For some reason it makes intuitive sense to me that high intensity exercise is better for fat loss than low intensity, but I do not quite know how this came to make sense.
High intensity exercise will cause a primary burning of glucose and so high intensity exercise will make our muscles better at taking up and burning glucose thus theoretically making us more glucose tolerant. Then again, during low intensity endurance exercise we primarily burn fat. Endurance exercise will make us better at burning fat, and fat is undoubtedly the thing to burn if we want to lose weight.
Generally speaking high intensity exercise will increase our muscle-neuron cooperation, improve factors related to anaerobic fuel utilization, improve our glucose metabolism and if performed with some resistance, increase our muscle size and strength. Lower intensity exercise will improve our aerobic fuel utilization, increase blood flow by increasing numbers of capillaries and increase the number and size of mitochondria – the powerhouse of the cells that are responsible for burning fat.
In the 2001 version of the ACSM position stand “Appropriate Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults,” the authors could say little on the effect of exercise intensity . Lucky for us, several studies have come out since then that help paint a clearer picture. Despite these recent publications, the new and updated 2009 ACSM position stand contain no discussion on exercise intensity at all and the subject seems forgotten .
No doubt, if physical activity is to affect weight loss or weight loss maintenance it invariably has to affect the very same factors we try to affect by diet. As overweight is a condition of excessive fat storage we want to increase lipolysis (the release of fat) from fat cells and we want to reduce storage of fat in the fat tissue. This does not however necessarily mean low intensity endurance exercise that increases our fat burning abilities.
The two major factors at play here are insulin and glucose levels in the blood. Because the fat tissue is guarding us against the dangers of high blood glucose by taking up excess it will grow as it is protecting us, making us burn glucose to reduce blood levels and storing fatty acids for later as they pose no immediate threat. If our muscle and liver glycogen stores are full (as they will be with high carb diets and little exercise) excess carbohydrate intake will force the body to turn glucose into fat to get it out of the way.
It is well known that even a single exercise session is associated with a significant improvement in insulin-stimulated glucose uptake. Training, in making us better at taking up glucose, will also improve glucose uptake into the fat cells by increasing GLUT-4. This is a good thing.
The antidiabetic drug Metformin (Glucophage) for example will increase glucose uptake by fat cells by increasing GLUT-4 mRNA expression and cellular protein content, leading to increased GLUT-4 protein content in the plasma membrane. GLUT-4 is a glucose transporter which allows glucose to travel across membranes. Metformin also stimulates glucose oxidation in fat cells.
It is also likely that increased lipolysis and fat oxidation will reduce hunger. The result is that a greater amount of the energy needed to sustain your daily life will come from your own fat stores rather than from food. Increased lipolysis and fat oxidation may even make you want to move around more. I think Albert Pennington hit the nail on the head when he said, «Energy expenditure is an index of calorie nutrition at a cellular level.«
First let’s look at some studies. (If you find yourself fighting the urge to fall asleep, just skip to the conclusion)
Coker et al tested the differential effects of moderate (50% of Vo2max) versus high-intensity (75% Vo2max) exercise in 18 overweight (BMI=30) elderly (71ys). The training was endurance exercise designed to expend 1000kcal per week. There was no significant change in body weight, BMI or percent fat, but the high intensity group experienced a significant reduction in visceral fat and a significant increase in thigh muscle attenuation .
A study published in the latest European Journal of Internal Medicine, reports putting twenty-two women with type 2 diabetes in a supervised group exercise program for six months. The program combined endurance and resistance exercise and the duration and intensity of exercise for each subject was recorded. The subjects were then divided with respect to training volume in a high training volume group and a low training volume group .
Although exercise capacity did not change significantly during the training period, insulin sensitivity increased significantly and HbA1c decreased significantly from baseline in the high volume group but not in the low volume group. A large part of the improvement in insulin resistance was explained by exercise intensity. It is likely that an improvement in insulin resistance will affect the amount of fat mass lost over time.
Contrary to the above finding, Emmanuel G. Ciolac found that equal volume of high-intensity interval (aerobic interval training 80–90% of Vo2max) and moderate-intensity continuous exercise training (50–60% of Vo2max) elicited equal improvements in insulin sensitivity in healthy women .
Grediagin et al  did a small study where twelve untrained, moderately overweight women were randomly assigned to a high-intensity (80% Vo2max) or low-intensity (50% Vo2max) exercise group. The subjects trained four times per week for 12 weeks in sessions with duration sufficient to expend 300 kcal. The subjects were further instructed to maintain their normal diet and activity patterns. By the end of twelve weeks there were no significant between-group differences for change in weight, percent body fat, fat mass, fat-free mass, skin fold measurements or circumference measurements.
The above study however, because of its small number of participants may have type 2 error in it. Although the differences did not reach statistical significance, the mean weight loss was 0.7 lb for the high-intensity group and 3.3 lb for the low-intensity group. Hydrostatic data revealed that both groups lost 5.0 lb of fat, but the high-intensity group gained more than twice as much fat-free mass (4.3 vs 1.8 lb).
