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It’s the question that almost everyone wants an answer to. Whether you pound the pavement or pump iron, burning some extra fat during your workout is a desirably aim for almost anyone. With the weight loss industry as big as it is, there no shortage of products and special diets which promise to help one melt away their dreaded fat. But as is often the case in any industry where there’s a lot of money at stake, it can be a challenge to find unbiased objective information that credible and comes off the back of proven, researched principles.

So what are the factors and variables that affect weight loss, particularly in the context of exercise? The most basic factors are diet and exercise. But each of these factors can be broken down into more specific areas. For instance, with exercise, you have variables such as exercise type, i.e. aerobic, anaerobic/high-intensity, weight training etc. Whereas for diet you have variables such as type of food and its ingestion proximal to exercise. Some lesser known factors affecting weight loss include an individual’s genetics and the effect of macronutrient diet composition on use of fat during exercise. This article will take a detailed look at each of these factors, as in so doing provide an overview of some of the contemporary factors considered in the battle of the bulge.

Rate of Fat Burning During Exercise

Many will have heard of the VO2max test, which measures the maximum volume of oxygen an individual can consume per minute. What many may not know is that as part of this test, it is possible to measure something called the respiratory exchange ratio (RER) or respiratory quotient (RQ). This important measure combines the volume of oxygen and carbon dioxide consumed at any one time, to calculate the respective amount of energy being generated from fat versus carbohydrate (glucose). For instance, an RQ value of 0.7 means that 100% of energy is coming from burning fat, while an RQ of 0.94 means only 20% of energy is coming from fat, with carbohydrate or glucose making up the difference.

Individual Differences in Fat Burning During Exercise

One of the most fascinating insights concerning fat loss and exercise is that there is a large individual variability in the percentage of fat burned during exercise at a range of intensities. For example, at the turn of the century, researchers from Cape Town, South Africa showed in a landmark study that the relative rate of fat oxidation in a group of competitive cyclists range from 23 to 95% at rest1. And even when these cyclists where put through their paces at a range of different exercise intensities, they still maintained a similar degree of difference in fat burning rates1.

So what determines how much fat one burns during exercise? This question was also answered by the researchers from South Africa. When not exercising, the major determinants of RQ or fat oxidation were muscle carbohydrate (glycogen) content, training volume, proportion of type 1 fibers, fatty acid levels, lactate levels and percentage dietary fat intake1. Once subjects start exercising muscle fiber composition was the only variably not to significantly affect RQ1. In the ensuing article we’ll take a look at two of the above factors (i.e. glycogen levels and % fat intake) which are both easily modifiable.

Training Empty or Full?

Muscle glycogen levels at the start of exercise are a major determinant of how much fat one burns during exercise2. Glycogen is simply the storage form of carbohydrate in muscle and its levels are directly related to the amount and timing of carbohydrate intake. Glycogen is also the body’s premium fuel, in that it can supply energy at a greater rate than fat or protein. So as the intensity of exercise increases so does the body’s reliance on glycogen. But if you want to reduce your level of muscle glycogen at the start of exercise, you either have to reduce your carbohydrate intake or make an effort to specifically reduce carbohydrate intake after an intense or prolonged exercise session. Whenever you do an intense (i.e. interval training) or prolonged (>1.5 hr aerobic exercise) training session you inevitably deplete your glycogen reserves. By deliberately restricting carbohydrate intake in the hours immediately following exercise, one can maintain low glycogen levels. This particular strategy has been termed ‘train low’ and is a common practice used by competitive endurance athletes because it has been shown to enhance training adaptations3. But it is also a very effective strategy for any active individual looking to maximise fat loss associated with exercise.

