Running speed and body fat, how are they related?

Having had a long term friendly argument with my coach over body weight (yes I’m happy with my weight, I don’t understand why you aren’t?!) I decided to do some research into how exactly body weight affects performance.

As far as I understand, body weight has little effect on swimming performance; in fact, I read that excess weight can in fact aid buoyancy. Seems logical, and it certainly would explain why that rather large man in my local pool can swim at least as fast as I can!

When it comes to running and cycling, body weight has a lot of influence. In cycling, power to weight ratio is what you will hear everyone saying. On the flat, it may not matter as much, but on an incline, there is a LOT to be gained if you are a lighter athlete. For example, I found this on an internet forum:

‘I've been cycling for several months now and saw some modest speed improvement when tackling hills. However the biggest boost in performance only began when I started dieting and shedding a lot of weight.’

Logical really if you think about it, take two riders. Equal power output, matched exactly in flat track speed, but if one was heavier than the other, then dragging that excess weight up the hill would make them slower, Right?

If you want a more exact formula, take a look here: http://www2.trainingbible.com/joesblog/2008/01/climbing-power-formula.html

The bit I was interested in more than anything though, was the running. Where is the science to support the logic that leaner is better?

So I went to look for it.

THE RESEARCH FINDINGS

A lot of research on this topic can be flawed. It is difficult to isolate all the variables. In a test situation, the leaner people are likely to be fitter anyway, and therefore probably train more. So finding reliable research is tricky. One measure of running fitness is VO2 Max, or the maximum amount of oxygen a given runner can use per unit time per unit body mass. More simply, the lighter a runner is at a given fitness level, the higher her VO2 Max and--at least in theory--the faster she will be. Not all "excess" weight, however, is created equal; fat for example, is merely dead weight, whereas muscle, at least to a point, can contribute to performance in distance running despite its high density.

So just how much can you expect to benefit from being lighter? Joe Henderson, the author of various books on running, has this to offer: "The loss of a single pound doesn't mean much for a single mile, but the effect multiplies nicely. Ten pounds equals 20 seconds per mile, which grows to a minute-plus in a 5K, more than two minutes in a 10K, nearly 4.5 minutes in a half-marathon and almost nine minutes in a marathon."

An article written by Ron Maughan is professor of sport and exercise sciences at Loughborough University had this to offer:

An excess of body fat, however, serves no useful function for the endurance athlete. It can help the sumo wrestlers, and perhaps may not even be a disadvantage for the shot putter, but not the runner. Extra fat adds to the weight that has to be carried, and thus increases the energy cost of running. Even in an event as long as the marathon, the total amount of fat that is needed for energy supply does not exceed about 200g for the average runner.

‘In a study of a group of runners with very different levels of training status and athletic ability, scientists observed a significant relationship between body fat levels and the best time that these runners could achieve over a distance of 2 miles(3). Although these results indicated that leaner individuals seem to perform better in races at this distance, some complicating factors have to be taken into account.

Although there’s an intimate link between body fat levels and running performance, it’s important to remember that reducing fat levels will not automatically guarantee success and may even be counter-productive. If you reduce fat by a combination of training and restricting diet, you are walking a fine tightrope. While a reduction in body fat may well boost running performance, cut down food intake too drastically and not only will training quality suffer, but the risk of illness and injury also increases dramatically.’

This is all very nice, but I need evidence. Statistics!

THE SCIENCE BIT

CURETON and SPARLING did some research in this field which seemed much more controlled.

Metabolic responses to running in was tested with a treadmill (TM) running (12-min run performance). 10 female and 10 male adults who regularly engaged in distance running were used. They were testing differences in % body fat, and differences between men and women.

The males were studied under two conditions: (1) with normal body weight and (2) with external weight added to the trunk so that the total percent excess weight (% EW) was equal to the % fat of a matched female. Under the added-weight condition, % excess weight of the males was increased by an average of 7.5%.

TM run time was reduced by 1.3 min (32%) and in 12-min run performance by 173 m (30%).

It was concluded that the greater sex-specific, essential body fat of women is one determinant of the sex difference in metabolic responses to running and distance running performance. Because of her greater body fatness, the average woman will utilize more oxygen per unit FFW to run at any given submaximal speed, will have a lower Vo2max expressed relative to body weight and, as a result, will maintain a speed on the 12-min run or other similar distance running event which is slower than her male counterpart. Since the sex-specific, essential fat of women cannot be eliminated by diet or training, it provides part of a biological justification for separate distance running performance standards and expectations for men and women.

Read more about this study here http://journals.lww.com/acsm-msse/Abstract/1980/24000/Distance_running_performance_and_metabolic.11.aspx

Although they are not testing body fat / speed directly, they do show quite clearly that the extra % body fat of women is directly related to the difference in performance between the sexes. Therefore it may be correct to conclude this result would be extended to include excess fat carried in runners, and its relation to decreased speed and power.

My conclusions:

·       There’s no getting away from it, being leaner will make it LESS EFFORT!

·       Being leaner will definitely give you the opportunity to be faster

·       Being leaner does not necessarily automatically mean you will be faster, there is still an element of fitness involved, but you’re increasing your chances for sure!

Just as a matter of interest, since I gave up on the argument and did lose that weight as coach instructed, I have noticed my run performances improve. I know it’s difficult to isolate all the factors, but I believe lean mass is something at least in part, to do with it.