Body weight, body composition and weight management
Body weight alone misleads athletes; body composition splits it into fat mass and fat-free mass. Weight change should be slow, planned and off-season where possible.
Methods to assess body composition
| Method | Principle | Merits and limits |
|---|---|---|
| BMI | Weight (kg) ÷ height (m)² | Quick and cheap; cannot separate fat from muscle, so muscular players may read 'overweight' |
| Waist and waist-hip ratio | Tape measure of abdominal fat | Simple; shows central fat linked to heart disease and diabetes |
| Skinfold calipers | Pinch of fat under the skin at fixed sites; Durnin & Womersley use biceps, triceps, subscapular and suprailiac; sum → body density → % fat by the Siri equation | Cheap and portable; needs a trained tester and careful site marking |
| Bioelectrical impedance (BIA) | A tiny current passes easily through water-rich lean tissue and poorly through fat | Fast and easy; affected by hydration, meals and recent exercise |
| DEXA | Two low-dose X-ray beams separate fat, lean tissue and bone mineral | Accurate, gives regional values and bone density; costly, small radiation dose |
| Underwater (hydrostatic) weighing | Body density from weight in air and in water (Archimedes) | Long used as the reference method; tiring and needs a tank |
| Air displacement (Bod Pod) | Body volume from air displaced in a closed chamber | Comfortable and accurate; costly equipment |
Body fat ranges (% of body weight; American Council on Exercise)
| Category | Men | Women |
|---|---|---|
| Essential fat | 2-5% | 10-13% |
| Athletes | 6-13% | 14-20% |
| Fitness | 14-17% | 21-24% |
| Average | 18-24% | 25-31% |
| Obese | 25% and above | 32% and above |
Why BMI misleads a muscular athlete
Ravi, a 20-year-old kabaddi defender, weighs 82 kg and is 1.75 m tall. Skinfolds show 14% body fat. Find (a) his BMI and Asian category, (b) his fat mass and fat-free mass, (c) his target weight if his coach wants 12% fat with no loss of muscle.
- (a) BMI = 82 ÷ (1.75 × 1.75) = 82 ÷ 3.0625 = 26.78, rounded to 26.8 kg/m². By the Asian cut-off (25 and above) this reads 'obese'.
- (b) Fat mass = 82 × 0.14 = 11.48 kg, about 11.5 kg. Fat-free mass = 82 − 11.48 = 70.52 kg, about 70.5 kg.
- (c) Target weight = fat-free mass ÷ (1 − target fat fraction) = 70.52 ÷ (1 − 0.12) = 70.52 ÷ 0.88 = 80.14 kg.
- Fat to lose = 82 − 80.14 = 1.86 kg, about 1.9 kg.
- Check: 80.14 × 0.12 = 9.62 kg fat; 9.62 + 70.52 = 80.14 kg.
BMI 26.8 calls Ravi obese, yet 14% fat sits in the healthy 'fitness' range for men: BMI misjudges muscular athletes. Target weight is about 80.1 kg, a slow loss of about 1.9 kg of fat (values rounded to one decimal).
Activity: skinfolds, BIA and tape measurements
- 1Prepare the subject: light clothing; for BIA, no food or big drink for 4 hours and no hard exercise for 12 hours before (ACSM pre-test rules); bladder emptied.
- 2Measure height (stadiometer) and weight (digital scale); work out BMI.
- 3Tape measurements: waist midway between the lowest rib and the top of the hip bone; hips at the widest point; also mid-upper arm, chest, thigh and calf. Keep the tape snug, not tight.
- 4Skinfolds on the right side: lift a double fold of skin and fat, place the caliper 1 cm below the fingers, read after about 2 seconds; take 2-3 readings at each site and average.
- 5Add the four skinfolds, find body density from the Durnin & Womersley table, then % fat = (495 ÷ density) − 450 (Siri).
- 6BIA: subject lies still or stands on the foot plates with hands on the grips; record % fat and fat-free mass.
- 7Observe a DEXA scan where available: the subject lies still on the table while the arm passes over; note fat, lean and bone values by region.
- 8Tabulate the results from each method and discuss why they differ.
Strategies for weight management in athletes
| Goal | Strategies |
|---|---|
| Losing fat | Plan it in the off-season; a deficit of about 500 kcal a day (about 0.5 kg a week); keep protein at the upper end of the athlete range; keep carbohydrate around training; more vegetables, dal and fruit; fewer fried snacks and sweet drinks |
| Gaining muscle | Extra 300-500 kcal a day with resistance training; protein spread over 4-5 meals; energy-dense snacks such as peanut chikki, banana milkshake, sattu drink, nuts; expect slow gains |
| Making weight (wrestling, boxing, judo, weightlifting) | Compete near natural weight; plan loss weeks ahead; avoid rapid weight loss: sauna suits, fasting, spitting, laxatives and diuretics (diuretics are banned by WADA) cause dehydration, low glycogen and weakness |
| Monitoring | Weigh weekly at the same time; track skinfolds or waist, not just the scale; refer early if signs of disordered eating appear |
For a wrestler or boxer the dietitian sets a target weight from fat-free mass, not from BMI, and plans a slow loss months before the weigh-in. Crash dieting before a meet drains glycogen and water and leaves the athlete weak on the mat.
2 marks: “Why is BMI not a good index of fatness in athletes?” BMI uses only weight and height, so it cannot separate fat from muscle; a lean, muscular player may be labelled overweight or obese.
5 marks: “Explain the methods of assessing body composition.” BMI, waist, skinfolds, BIA, DEXA and underwater weighing: principle, one merit and one limit for each.
10 marks: “Discuss body composition for health and sport. Add the strategies for weight management in athletes.” Two-compartment model, methods, fat ranges, why composition matters in sport, then fat loss, muscle gain, making weight and monitoring.
Weigh the parts, not the whole: weight = fat + fat-free mass. Target weight = fat-free mass ÷ (1 − target fat fraction).