Macronutrient Composition of Diet

One of the other modifiable factors mentioned in the study above that had a significant effect on fat oxidation during exercise was the percentage of dietary fat intake. In a nutshell, the higher an individuals’ fat intake, the higher their rate of fat burning during exercise. With a number of other studies showing this to be true4-7, it appears the body is able to recognise the ratio of macronutrients it consumes and make metabolic adjustments accordingly. While eating a high fat diet leads to more fat burning during exercise, the real question is whether a high fat diet can lead to more weight loss compared with a more normal low fat diet.

The most common approach to weight loss from a dietary perspective is low fat or low calorie diets. While low calorie diets have proven effectiveness for weight loss, the challenge is adhering to them in the long term. There is also the tendency for people to rebound weight when going back to their normal diet. An alternative dietary approach to weight loss that is gaining in popularity is called low carbohydrate-high fat. In contrast to low calorie diets, the low carb-high fat diet does not enforce calorie counting or calorie restriction – more just calorie modification. The diet centres on the premise that dramatically reducing carbohydrate intake while significantly increasing fat intake teaches the body to derive a much higher portion of its energy needs from fat8. As discussed in our article on low carbohydrate high fat diet, Professor Timothy Noakes is one of the most high profile proponents of the low carb-high fat diet for weight loss. He has authored a recent publication compiling personal reports from 127 individuals8 and his 35,000+ twitter followers’ regular tweet their positive reports.

Research on Low Carb-High Fat

As far as supporting research for low carb-high fat goes, some of the best support comes from research comparing diets identical in total calories, but significantly different in their macronutrient composition. One such example is a study by Dr Jeff Volek and colleagues from Connecticut, USA. In this study, which spanned 12 weeks, subjects were fed either a low carbohydrate high fat diet or a more traditional low fat diet.  The low carbohydrate diet had a macronutrient composition equivalent to: 12% carbohydrate, 59% fat and 28% protein, while the low-fat diet had a macronutrient composition equivalent to 56% carbohydrate, 24% fat and 20% protein9. Both diet provided 1500kcal/day and subjects were 40 individuals with atherogenic dyslipidemia9; a type of cardiovascular disease characterized by increased plasma triglycerides and low-density lipoprotein levels and decreased high-density lipoprotein levels. Despite consuming over twice the amount of fat, subjects in the low carbohydrate group lost on average 5kg more than the low fat group. The other notable markers to improve significantly in the low carb group were glucose and insulin concentrations as well as insulin sensitivity. Improved insulin sensitivity is thought to be the underlying cause of better weight loss for subjects on low carbohydrate high fat diets. Poor insulin function results in the body synthesising higher levels of fat in response to carbohydrate than in an individual with normal insulin function and is thought to be the major underlying cause of obesity and diabetes.

Genetically Determined Responses to Exercise

One other broad factor to consider is genetic influences on weight loss responses to exercise. For example, is it true to assume that everyone responds to exercise in the same way (as far as fat loss is concerned)? One could certainly be forgiven for thinking so, particularly given that public recommendations for exercise are very broad and general. For example, at the time of writing, new Australian physical activity guidelines have just been released and recommend that adults between ages of 18-64 accumulate:

“150 to 300 minutes (2 ½ to 5 hours) of moderate intensity physical activity or 75 to 150 minutes (1 ¼ to 2 ½ hours) of vigorous intensity physical activity, or an equivalent combination of both moderate and vigorous activities, each week.”12

AND

“Do muscle strengthening activities on at least 2 days each week”12

While these guidelines do recognise three distinct types of physical activity, namely, moderate aerobic exercise, high-intensity exercise and weight training; they don’t offer any guidelines on which type of exercise is most appropriate for a given individual. The hard facts are that different types of exercise elicit different physiological responses (especially in terms of fat loss) for different people10. For more detailed information on this topic, please see our corresponding article on 'The Best Exercise for Fat Loss'.

Exercise Non-Responders

But an even harsher truth is that some people are simply poor responders to exercise; when successful response is measured in terms of fat loss10. These findings were uncovered over two decades ago in a fascinating series of studies measuring the impact of genetic factors on weight loss response to exercise10, 11.

