I have this theory. In the last post I concluded that there is sufficient data for recommending high intensity exercise in favor of low intensity for fat loss. Admittedly, the data in support of this is not strong. It is there, but I would not bet my life on it yet.
I might be a bit biased in favor of high intensity because of my theory that seems rigidly stuck in my brain. From the world of resistance exercise, we know that you can use a number of strategies to build muscles. Some use slow movements with relatively low resistance where the muscles are activated for a long time. Others use quick movements with high loads allowing for a greater workload to be performed in a shorter amount of time. Some simply cut blood flow to the working muscles which also will make them grow. The bottom line is that the one important factor when it comes to muscle hypertrophy seems to be the amount of physiologic stress placed on the working muscles during exercise. So you can stress a muscle or muscle group by making it work for a long time like in static strength training or you can do the standard high load high repetition fast movement exercise. You can even do pure eccentric exercises rather than the usual concentric/eccentric. All of this and pretty much everything in between will cause muscle growth. It is the total workload, the time multiplied by load, that matters.
The concept is very similar to the energy hypothesis that claims it is the amount of energy spent during exercise that determines fat loss, rather than intensity or time by itself. The difference is that the hypertrophy theory is supported by science whereas the energy hypothesis is not.
Although hypertrophy can be achieved in many ways, there is a limit to the equation, workload = time x load. It is not only the amount of work done that determines muscle growth. Running, for example will not build significant amounts of muscle even if some of the muscles involved perform a great workload. The reason is that somewhere in the area between multiple repetition resistance exercise and endurance exercise the stimulus changes and thus the physiological adaptations.
Most people are aware of this and know that walking or running is a poor strategy for building muscles.
My theory is that the whole «exercise intensity and fat loss» issue is of a similar nature, that to a certain degree, it is the amount of work performed that matters the most. Both low and high intensity exercise cause fat loss, but somewhere in the declining intensity area the stimulus becomes different and fat loss is reduced. Shopping, gardening and other everyday activities with low intensity, but often of long duration seem poor strategies for weight loss. Likely, we are working to close to the body’s comfort zone to make it adapt. Or put in better words, the body does not adapt to these activities because it doesn’t need to, it already has.
This means that high or low intensity might not matter that much, as long as the tissues are exposed to new stimuli, forcing them to adapt. However, this is not to say that high intensity is not better than low, the question is how much better.
As with muscle hypertrophy, if the goal is to maximize all factors, then high intensity, high workloads and variation in stimuli is important. I believe it is the same with exercise intensity and fat loss. This might explain some of the variation in results found in different trials.
The losing of fat and not muscles
Loss of muscle mass is common with weight loss, independent of dietary approach. Some diets seem to lead to better muscle mass retention and some exercise forms will prevent muscle loss. But we must make sure not to assume that all muscle loss is a negative thing. The size of our muscles will determine our appearances while their function is what will affect our health.
In school I was taught that one important factor for skeletal muscle hypertrophy was positive energy balance. You had to take in more than you expended. This however, turned out to be wrong. The muscles might need a readily supply of fuel, but they couldn’t care less if the energy comes from glycogen and fat stores or from food. Thus it is theoretically possible to lose fat mass while increasing muscle mass.
Fleck and Kraemer  concludes that in untrained individuals doing regular resistance exercise you can expect a 2kg increased muscle mass in 14 weeks. That’s about 0.06kg per exercise session. If we consider the women only trials the number is about 0.04kg per session.
Resistance exercise builds muscles when we’re not dieting, but does this hold true when we are simultaneously losing fat mass? Short answer, yes. We can. But, you would not come to that conclusion if you put together a meta-analysis of decent trials. The reason is that in several studies fat loss is significant while muscle growth is not. However, when the mean is zero change, the individual data will show that many did increase their lean mass while losing fat.
I know this is a statistical quagmire, but part of my conclusion will still be based on individual results rather than the reported means. The reason is that when several people in a resistance exercise study build muscle, it is very likely that the exercise is to blame. This is not a strategy I would use when for example looking at weight loss results from low fat diet studies that show a mean zero improvement. If finding that half the participant lost weight and the other half did not, it would not be smart to claim that low fat diets works. There are a great many things that lead us to lose weight, although it almost always involves some reduction in dietary carbs. But as I said, when people gain muscle in a resistance exercise trial, the gain is extremely likely to come from the resistance exercise.
There is another issue as well. Many body composition studies are done with DEXA (dual energy x-ray absorptiometry). Because adipose tissue mass is considered 85% lipid and 15% water, and because DEXA measurement of fat free mass includes this adipose tissue water, a loss of adipose tissue without loss of fat free mass suggests favorable changes in muscle mass.
When I did a small study in 2009 I found that in eight women who did regular resistance exercise in combination with a ketogenic diet, four of them increased lean body mass, while four reduced it. The study was not large enough to indicate what factors were responsible for the lack of muscle mass gain in the other four. I guess anything from dietary protein (amino acids), thyroid issues to stress might contribute .
|Figure 2 from Jabekk et al 2009. Lc+Ex= Low carb + resistance exercise. Ex=resistance exercise.
