Body Composition Powers & Howely pp 344-356 Lecture Overview Methods of measuring body composition Two-component system Body fatness for health & fitness Obesity and weight control Diet, exercise, and weight control Energy and nutrient balance Diet and weight control Energy expenditure and weight control Measuring of Body Composition Height-weight tables Isotope dilution Photon absorptiometry Potassium-40 Hydrostatic weighing Dual energy X-ray absorptimetry Near infared interactance Radiography Ultrasound Nuclear magnetic resonance Total body electrical conductivity Bioelectrical impedance analysis Skinfold thickness 1
Height-Weight Tables Metropolitan Life Insurance Company (1959 & 1983) Divide a person s weight by midpoint = relative weight or obesity index Person with RW of 1.0 would have weight that is = to midpoint BMI = wt [kg] / ht [m 2 ] Classify people into weight categories (underweight, normal, overweight, 3 classes of obesity) Isotope Dilution Total body water determined through ingestion of a water isotope People w/large TBW volumes possess more lean tissue & less fat tissue Photon Absorptiometry Determines mineral content & bone density Greater absorption of photons = greater bone mineral density Potassium-40 Potassium isotope is proportional to mass of lean tissue Near Infared Interactance Based on the absorption of light, reflectance, & near infared spectroscopy Radiography X-ray of limb allows measurement of fat, muscle, & bone width used to estimate total body fat 2
Ultrasound Use of sound waves to measure thickness of subcutaneous fat Nuclear Magnetic Resonance Electromagnetic waves transmitted through tissues to estimate volumes of specific tissues Total Body Electrical Conductivity Lean tissue & water conduct electricity better than fat uses an electromagnetic coil Hydrostatic (Underwater) Weighing Density of water = 1 g/ml Density of fat = 0.900 g/ml and will float in water Density of lean tissue = 1.100 gml Determines whole body density to estimate lean and fat mass Air Displacement Plethysmography - Bod Pod Based on the same wholebody measurement principal as hydrostatic weighing, but using patented air displacement technology instead of water. The subject sits inside the BOD POD while computerized pressure sensors determine the amount of air displaced by the person's body. A complete analysis takes about 5 minutes. 3
Dual Energy X-Ray Absorptiometry (DEXA) Single X-ray source determines wholebody & regional estimates of lean tissue, bone, mineral, and fat Bioelectrical Impedance Analysis (BIA) Small portable instrument similar to TOBEC Total body water is calculated & used to estimate percent body fatness Skinfold Thickness Estimate of total body fatness is made from a measure of subcutaneous fat Values are entered into equations to calculate body density 4
Four-Component Model Mineral, water, protein, fat Three-Component Model a) Body water, protein & mineral, fat b) Body water & protein, mineral, fat Two-Component Model Fat mass & fat-free mass Two Component System Whole-Body Density Density constants Adults: % Body Fat = (495/Density) 450 African American Adults: % Body Fat = (437/Density) - 393 Two Component System Underwater Weighing Density = mass/volume = M air [(M air M water )/D water (V R V GI )] The percent body fat value has an error of about ± 2.0% due to the normal biological variation of the fat-free mass 5
Two Component System Sum of Skinfolds Assumption that a certain percentage of total body fat lies beneath the skin Jackson & Pollack and then Jackson, Pollack, & Ward developed generalized equations Equations have an error of about ±3.7% Body Fatness for Health & Fitness Recommended range of 10-20% for men & 15-25% for women Calculate fat-free weight: 100%-30% fat = 70% fat free 70% x 142 lb = 99.4 lb fat-free weight Calculate optimal weight: = fat-free weight/(1-optimal % fat {fraction}) i.e. 15% = 99.4 lb/(1-.15) = 117 lb Obesity If everyone were at optimal weight there would be a 25% decrease in CHD & a 35% decrease in congestive heart failure & stroke Obesity associated w/fat mass in excess of 30 kg is due primarily to an increase in fat cell number (vs. hypertrophy) Genetic factors account for about 25% of the transmissible variance for fat mass and % body fat; culture accounts for 30% 6
Physiological Set Point & Obesity Input (±) Hypothalamus: Body weight set point Output (±) Physiological signals: Glucose concentration (glucostat) Fat cell size (lipostat) Body weight (ponderostat) Type of diet (±) Feeding (±) Exercise (±) Cognitive Set Point & Obesity Input (±) Ideal cognitive set point for body weight Cognitive Signals How do I look? Size of clothing Perception of effort Body weight Concerns about health Output (±) Type of diet (±) Feeding (±) Exercise (±) Energy & Nutrient Balance Change in E stores = E intake E expenditure Rate of change of E stores = Rate E intake - Rate E expenditure Nutrient balance: New theory = Exists for protein & CHO excess intake is oxidized and is not converted to fat Excess fat intake does not drive its own oxidation excess is stored in adipose tissue Measured by ratio of Food Quotient (FQ) to the Respiratory Quotient (RG) 7
Diet & Weight Control Diets w/a high fat-to-cho ratio are linked to obsesity Nutrient balance for fat can be most easily achieved w/a low fat diet (high FQ) Calories do count & they must be considered in any diet aimed at achieving or maintaining a weight loss goal Energy Expenditure & Weight Control Basal metabolic rate vs. resting metabolic rate The BMR represents the largest fraction of total E expenditure in sedentary persons The BMR decreases w/age & women have lower BMR values than men The fat-free mass is related to both the gender difference & to the decline in BMR w/age A reduction in caloric intake by dieting or fasting can reduce the BMR, while physical activity is important in maintaining it Thermogenesis Thermogenesis (heat generation) is associated w/the ingestion of meals (thermic effect of feeding), brown adipose tissue, & futile cycles The thermic effect of feeding represents a small part of total E expenditure & is not predictive of obesity 8
Physical Activity & Obesity Humans increase appetite over a broad range of E expenditure; however, formerly sedentary individuals show a net loss of appetite when they undertake an exercise program When weight loss occurs w/an exercise & diet program, less lean body mass is lost than when the same weight loss is achieved by diet alone Weight Loss vs. Weight Maintenance Light-to-moderate exercise is an appropriate choice for most Americans to achieve healthrelated and weight-loss goals. Plasma free fatty acids make up a large fraction of E supply for that level of PA Moderate exercise promotes the expenditure of large amounts of fat & calories, consistent w/achieving weight-loss & fitness goals Vigorous activity is effective in expending calories & achieving performance & fat-loss goals 9