases and that’s what’s
Indirect method: inhibit Ca leak which indirectly inhibits Ca pumps because the pump would not
need the actin to pump the Ca back in.
High concentration of MgCl2 = binds to Ca release channels and maintains it in the closed state
o Prevents Ca from leaking out of the channel
Reduction of VO2 content in comparison to control was 41% soleus (slow twitch), 46% EDL (fast)
o ATP consumption contributes >5% significantly more (~40-50%)
Variable, regulated by the brain
Responds to temperature and diet
Occurs in brown adipocyte mitochondria
o Cold or excess energy is sensed in the brain
o Sympathetic nerves are activated – chronic elevation of SNS activity = good for adaptive
thermogenesis but bad for CVD and high BP
o NE binds to beta-adrogenic receptor (B-AR) on BAT
o Stimulates lipolysis, activates UCP1 and generates heat/burns energy Altering SR Ca transport efficiency
Physiologically – phospholamban, sarcolipin, HUFAs and cold acclimation all decrease efficiency
of Ca pumps.
o Cold increases thyroid hormone which decreases SERCA pump efficiency.
o T3 levels also increase heat/EE
Pharmacologically – hyper/hypothyroid, tamoxifen, curcumin = decrease Ca pump efficiency
o Fluoride increases Ca pump efficiency
Integral membrane protein, 31 aa
Interaction within Ca binding domain of SERCA
o Affects rate and efficiency of Ca pumping (reduces it) = reduces affinity of Ca binding
o Inhibits uptake more than ATPase activity = more ATPase is required.
SLN uncouples hydrolysis of ATP from accumulation of Ca by the Ca-ATPase of skeletal-muscle
o Took artificial membranes and reconstituted them with pure proteins with different
rations of SERCA to SLN and they measured Ca uptake.
o The more SLN was reconstituted, the more uncoupling they got
The presence of SLN results in increased heat production by Ca-ATPase
o Same prep in artificial membrane
o Proposed a model where SLN causes slippage during Ca pumping.
o The reaction cycle of SERCA:
E1 confirmation – ATP is hydrolyzed, ADP is released
Ca binds at the binding site with high affinity
Cytoplasmic gate is open, lumen gate is closed
Ca binding site faces cytoplasm
E2 confirmation – free energy released by ATP hydrolysis is used to drive the
Cytoplamsic gates close, lumen gates open
Cytoplasmic gates close before Ca can get through the pump and it slips
out back into the cytosol.
SLN leads to slippage and promotes inefficient Ca pumping
Ca no longer binds with high affinity therefore released in cytoplasm
Coupling ratio < 2Ca: 1ATP and increased heat release
Transgenic mouse models (SLN KO) Breed mice that have 1 copy of the mutated gene and 1 normal gene (f1 generation will be bred
by Mendelian genetics, so you always have to test the offsprings)
o WT will only have the WT allele,
o Heterozygotes will express both alleles
o KO will only have the targeting allele
No mRNA in KO mice
Diaphragm, soleus, WG, Artia = tissues that express high levels of SLN
In EDL, KO and WT should be similar.
SLN ablation increases Ca transport efficiency in soleus (High SLN)
o Measured Ca uptake by transport
o Measured ATPase activity by ATP hydrolysis
For the same amount of Ca uptake, the KO mice required less ATP to achieve it =
coupling ratio in KO mice higher = more efficient
o Physiological levels of SLN in skeletal muscle reduces the efficiency of Ca transport (WT
Conclusion: mice with varying levels of SLN expression are ideal for studying the effects of
altered Ca pump efficiency on metabolic rate, energy balance and susceptibility to diet-induced
The relative contribution of SERCA to resting soleus VO2 is lower in chow-fed SLN KO mice
o Lower in KO because more efficient but not statistically significant
o Difference in % reduction in VO2 when MgCl2 is used:
45% in WT, 35% in KO.
Less in KO because SERCA pumps use less energy to RMR because they
are more efficient
o The resting VO2 metabolic rate did not go down as expected because it could be due to
some other mechanism kicking in to compensate.
o Skeletal muscle contributes ~ 30% of total body BMR. If there was a significant
difference here, it would be expected to be noted at the whole body level
Body weight, whole body VO2, food intake and activity are not different between SLN KO and
WT chow-fed mice.