Whole Body Energy Balance

1
Lecture 6

• Whole Body Energy Balance

2
Intake and Expenditure

• Fuel oxidation rate is beautifully matched to ATP
  demand
• Well coupled at the molecular level
• But fuel intake is totally mismatched from ATP
  demand
• Ie, we dont eat at the same time as we consume
  energy!
• On a daily basis, energy intake does not match
  energy expenditure
• Excess in energy consumed over energy expenditure
  will be stored as fuel reserves
• Mainly fat!
• Despite this, weight (fuel reserves) stay
  remarkably constant over time

3
Lunch
150
Dinner
B/fast
100
Rate energy Intake (kJ/min)
50
0
30
Activity
20
Rate energy output (kJ/min)
Sleep
10
0
0
4
8
12
16
20
24
Time of Day (24 hr)
4
Intake and Expenditure

• We seem to have a set point
• Over months the two sides are VERY finely matched
• We are good at defending our weight
• Weight normally regulated to within about 1
  kg/year
• A 1 kg rise would represent an imbalance of about
  30 MJ per year
• Ie, 70 kJ per day
• Equivalent to a couple of ml of fat
• Or 5 g carbohydrate (a teaspoon of sugar!)
• These are imperceptible amounts in dietary
  analysis
• lt1 of daily energy intake
• Over that year we would have consumed about 4000
  MJ
• Conversely, putting on weight requires SOME
  EFFORT!!!

5
Weight Gain Takes Effort!

• BMI mass (kg) divided by the height (m) squared
• eg, 83/1.8 x 1.8 25.3
• Normal 20-25 Overweight 25-30 Obese gt30
• A person with BMI of 22 needs to gain 25 kg to
  get to a BMI of 30
• e.g., a 1.8 m tall person going from 72 kg to 96
  kg
• Of that, about 19 kg is pure fat and 6 kg is
  lean mass
• Fat is 38 MJ/kg, lean is 4.8 MJ/kg
• So this extra weight represents an excess of 750
  MJ
• Would need 2 MJ/day EXTRA to do it all in a year
• A person normally consuming 8 MJ/day would need
  to eat 25 extra EVERY DAY for a whole year
• So this sort of weight gain generally takes some
  time!!
• More like 10 years!

6
Energy Expenditure

• Three main components
• Basal metabolic rate (BMR) 60
• Resting metabolic rate (RMR)
• Physical activity - 30
• Voluntary and Non-exercise activity thermogenesis
  (NEAT)
• Diet-induced thermogenesis 10
• Thermic effect of food
• Obviously the contributions will vary in
  individuals
• We can do something about physical acitvity but
• Can we do anything about BMR
• Does BMR vary between individuals?
• How do we measure energy expenditure?

7
BMR

• BMR is almost totally dictated by LEAN BODY MASS
• Ie, the mass of metabolically active tissue
• Muscle metabolically active because it is
  continually pumping ions, even when it is still
• Adipose tissue is relatively inactive
• So plots of metabolic rates vs fat free mass
  (FFM) are linear
• Slope is 4 ml O2 consumed per min per kg FFM (ie,
  about 100 kJ/day/kg FFM)
• Overweight people do not have lower BMRs
• Indeed the extra weight makes whole body
  metabolic rate higher
• Higher fat mass is accompanied by higher fat free
  mass
• But metabolic rate vs actual mass not linear at
  high weights because fat is not as metabolically
  active

8
Non-obese
10
Obese
9
8
Basal Metabolic Rate (MJ/day)
7
6
5
4
35
50
60
70
Fat free mass (kg)
9
Changing BMR

• Metabolic INEFFICIENCIES
• Uncoupling proteins
• Brown adipose tissue UCP-1, but also maybe UCP-2
  and UCP-3 in muscle
• More lean body mass
• Substrate cycles (futile cycles)
• Fuel synthesis then breakdown
• Eg, make protein from amino acids consuming ATP,
  then dismantle the protein (with no gain of ATP)
• Leaky membranes
• Thyroid hormone
• T3 strongly affects metabolic rate
• Lack of thyroid hormone reduces BMR
• But how?
• We know its transcriptional but exactly which
  genes change and how this then changes metabolic
  rate is not known
• Leaky membranes, inefficiencies of NADH transport

