You’re not happy. It’s understandable. You’ve been spending your hard-earned cash on your gym membership and not to mention the sweat and tears you’ve been putting in. But you’re just not losing weight. Worst still – you may have actually put on weight!4
Rest assured – you are not alone. Studies back in the 1990s confirmed that fat loss response to exercise varies largely between individuals1, 2. What’s more, there is no single reason or simple explanation as to why. It’s different for different people.
Most people are quick to assume someone’s not losing weight simply because they’re not doing enough exercise, or the right type for that matter. But there are several studies that show even when one accounts for the differences in compliance to a given exercise program, there are still significant differences in weight and fat loss between individuals2, 3.
Type 2 diabetics are another group of individuals for whom it’s been proven between 15-20% fail to improve their glucose control, insulin sensitivity and muscle mitochondrial density following supervised exercise training interventions.5 This lack of response to exercise has been termed ‘exercise resistance’.5
So what are the key factors governing weight loss response (or lack thereof!) to a given exercise program? The answers make for some fascinating reading and contradict a lot of common held beliefs around exercise and weight loss.
Effect of Exercise on Metabolic Rate
Arguably one of the most widely held beliefs concerning exercise and fat loss is that a good bout of exercise will increase your metabolic rate (and therefore the amount of fat one burns). While this does happen for some, there are a large majority for whom there is no change in metabolic rate (also referred to as ‘energy expenditure’) following exercise3. Worse still, some even exhibit a decrease3. So making the assumption that doing some exercise will provide a boost to your metabolism and associated fat burning is not a given for every individual. While some botanical extracts such as green tea6 have good evidence supporting their ability to increase metabolic rate and fat oxidation, it’s clear one can’t rely on exercise alone for this.
Effects of Exercise Type on Fat Loss
The next contentious area as far as exercise and weight loss is concerned is that of exercise type. Theoretically, aerobic exercise has always been considered the best for fat loss simply because aerobic exercise is typically performed at lower intensities (than anaerobic or high intensity exercise), which is associated with increased use of fat as fuel. However, aside from the use the relative proportions of fat and carbohydrate that are used during exercise, it’s just as important the effect a given exercise has on substrate use and energy expenditure following exercise. There have been some major studies done in this area, which are worth mentioning. The first one worth mentioning was conducted in 2010 and was a large-scale randomised controlled clinical exercise trial in which individuals with type 2 diabetes participated in either aerobic exercise, resistance exercise or a combination of aerobic and resistance exercise for 9 months7. Only the combined aerobic and resistance exercise group lost the most fat -1.7 (-2.3 to -1.1 kg; P < .05) and exhibited the greatest reduction in HbA(1c) levels. Similarly, a later study published in 2013 by researchers from Orlando found combined aerobic and resistance exercise was most effective in improving muscle mitochondrial content and associated fat oxidation8. Both of these studies ran for 9 months and all participants performed the same quantity (i.e. time) of exercise, which suggests that the type of exercise rather than the volume is critical to improvements in metabolic health.
Effect of Genetics on Fat Loss Response to Exercise
Another major factor that effects fat loss response to exercise is genetics. Unfortunately, there is nothing one can do to alter this, but it’s vital to acknowledge the powerful role genes can play. Research has uncovered a multitude of gene variations that affect individuals’ response to critical weight-controlling variables like insulin resistance9 and appetite10. There is the very distinct possibility in the future that medicine will allow patients to undergo genetic testing to determine the best diet and exercise regime to minimise risk of weight gain11-13.
Effects of Exercise on Food Reward
A lesser known factor that can affect an individuals’ weight loss response to an exercise program is changes in the exercise-induced reward value of food. Recent studies into this area have lead to the theory that ‘exercise responders’ tend not to display significant changes in food reward (ratings of liking and wanting, and relative preference by forced choice pairs) following exercise. While exercise non-responders tend to display an increased liking for all foods following exercise. What’s more, non-responders show increased wanting and relative preference for high-fat sweet foods14. For individuals falling into this category, some suggest changing to a low carbohydrate and higher fat diet may be warranted as a means of decreasing one’s craving for sugar and sweet foods.
Effect of Exercise of on Fat Oxidation
One of the most useful measures of an individual’s metabolism is something called their respiratory quotient (sometimes also referred to as respiratory exchange ratio or RER). The RER measures the relative contribution of fat and carbohydrate to energy production in the body. There are two types of RER measures that are used in research; one is fasting RER and the other postprandial (i.e. 2 hours after eating). Both have been shown to increase for at least 24 hours after an exercise session, but this is highly variable between individuals15-17, 19. Some people even at rest will derive most of their energy requirements from the oxidation of carbohydrate. This obviously means, such individuals don’t burn much appreciable fat. Conversely, studies have shown that endurance trained athletes tend to exhibit higher fat oxidation (i.e. lower RER) at rest than untrained individuals18. That being said, even in endurance-trained athletes, studies have shown a large variability in RER at rest and during exercise at different intensities19. But what can one do to lower their RER and increase the proportion of fat they burn?