Exercise Studies in Twins

Canadian researchers from Quebec took seven pairs of young adult male identical twins who exercised twice a day for a period of 93 days while being kept on a constant daily energy and nutrient intake. As you would expect, the group as a whole lost 5kg of body weight on average11. But one of the most significant findings to emerge from the study was the large individual differences (both within-pair and between-pair) in response to exercise. This was despite the fact that all subjects consumed the same diet and were under surveillance 24-hours a day for a period of 4 months and housed as inpatients in an experimental research station so as to control for other confounding factors such as environment, occupation and physical activity. The graph below shows the variability in weight loss response.

Weight changes in identical twins in response to exercise

While it’s evident that there was a general trend for identical twins to lose a similar amount of weight (within pairs), the pair of twins highlighted in red above illustrate how in certain twins there was significant differences in weight loss. The graph also illustrates the wide variability in weight loss between pairs, with the green highlighted example showing a pair that only lost 1-2kg versus 8-9kg lost by other sets of twins.

Even though this study was conducted nearly two decades ago, it provides some of the best evidence to date that there are large individual differences in weight loss response to exercise. So the first thing to establish in the fight to burn more fat is simply that certain individuals are poor responders to exercise when it comes to fat loss. If you happen to be one such individual, who has experienced less than optimal weight loss despite regular adherence to an exercise routine, you may be an individual who for genetic reasons responds poorly to exercise as a weight loss tool. But despair not as diet may be another avenue that can offer more promise. Such individuals might be expected to see proportionally bigger fat loss from appropriate dietary changes.

Finding the Right Approach

In summary, fat loss in response to different exercises and diets is a complex and fascinating science. From some of the information presented in this article, it is apparent that basic intuition does not always play out when it comes to successful weight loss. Approaches such as high fat diets and issues such as poor genetics are some of the key issues that can have powerful effects on an individual’s response to any given weight loss program. Doctors and health practitioners will do well to be aware of such issues when devising weight loss programs for their patients.

 

References
1. Goedecke JH, et al. Determinants of the variability in respiratory exchange ratio at rest and during exercise in trained athletes. Am J Physiol Endocrinol Metab. 2000;279:E1325–E1334.
2. Van Proeyen K, et al. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. J Appl Physiol (1985). 2011;110(1):236-245.
3. Burke LM. Fueling strategies to optimize performance: training high or training low? Scandanvian Journal of Medicine and Science in Sports. 2010;20 Suppl 2:48-58.
4. Lambert EV, et al. High-fat diet versus habitual diet prior to carbohydrate loading: effects of exercise metabolism and cycling performance. Int J Sport Nutr Exerc Metab. 2001;11(2):209-225.
5. Goedecke JH, et al. Metabolic adaptations to a high-fat diet in endurance cyclists. Metabolism. 1999;48(12):1509-1517.
6. Burke LM, et al. Adaptations to short-term high-fat diet persist during exercise despite high carbohydrate availability. Med Sci Sports Exerc. 2002 Jan;34(1):83-91.
7. Stellingwerff T, et al. Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration. Am J Physiol Endocrinol Metab. 2006;290(2):E380-388.
8. Noakes TD.  Low-carbohydrate and high-fat intake can manage obesity and associated conditions: Occasional survey. S Afr Med J. 2013;103(11):826-830.
9. Volek JS, et al. Carbohydrate restriction has a more favorable impact on the metabolic syndrome than a low fat diet. Lipids. 2009;44:297–309.
10. Blouchard C & Tremblay A. Genetic influences on the response of body fat and fat distribution to positive and negative energy balances in human identical twins. J Nutr. 1997;127(5 Suppl):943S-947S.
11. Bouchard C, et al. The response to exercise with constant energy intake in identical twins. Obesity Research. 1994;2(5):400-410.
12. Australian Government, Department of Health 2014, accessed 20th February 2014, < http://www.health.gov.au/paguidelines#apaadult>

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