10
Indirect Calorimetry

• Measure rate of O2 consumption
• Because energy expenditure linked to the rate of
  the electron transport chain and the latter
  involves oxygen consumption
• Assume that energy released when O2 is used
  20.2 J/ml (for fat, CHO, protein)
• Ratio of O2CO2 gives an indication of which type
  of fuel is burning
• The respiratory quotient
• RQ for carbs 1
• RQ for fat 0.7
• Can also tell us if someone is making fat
• RQ gt 1
• Inconvenient
• Not appropriate for free living
• Can be done during exercise

11
Doubly Labelled Water

• Subject consumes 2H218O (D218O)
• Like normal H2O This reacts with CO2 produced in
  fuel oxidation to form H2CO3
• All the oxygen atoms in H2CO3 are equivalent, so
  during the reverse reaction, some oxygen goes
  into CO2 and will be lost at the lungs.
• The rate of 18O loss could then be used to guage
  how much CO2 was produced
• And hence the rate of fuel oxidation
• Since oxygen could also be lost through water
  excretion, we need the 2D to indicate depletion
  through excretion, sweating, etc
• The difference indicates the true rate of carbon
  dioxide production.
• Very good for long term assessment
• But expensive and needs specialised equipment
  (mass spectrometer!)
• Not a measure of BMR

12
The Harris Benedict Equation

• Estimate BMR 
• Women
• 655 ( 9.6 x weight in kilos ) ( 1.8 x
  height in cm ) - ( 4.7 x age in years )
• Men
• 66 ( 13.7 x weight in kilos ) ( 5 x height
  in cm ) - ( 6.8 x age in years )
• The only factor omitted by the Harris Benedict
  Equation is lean body mass. So it is quite
  accurate in all but the very muscular (where it
  will under-estimate BMR) and the very fat (will
  over-estimate BMR)
• Multiply by 4.184 to get into kJoules.

13
Daily Energy Expenditure

• Multiply BMR by the appropriate activity factor
• Sedentary (little or no exercise) BMR x 1.2
• Lightly active (light exercise/sports 1-3
  days/week)   BMR x 1.375
• Moderatetely active (moderate exercise/sports 3-5
  days/week)   BMR x 1.55
• Very active (hard exercise/sports 6-7 days a
  week)   BMR x 1.725
• Extra active (very hard exercise/sports
  physical job   BMR x 1.9 Note how the
  activity doesn't make as much difference as you
  might expect.

14
Regulation of Food Intake

• Controlled by many hormones and neuropeptides
• In animals but in humans more by norm
  behaviour
• Leptin
• The Adipostat or Lipostat
• Communicates size of fat stores to the brain
• Secretion of leptin is proportion to the amount
  of fat stored in white adipose tissue
• Leptin binding to receptors in the hypothalamus
  elicits satiety
• Mice without leptin are hyperphagic, i.e. eat
  without control
• when leptin is injected to these mice
• ? food intake
• ? energy expenditure in brown adipose tissue
• People without leptin are hyperphagic
• ..and they respond to leptin injections

15
120
leptin
100
80
Body Weight (kg)
60
40
50 percentile
20
0
2
4
6
8
10
0
Age (years)

• So could leptin injections be the cure for
  obesity?

16
No!

• Obese people have higher blood leptin
• More and bigger WAT cells
• So extra leptin does not help
• Indeed, they may be leptin-resistant
• Also humans have a small amount of brown adipose
  tissue (ie, cant respond to the leptin by
  increasing EE)
• A lack of leptin may tells us to start eating,
  rather than a excess of leptin telling us to stop
  eating

120
100
80
Leptin (ng/ml)
60
40
20
0
0
10
20
30
40
50
60
70
Body Fat ()