Diet and the Respiratory Exchange Ratio
There are genetic factors such as muscle fibre type and proportion that affect RER, but one of the most powerful factors affecting it is the percentage dietary fat intake. It seems the body has an ability to adjust what type of fuel it predominantly uses for energy based on the macronutrient composition of one’s diet19. For example, higher fat diet tends to result in higher rates of fat oxidation19. The same is true with carbohydrate19. Low carbohydrate diets with proportionally higher fat intakes have received increasing supporting scientific for their ability to induce fat loss and better blood glucose control in individuals with obesity and insulin resistance20. This same diet has been associated with lower RER during exercise, which implies higher rates of fat oxidation20. If you have traditionally enjoyed a low fat and high carbohydrate diet, and have experienced less than optimal weight loss with your existing diet, a low carbohydrate high fat diet may be an option to consider. Individuals who have insulin resistance seem to benefit the most from a low carbohydrate-high fat diet21.
In closing, it’s evident that there are a lot of variables that affect how much weight one can expect to lose in response to a given exercise program. There is a large body of literature to support the notion that the body has a set ‘fatpoint’ which it protects by strictly regulating various body processes. These might include factors such as the reward value of food, compensatory decreases or increases in energy expenditure and compensatory changes in appetite. Whatever the case, it’s important to acknowledge that the individual fat loss response to exercise is highly variable. For this reason it’s important not to compare oneself with others, but rather focus on the factors under your control and make informed decisions about the diet and exercise you chose as part of your weight loss program.
- Katzel LI, et al. Effects of weight loss vs aerobic exercise training on risk factors for coronary disease in healthy, obese, middle-aged and older men. A randomized controlled trial. JAMA. 1995;274(24):1915-21.
- Snyder KA, et al. The effects of long-term, moderate intensity, intermittent exercise on aerobic capacity, body composition, blood lipids, insulin and glucose in overweight females. Int J Obes Relat Metab Disord. 1997;21:1180-1189.
- King NA, et al. Individual variability following 12 weeks of supervised exercise: identification and characterization of compensation for exercise-induced weight loss. Int J Obes (Lond). 2008;32:177-184.
- Bouchard C, Blair SN, Church TS, et al. Adverse metabolic response to regular exercise: is it a rare or common occurrence? PLoS One. 2012;7:e37887.
- Stephens NA & Sparks LM. Resistance to the beneficial effects of exercise in type 2 diabetes. Are some individuals programmed to fail? J Clin Endocrinol Metab. 2014;99:0000-0000.
- Huang J, et al. The anti-obesity effects of green tea in human intervention and basic molecular studies. Eur J Clin Nutr. 2014;68(10):1075-87.
- Church TS, et al. Effects of aerobic and resistance training on haemoglobin A1c levels in patients with type 2 diabetes: a randomised controlled trial. JAMA. 2010;304:2253-2262.
- Sparks LM, et al. Nine months of combined training improves ex vivo skeletal muscle metabolism in individuals with type 2 diabetes. J Clin Endocrinol Metab. 2013;98:1694-1702.
- Qi Q, et al. Insulin receptor substrate 1 gene variation modifies insulin resistance response to weight-loss diets in a 2-year randomized trial: the Preventing Overweight Using Novel Dietary Strategies (POUNDS LOST) trial. Circulation. 2011;124(5):563-571.
- Papazoglou D, et al. Serotonin receptor 2A -1438G/A promoter polymorphism in relation to obesity and response to sibutramine. Genet Test Mol Biomarkers. 2012;16(2):109-12.
- Ng MC & Bowden DW. Is genetic testing of value in predicting and treating obesity? N C Med J. 2013;74(6):530-3.
- Goni L, et al. A genetic risk tool for obesity predisposition assessment and personalized nutrition implementation based on macronutrient intake. Genes Nutr. 2015;10(1):445.
- Qi L. Gene-diet interaction and weight loss. Curr Opin Lipidol. 2014;25(1):27-34.
- Finlayson G, et al. Low fat loss response after medium-term supervised exercise in obese is associated with exercise-induced increase in food reward. Journal of Obesity. 2011; 2011 pii: 615624.
- Votruba SB, et al. Prior exercise increases subsequent utilization of dietary fat. Med Sci Sports Exerc. 2002;34:1757-1765.
- Burton FL, et al. Energy replacement attenuates the effects of prior moderate exercise on postprandial metabolism in overweight/obese men. Int J Obes (Lond). 2008;32:481-489.
- Hansen K, et al. The effects of exercise on the storage and oxidation of dietary fat. Sports Med. 2005;35:363-373.
- Romijn JA, et al. Strenuous endurance training increases lipolysis and triglyceride-fatty acid cycling at rest. J Appl Physiol. 1993;75:108-113.
- 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.
- Wycherley TP, et al. Long-term effects of a very low-carbohydrate weight loss diet on exercise capacity and tolerance in overweight and obese adults. J Am Coll Nutr. 2014;33(4):267-73.
- Maekawa S, et al. Retrospective study on the efficacy of a low-carbohydrate diet for impaired glucose tolerance. Diabetes Metab Syndr Obes. 2014 Jun 13;7:195-201.