SWIMMERS: Body fat mystery!
|Louise Burke, Australian Institute of Sport, Canberra, Australia|
Swimmers, especially female swimmers, face an energy balance conundrum. Elite swimmers typically undertake 4000-20,000 m per day in training, burning thousands of calories. However, the typical body fat levels of these athletes are significantly higher than runners or cyclists who expend similar or even smaller amounts of energy in their training. Many female swimmers have fought well-publicized battles with their body fat levels and with their coaches! They are generally prescribed "land training" (running or cycling) in addition to their many laps of the pool in the belief that it is a necessary treatment to produce lower skinfold levels.
Do energy discrepancies really exist in swimming? Why do swimmers seem to have drawn the short straw of body fat management? The following theories have been suggested:
Two studies from Costill's Human Performance Laboratory at Ball State University have tried to address the energy balance oddity of swimmers. Jang et al.(1987) attempted to gain a crude measure of daily energy balance by comparing collegiate swimmers and collegiate distance runners. Ten athletes of each sex from each sport participated in the study. The findings: runners had lower body fat levels than swimmers (7% v 12% for male runners v swimmers, and 15% v 20% for females). All subjects kept detailed 3-day food records, and 1-day activity records were kept by half the subjects in each group. These records noted the amount of time each individual spent sleeping, sitting, walking, standing or training. The energy cost of these activities was estimated individually for each athlete by duplicating the activity in the laboratory and collecting oxygen consumption data. This factor multiplied by the time spent in each activity produced an estimate of total daily energy expenditure.
Results showed that both groups reported similar daily energy intakes: 3380 kcal and 3460 kcal for male swimmers and runners; 2490 kcal and 2040 kcal for female swimmers and runners, respectively. Estimated energy output was in agreement for each group, with the values for the groups of male athletes being roughly equal and similar to their reported intake. The female swimmers had a higher energy expenditure than female runners, and in fact were in slight negative energy balance. These results were not helpful in finding or explaining an energy dilemma, or major differences between types of athletes. The theories above might explain the problems experienced by some individual swimmers, but the theories were not supported by evidence from the study.
One of the limitations of this study is that each method of measuring energy balance is subject to considerable flaws. It is almost impossible to measure usual energy intake from diaries. Apart from the errors in translating descriptions of food into calorie counts, it is unlikely that people eat "normally" while they are recording. It is well-known that those who are conscious of their body fat underreport their food intake. It is also hard to complete and describe "normal" by record. In reporting, athletes try to appear as "good" as possible and thereby cover-up the clues to any energy balance problems. The behavior of individuals may also be masked by the "averaging" of results.
The other study by Flynn et al.(1990) examined energy and fuel usage during training sessions and recovery in swimming and running. It theorized that differences in hormonal patterns and the oxidation of fat might explain differences in body fat levels. Swimmers and runners trained for 45 minutes at 75-80% V02max then recovered for 2 hours. Triathletes did one session of each so that results could be compared for the same individual. During these periods, blood hormone levels, glucose and fatty acid levels, and gas exchange were measured and oxidation of various body fuels monitored.
The results showed no differences in total energy expenditure during training or recovery between groups. There were some differences in substrate utilization and hormone levels. For example, swimming resulted in lower blood glucose levels than running, with some evidence of a greater reliance on carbohydrate as a fuel during swimming. This is likely to be further accentuated in the real life training of swimmers who undertake a high proportion of high-intensity interval work. During recovery, fat oxidation tended to be greater after swimming than running. Overall, these differences were small and could not explain why swimmers have higher body fat levels.
While theories abound, no studies can verify or explain a real difference. These studies clearly leave the way open for further research. Techniques such as the double-labelled water method of energy expenditure estimation might provide a new way to measure energy balance issues. A final idea that needs to be explored is whether a selection process is at hand. Elite swimmers may be predisposed to have higher body fat levels because it is a help, or at least less of a disadvantage, to their swimming. Rounded shoulders and smooth curves may be more biomechanically sound than bony angles. Higher body fat levels are a greater disadvantage to weight-bearing sports like running. So perhaps those who aren't genetically inclined to very low body fat levels, but are otherwise possessive of high-level endurance qualities, should head for the water at an early age!
Flynn, M.L., Costill, D.L., Kirwan, J.P., Mitchell, J.B., Houmard, J.A., Fink, W.J., Beltz, J.D., D'Acquisto, L.J. (1990). Fat storage in athletes: metabolic and hormonal responses to swimming and running. International Journal of Sports Medicine, 11, 433-440.
Jang, K.T., Flynn, M.G., Costill, D.L., Kirwan, J.P., Houmard, J.A., Mitchell, J.B., D'Acquisto, L.J. (1987). Energy balance in competitive swimmers and runners. Journal of Swimming Research, 3, 19-23.