Brian Irving and colleagues  did a study where twenty-seven middle-aged obese women (51ys, BMI=34) with the metabolic syndrome completed one of three 16-week aerobic exercise (walk/run) interventions: no-exercise training (Control), low-intensity exercise training (<lactate threshold 5days per week) and high-intensity exercise (>lactate threshold, 3days per week). Exercise time was adjusted to maintain caloric expenditure (400 kcal per session). The researchers found that high-intensity significantly reduced total abdominal fat, abdominal subcutaneous fat and abdominal visceral fat. These changes were not observed in the control or the low intensity group.
Similar to Irving, Bryner et al  found that in 15 women randomized to exercise with a mean heart rate of 132 or 163 for 45 minutes 4 times per week, high intensity did cause a significant reduction in percent body fat not observed in the low intensity group.
Nicklas et al  randomized 112 overweight and obese postmenopausal women to one of three 20-week interventions of equal energy deficit: calorie restriction, calorie restriction plus moderate-intensity aerobic exercise (treadmill walking at an intensity of 45-50% heart rate reserve), or calorie restriction plus vigorous-intensity exercise (70-75%). Exercise was 3 days per week.
The exercise groups lost significantly less lean mass than the diet only group. The vigorous intensity group increased Vo2max more than the other groups and changes in visceral fat were inversely related to increases in Vo2max. However, the groups lost equal amounts of total and regional fat.
Slentz, who coauthored the Houmard study mentioned in the last post, reported that low-amount/moderate-intensity and low amount/vigorous intensity endurance training (i.e., activity equivalent to ~12 miles per week of walking or jogging) were equally effective in reducing percent body fat mass, waist circumference and abdominal circumference in overweight middle-aged adults. They also reported that high-amount/vigorous-intensity endurance training (activity equivalent to ~20 miles per week of jogging) was more effective in reducing percent body fat and fat mass compared with the two low-amount training groups .
These results are of course used to justify the claim that it’s the amount of calories burned or the volume of exercise that matters.
In a more recent trial Slentz et al  demonstrated that in middle-aged overweight/obese individuals who were randomized to three 8-month exercise programs: low amount/moderate intensity, low amount/vigorous intensity, or high amount/vigorous intensity, the moderate intensity improved pancreatic β-cell function more than vigorous intensities. However, vigorous intensity caused a 7% reduction in visceral fat whereas moderate intensity gave no reduction. Also, only the high-amount/vigorous-intensity group showed a decreased insulin response to a glucose challenge. There was no decrease in insulin response in the low amount/moderate intensity group that had the greatest improvement in insulin sensitivity index.
Whyte et al  tested the effect of 2 weeks of sprint interval training on ten overweight (BMI=31) men. The exercise comprised of 6 sessions of 4 to 6 repeats of 30-second Wingate anaerobic sprints on a cycle ergometer, with 4.5-minute recovery between each repetition. Despite the short intervention time, both waist and hip circumferences decreased compared with baseline.
It is important to remember that these acute (2 weeks) responses to exercise are not necessarily transferable to long term responses. However, acute responses are sometimes all we’ve got and should be considered.
Chambliss et al did a 1 year intervention study with four different exercise interventions as a part of a standard calorie restricted diet. They found no significant differences between low and moderate intensities on weight loss .
Jakicic et al  also did a 1 year study where 201 women were randomized to 1 of 4 exercise groups: vigorous intensity/high duration; moderate intensity/high duration; moderate intensity/moderate duration; or vigorous intensity/moderate duration. The participants also followed a low energy diet. The authors did not find any differences in weight loss after one year.
There are a few things I find important to keep in mind when interpreting results from exercise trials. First, an improvement in a factor related to metabolic health may not cause a significant short term effect on weight or fat mass. It is still theoretically plausible that improvements in fat mass may come in time because of the improvements in the related metabolic parameters. Simply speaking, I don’t expect big results on fat mass from any form of exercise lasting only a few months.
Also, some of these studies use intension to treat analyses, and so the actual effect of doing the different exercises is not reported.
All in all, and as far as I can tell, there is sufficient data to support a recommendation of high intensity exercise over low intensity exercise for maximal loss of body fat. Not all trials find a difference, but several do, and some of those that do not find anthropometric differences find metabolic differences likely to affect weight loss over time. In addition, risk factors for cardiovascular disease are improved more with high intensity than low intensity.
Although most studies looking at exercise intensity in relation to fat loss use endurance exercises some studies still show an increased growth or reduced loss of muscle tissue with high intensity training. In fact, it is quite possible to lose fat while gaining muscle even while doing low resistance high intensity exercise. I think the whole topic of gaining muscle while losing fat deserves a separate post so…
Next: The losing of fat, not muscles.
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