squanto said:
i disagree. i swim all the time and it works for me. it may not be the BEST cardio, but do any of you really have any evidence to back this up?
The Paradox of Body Fat and Swimmers
Swimmers are often maligned in fitness and sports circles as not being able to decrease their bodyfat levels. Some of the research documents that swimmers, especially female swimmers, typically have higher levels of bodyfat than their counterparts in other sports, such as runners and cyclists. Is there really an inherent reason why swimmers can't reduce bodyfat levels? Or are there other discrepancies that factor into bodyfat management in swimmers?
Let's take a look at what some of the studies suggest:
* One study showed that moderately obese women gained body fat during six months of daily swimming (60 minutes per day). Another similar group of women lost body fat with walking or cycling for the same time period. This has been challenged with the observation that recreational swimmers typically do not exert the same physical intensity that other forms of aerobic activity may require. Although another study reported that strenuous swimming was ineffective in reducing body fat levels in children, extrapolations to adults may be presumptuous.
* When two different swimming and calisthenics exercise programs were compared in college students, there were no significant differences in body fat levels. Exercising sessions involved subject entirely immersed in the water or combined water and land exercises. Each session was performed for 35-40 minutes three times per week for eight weeks. Since this training volume does not equally compare to that performed by elite swimmers, extrapolation is limited. However, it may allay the fear of increased body fat levels by including swimming in a weight control program for non-athletes.
* One author suggested that swimming induced increases in energy intake. In contrast, a research group examined the energy intake and output of a group of collegiate swimmers, and compared it to that of collegiate long distance runners. Male and females in both groups had similar caloric intakes. Nonetheless, one has to question how accurate the energy intake measurements were from the food diaries. It is well known that individuals, especially athletes, often underreport their food intake. A recent study using more controlled measurements of food intake for 2 hours post training determined that the there was no difference in hunger or energy intake between runners or swimmers.
* Elite swimmers typically have an enormous energy output in their training programs. They often burn thousands of calories in a day with sprinting and long-distance swimming. Many often partake in dry land training to increase strength, adding to their caloric expenditure. However, some studies have reported that swimmers also have a higher energy intake than other athletes, possibly eating more energy than they expend. While training for other sports often suppresses athletes' appetite, swimmers often report that their appetite increases after swimming. Some researchers suggest this may be due to a decrease in body temperature during swimming sessions. Runners and cyclists, however, usually experience a rise in body temperature. This temperature rise may account for their short-term appetite suppression. Nevertheless, an increased appetite in swimmers has not been verified.
* Differences in substrate utilization during swimming and running were revealed in several studies. An early study suggested that swimming results in reduced fat oxidation during swimming when compared to running, possibly contributing to swimmers' body fat levels. In contrast to this, Flynn1 and colleagues demonstrated an increase in fat oxidation during recovery after swimming compared to running. Thus, this suggests that a greater body fat level in some swimmers is not due to reduced fat oxidation during recovery from training.
While there are several hypotheses of why swimmers may have more body fat, no studies can verify or define an indisputable difference. While some evidence exists that there are metabolic changes in swimmers in comparison to other athletes, they do not support the contention that swimming induces body fat gain. Actually, swimmers need body fat.
Two important factors that influence the resistance of the body through water are body shape and composition. The ideal body shape for swimming is one shaped like a javelin because it offers little resistance to movement through water. Body fat determines the buoyancy and the amount of energy expended to stay on the water surface. A lean body must work harder to stay high on the water, whereas having some body fat allows the body to float. However, too much total body mass increases the resistance through the water.
No one really knows what the ideal body weight should be for a swimmer. Body fat percentages in elite swimmers vary from 6% to 25% with women having the higher percentage of body fat than male swimmers. This gives female swimmers a buoyancy advantage - especially since a higher amount of their body fat tends to be distributed in the lower body. This gives more of a lift to the legs, which reduces drag. As a result, the energy expended for a female to swim at the same pace as a thin man is less.
Men tend to store body fat around or above their waist, so increasing their bodyfat would shift their buoyancy forward and make their legs sink. This would result in increased drag in their movement through the water. Swimming performance of female swimmers has been shown to positively correlate with body fat levels, whereas such correlations were lacking in men. This suggests that body composition may be a predictor of swimming performance more so in women than in men.
Another consideration of body fat in swimmers is insulation. Body fat influences the metabolic changes of the body depending on water temperature. Insulation provided by body fat is needed to diminish the likelihood of swimmers suffering from hypothermia in cold water. For instance, for one subject studied2 to successfully swim the English Channel, he would require a body fat gain of 13.88 pounds to resist heat loss and supply the energy needed at a given pace and level of fitness. Swimmers generally thermoregulate their body temperature in cold water better than non-swimmers. This may be due to differences in adaptation or amount of body fat.
Dr. David Costill3, a professor of exercise physiology and an elite Master's swimmer, suggests a body fat range for men within 10% to 20%; for women, 15% to 25%. Although he also claims that being too thin is generally not a problem for most swimmers, dieting to lose body weight may jeopardize swimming performance. Those who wish to decrease their body weight should do so during the off-season when training is at a low intensity and there are no competitions.
References
1. Flynn, MG, Costill DL, Kirwan JP, et al. Fat storage in athletes: metabolic and hormonal responses to swimming and running. Int J Sports Med, 11:433, 1990.
2. Acevedo, EO, Meyers MC, Hayman M, et al., Applying physiological principles and assessment techniques to swimming the English Channel. A case study. J Sports Med Phys Fitness 37:78, 1997.
3. Costill, D. The Right Stuff for Masters Swimming.
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Take another look at an olympic swimmer! shoulders? I think so. They may be lean, but by no means small muscles. 
