Energy Balance And Body Composition

26.2.07

Body weight = Fat + Fat-Free Mass
0.900g/cm3 1.100g/cm3

Estimating total body fat: whole body density (typical figure: 1.024g/cm3). Methods:
• Underwater weighing - gold standard.
• Skinfolds - not direct measures of body fat - callipers - four sites:
-Biceps.
-Triceps.
-Subscapular.
-Iliac.
• Total body volume.

Estimating total body fat
• Total body water: inverse relationship between body water and fat.
-FFM = TBW/0.73.
• Total body potassium: gives idea of FFM - radioactive K40.
-Women: FFM = TBK/60.
-Men: FFM = TBK/66.
• Bioimpedance: more fat, greater resistance to electrical climate - if subject dehydrated, poor result.
• Ultrasound.
• X-rays.
• Dual energy x-rays (DeXA).
• MRI scans.
• CAT scans.

Estimating lean body mass
• Total body potassium (K40).
• Total body nitrogen: in vivo neutron activation analysis.

Estimating carbohydrate stores:
• Muscle biopsy.

Assessing body composition using BMI
• BMI = Weight (kg) / Height2 (m2).
• Not direct measure of body fat, but useful.
• Cutoffs:
-<20: underweight.
-20-24: "normal."
-25-29: overweight.
-30+: obese.
• NB1: athletes.
• NB2: children - Cole et al (2000) BMJ 320; 1-6.
• NB3: it's not just how much fat - it's where!

Energy balance
• Intake - output = Δstore
Protein BMR (basal metabolic rate) glycogen
Fat HIF (heat increment of feeding) fat
Carbohydrates activity
Alcohol
• Metabolisable energy.
Protein Fat Carbohydrate Alcohol
4 9 3.75 7 kcal/g
17 39 16 27 MJ/g

Losing weight
• Negative energy balance:
-Decreased intake.
-Increased expenditure.
-Decreased intake AND increased expenditure.

What is feasible?
• Energy deficit of 4.2MJ (1,000kcal)/day for 3-6 months.
• -29MJ (7,000kcal)/week = 1kg loss/week.
• Total loss ~5.20kg.
• After: Lean (1998) Handbook of Weight Management.
• NICE Guidelines: December 2006.