Shadid and Jensen  compared 20 weeks of pioglitazone with exercise and diet. Diet and exercise resulted in 11.8kg mean weight loss. The group was instructed in a 500kcal deficit diet and in an exercise program starting with 15min of aerobic exercise three times per week at 50% of each individual’s heart rate reserve. This was gradually increased to 45min aerobic exercise four times a week at 60–70% of heart rate reserve. The surprising find in the study was that the intervention caused a mean 9.5kg fat loss and a non significant 0.5kg reduction in fat free mass, which is an unusually small reduction.
Barbara A. Gower et al  also got surprisingly good results from a traditional very low energy diet in overweight women. Mean fat mass lost was 11.2kg. White women in the trial lost a non significant 0,77kg lean mass while the African American women lost 1.53kg (non significant) lean mass.
In both the above trials, several individuals managed to increase their lean mass while reducing fat mass. This was even done without any reported resistance exercise. Most if not all of the participants in the trials were insulin resistant, which if improved, I suspect can cause muscle growth by itself.
Gary Hunter  who coauthored the Gower trial found that in women who lost 12kg of weight, resistance exercise conserved lean mass. The women were randomized to diet only, diet and resistance exercise or diet and aerobic exercise.
|Figure 1 from Hunter et al 2008. FFM=fat free mass. AA=african-american. EA=european-american
In a study from 1988 Douglas L. Ballor et al  concluded similarly to Hunter and stated that “In conclusion, data from this study indicate that weight training added to a caloric restriction program results in maintenance of LBW [Lean Body Weight] and regional increases in muscle area. When diet plus weight-training exercise is compared with exercise without caloric restriction, there is no difference in the rate of strength gain or magnitude of upper-arm muscle area increase.»
|From Ballor et al 1988. C=control. DO=diet only. DPE=diet plus exercise. EO=Exercise only.
It might seem counterintuitive, but finding that endurance exercise increases lean mass or attenuate lean mass loss is quite common. Janssen et al  found that aerobic and resistance exercise did equally well in preserving muscle mass with dieting in premenopausal obese women. This finding reflected their previous (1999) trial with both men and women, which also found preservation of lean mass with both resistance and aerobic exercise.
Contrary to Janssen et al, Geliebter et al  and Bryner et al  did find that resistance exercise was more effective in improving lean mass with weight loss than aerobic exercise.
The combination of resistance training with aerobic training has been shown to be superior for body weight and fat loss and to result in greater lean body mass compared to aerobic exercise alone in several randomized controlled trials.
Park et al  for example, found that aerobic exercise reduced fat percentage by 9.2 while combining it with resistance exercise reduced fat percentage by 10.3. Lean mass increased by 0.9kg with aerobic exercise and by 5.6kg with combined exercise. This trial did not use a dietary intervention and the subjects had a mean BMI of 25. But it is an interesting example of how the exercise did not cause a weight change but still caused a significant body composition change.
Using a combination of high protein diet and resistance exercise Wycherley et al  demonstrated an 11.4kg fat mass reduction in 16 weeks. The reduction was accompanied by a 2.4kg reduction in fat free mass. The body composition results are impressive, but as I said we rarely if ever find a significant mean fat loss in combination with significant mean fat free mass gain.
Saving the best for last, Kevin David Ballard and colleagues is responsible for a meeting abstract from 2008 titled “Effects of diets restricted in fat and carbohydrate with and without resistance training on body composition and cardiovascular risk.” Ballard reports results from the Department of Kinesiology at University of Connecticut were heavyweights like Jeff Volek and William Kraemer reside. They report having examined the effects of carbohydrate restricted (CRD) and low fat diets (LFD) alone and in combination with resistance training (RT) in 47 overweight men.
The results state that “Reductions in body mass, fat mass, percent body fat and abdominal fat were greater in the CRD than the LFD (p<0.001).” and that “Gains in lean body mass were demonstrated in the RT groups only (p=0.004).” Their conclusion: “Resistance exercise is an effective strategy to combat diet induced losses of lean body mass.”
On the matter of resistance exercise and fat-free mass Donnely and ACSM  concludes thusly: “…most studies combining resistance training with energy restriction report improved lean body mass compared to dieting alone.” They continue; “In summary, resistance training does not seem to be effective for weight reduction in the order of 3% of initial weight and does not add to weight loss when combined with diet restriction. Resistance training increases fat-free mass when used alone or in combination with weight loss from diet restriction. Resistance training may increase loss of fat mass when combined with aerobic exercise compared to resistance training alone.”
Petra Stiegler and Adam Cunliffe  did a very good job addressing the evidence for the effect of exercise and diet on anthropometric variables. They conclude that: “…it becomes clear that a combination of modest caloric restriction combined with endurance exercise or endurance and physical training of different modes is preferable over dietary modification alone to induce favorable changes in body composition accompany ing weight loss.”
It is possible to increase lean mass while losing fat mass. There are many factors at play but a high animal protein diet and resistance exercise is the best strategy we know of. Resistance exercise is very effective at increasing lean body mass, and it seems that this effect remains even if people are dieting and the effect is observed in both low and high carbohydrate diets. The addition of endurance exercise to diet and resistance exercise seems to slightly increase fat mass loss, but has little effect on lean mass.
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