Fat and body fat levels – cyclists

As competitive cyclists search for ways to improve their workouts and performances, they often focus on their body-fat levels. Most cyclists want to get rid of surplus body fat so that they will have less ‘excess baggage’ to carry as they train and compete.

Paradoxically, although cyclists hate flabbiness, many of them actually take in fat as they exercise. You undoubtedly recall the cycling team which had to withdraw from a recent Tour de France after their badly contaminated, intravenous infusions of fat produced nausea and diarrhoea. The theory behind the practice of ‘fat loading’ is that the fat taken on board during exercise will conserve an athlete’s somewhat sparse supply of muscle glycogen and therefore heighten endurance.

Since cyclists are preoccupied with fat, exercise scientists have struggled with a couple of key questions: if cyclists want to burn away excess body fat during training, which exercise intensity should they use? Also, is it really wise for cyclists to take in fat during races or long workouts?

Recent research at the University of Texas addressed both questions. At a Texas laboratory, five experienced cyclists worked out for at least 30 minutes at three different intensities – 25% V02max, 65% V02max, and 85% V02max. These three intensities correspond with about 40-50 per cent, 76 per cent, and 92 per cent of maximal heart rate, respectively. At each exercise level, scientists carefully studied the cyclists’ rates of fat metabolism.

The trends in fat breakdown were clear. As exercise intensity increased from 25% V02max to 85% V02max, the amount of fat pouring out of the athletes’ fat cells into their bloodstreams steadily declined. As a result, fat originating in fat cells made a huge contribution to the energy required for exercise at 25% V02max, providing about 80 per cent of the needed energy! By contrast, fat coming from fat cells contributed only 30-40 per cent of the total energy at 65% V02max and just 10-15 per cent of the total energy at 85% V02max.

Why did fat cells pay so little regard to the muscles’ need for energy at 85% V02max? Actually, the chubby little cells were quite busy breaking down their internal fat molecules at that intensity; the real problem was in the blood. During high-intensity exercise, blood pours toward the muscles in flood-stage quantities but avoids the fat cells as much as possible. As a result, there’s little blood available to ‘pick up’ fat from the fat cells, and the fat has to wait until after a workout is over to move into the bloodstream.

However, some additional fat is always locked away inside muscle cells. This second supply of fat doesn’t have to move through the blood to get to the muscles, and it can provide a decent share of the fuel required for exercise. When the inside-muscle fat was factored in, fat contributed a steady 90 per cent of the required energy at 25% V02max, versus 50-60 per cent at 65% V02max.

So which intensity is best?
That may make it seem that 25% V02max is by far the best intensity for breaking down fat, but bear in mind that little energy is really needed to exercise at that paltry intensity, so the actual amount of fat burned was roughly 33-per cent greater at 65% V02max than at 25% V02max! At 25% V02max, the cyclists were burning energy at a rate of only about seven calories per minute; at 65%, the rate was roughly 14 calories per minute. Since fat supplied 90 per cent of the calories at the lower intensity, 90% X 7 = 6.3 calories of fat per minute. At 65% V02max, 60% X 14 = 8.4 calories per minute, a 33-per cent upswing in fat combustion.

In fact, total fat degradation was actually as high at 85% V02max as it was at 25% V02max, even though the former intensity is noted for its reliance on carbohydrate and the latter intensity is often referred to as a special ‘fat-burning’ zone of exercise. At 85% V02max, the cyclists used up energy at such a high rate that fat’s contribution, even though it amounted to a small percentage, added up to a sizable number of calories. To put it another way, a small piece of a huge pie (the 85% ‘pie’) can be just as big as a large piece of a small pie (the 25% ‘pie’).

The research suggests that 65% V02max (or around 76 per cent of maximal heart rate) is a wonderful intensity for breaking up fat, leading to 33-per cent greater fat metabolism than higher or lower intensities. Thus it’s a good exercise level for long workouts in which cyclists are attempting to ‘teach’ their muscles to get better at using fat.

What about losing weight?
However, for purposes of weight loss, calorie burning seems to be what really matters. Since an 85%V02max training sessions burns about 50 per cent more calories per minute than an equal-duration, 65%V02max workout, the former is better in a calorie-counting sense. It’s also nice to know that there is a sizable upturn in fat metabolism RIGHT AFTER AN 85% WORKOUT IS OVER. Remember that extra fat lurks inside fat cells during an 85% session. As blood returns to the fat cells when the workout ends, it picks up this fat and takes it to the muscles, where it’s readily gobbled up for energy.

If you’ve got about 30 minutes available for a cycling workout and you want to lose weight and get fitter, you’re better off with the 85% workout, rather than the 65% session. However, the 85% workout is much tougher (it corresponds to a heart rate of 90-95 per cent of maximal), so a more realistic exertion level is at about 75-80% V02max (83-88 per cent of maximal heart rate), an intensity which you’re more likely to maintain for 30 minutes and which also burns plenty of calories and a decent amount of fat. For workouts lasting significantly longer than 30-40 minutes, you’ve really got little choice but to rely on exertions below 85 per cent of maximal heart rate, but bear in mind that it’s worthwhile throwing in two- to five-minute intervals at an increased intensity as often as you can bear it.

We now know which cycling intensities are best for fat and calorie burning, but what about the other question: should cyclists really ingest fat as they exercise? There’s obviously no need to do so at the lower intensities, but there’s an intriguing possibility that it might be beneficial at 85% V02max – if you’re fit enough to cycle for a long time at such a high intensity. Remember that 85% V02max is the exercise level which creates problems with fat utilization: there’s plenty of fat available in the fat cells but only a few drops of blood to pick it up. If cyclists could get more fat into their blood without it having to actually collect it from the fat cells, perhaps they would have more energy.

It’s possible that some ingested fat could be absorbed from the digestive system and carried to the muscles during 85%-V02max efforts, but blood flow to the small intestine is rather meagre during such high-intensity efforts, so absorption rates could be low. That’s why the Tour de France cyclists avoided oral doses of lipid and tried intravenous shots instead, a practice that few of us would want to consider. At any rate, it’s clear that taking in fat at moderate to low exercise intensities is stupid; even a slim athlete’s body has plenty of stored fat, and that fat is made available to the muscles quite readily. We’ll have to wait for further research to find out whether a little fat taken orally can aid a cyclist pedaling long distances at 85% V02max.

‘Regulation of Endogenous Fat and Carbohydrate Metabolism in Relation to Exercise Intensity and Duration,’ American Journal of Physiology, vol. 265, pp. E380-391, 1993

Owen Anderson

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