While the prospect of living a healthier life certainly motivates a lot of people to engage in some type of regular physical activity, most gym goers also work out because they want to improve their physical appearance.
Working as a personal trainer has taught me that the number one goal in the fitness scene is to lose weight, but how effective is exercise really at promoting fat loss? In this paper I look at the relationhsip between exercise and weight loss!
Table of contents
- The history of physical activity and weight loss
- How does physical activity affect bodyweight?
- Physical activity, increased appetite and food intake
- Exercise and weight gain
- Does exercise promote fat loss?
Physical activity is considered an important part of a healthy lifestyle, and studies show that regular physical activity can help prevent several chronic diseases (Warburton, Nicol & Bredin, 2006). Conventional wisdom also entails that regular exercise is effective when trying to lose or maintain weight.
The first law of thermodynamics states that changes in the body’s energy stores equals energy in minus energy out. This implies that individuals who are eating a calorie restricted diet will experience additional weight loss when they engage in some type of physical activity that increases energy expenditure (Novella, 2008).
It is often suggested that calorie restriction is necessary to lose weight, and that someone who eats whenever desired and as much as desired (ad libitum) won’t lose any weight by exercising more. This notion is based on the belief that the increased energy expenditure during exercise, elevated energy metabolism after exercise and increased basal metabolic rate results in more calories consumed; thereby compensating for the calories burned during exercise (Taubes, 2007a).
The human body adapts in order to make it more effecient at handling various types of stimulus. Weight training promotes muscle growth and strength gains, and it’s often assumed that aerobic exercise induces fat loss so the body can improve it’s aerobic performance.
Despite the general belief that exercise is effective for someone who’s trying to lose weight, studies on isolated exercise or a combined approach of both exercise and diet show mixed results. It’s well established that diet is the most important modifiable factor in regards to weight loss, but is exercising for weight loss worth the sweat?
Reviews, systematic reviews and meta-analysis available on PubMed and The Cochrane Library were included in this short paper. The literature was used to investigate the metabolic improvements achieved through physical activity, compensatory mechanisms to exercise and the effects of exercise on body composition, fat loss and overall weight loss. Individual randomized controlled trials have also been studied to understand the individual responses to exercise and varying results between different studies.
Search terms include:
(exercise OR physical activity) AND (epoc OR metabolic rate)
(exercise OR physical activity) AND (metabolic OR leptin OR insulin OR inflammation)
(exercise OR physical activity) AND (appetite OR compensatory mechanisms OR hunger OR food intake)
(exercise OR physical activity) AND (fat loss OR weight loss OR bodycomposition OR bodyweight)
The purpose of this short paper is to investigate the history behind physical activity and weight loss, how physical activity influences body composition, the compensatory mechanisms to exercise and how effective aerobic and anaerobic exercise is at promoting weight loss in humans.
The history of physical activity and weight loss
In the 1950s, the French-American nutritionist Jean Mayer was one of the first to propose a relationship between physical activity and weight management. Prior to the 1950s there were few studies done on the relationship between bodyweight and physical activity, and few overweight and obese individuals were recommended to exercise more. Jean Mayer showed through his studies that people with a sedentary lifestyle usually had a higher body mass index, and he quickly became a leading spokesperson in the field of fitness and public health. Since then, physical activity has become synonymous not just with a healthier life, but with a leaner physique, and most medical doctors, personal trainers and health and fitness “gurus” recommend exercise for weight loss. However, the scientific literature is showing mixed results and several researchers argue that the link between a sedentary lifestyle and overweight doesn’t prove causality (Taubes, 2007b).
How does physical activity affect bodyweight?
Aerobic exercise includes light-moderate intensity activities that depend on oxygen to meet energy demands during training. While aerobic activities such as jogging and cycling can be sustained for long periods of time, anaerobic, high-intensity activities such as sprints and weight-lifting recruit more fast twitch muscle fibers and involve shorter work periods. Most activities depend on both anaerobic and aerobic energy (Novella, 2008).
Both regular aerobic and anaerobic activity potentially influence bodyweight by several different mechanisms. Energy expenditure during exercise is often considered the main component in regards to weight loss, but research shows that other aspects of physical activity might be equally as important.
Excess post-exercise oxygen consumption (EPOC) refers to the body’s increased rate of oxygen consumption after exercise in an attempt to return itself to the pre-exercise state. The magnitude and duration of EPOC depends on the intensity and duration of the exercise, and also training status and gender (Borsheim & Bahr, 2003). EPOC and increased metabolic rate following both aerobic and anaerobic exercise further increase the total energy expenditure achieved from training (Knab et al., 2011; LaForgia, Withers & Gore, 2006; Speakman & Selman, 2003).
Both regular aerobic and anaerobic physical activity can also influence “long-term” basal metabolic rate by several different mechanisms (Singh, Dureha, Yaduvanshi & Mishra, 2010; Speakman & Selman, 2003). Resistance training promotes muscle growth, and the increased muscle mass results in a higher resting energy expenditure. Increased muscle mass will also lead to a higher calorie expenditure in the hours after exercise, most likely due to the elevated protein synthesis.
Physical activity indirectly influences body composition by improving insulin sensitivity (Balkau et al., 2004; Borghouts & Keizer, 2000; Venables & Jeukendrup, 2008), reducing serum leptin levels (Hulver & Houmard, 2003; Jiménez-Pavón et al., 2012; Pasman, Westerterp-Plantenga & Saris, 1998) and preventing oxidative stress and inflammation (Abramson & Vaccarino, 2002; Szostak & Laurant, 2011). Physical activity also promotes other metabolic improvements that might be important in regards to weight loss.
Insulin is a hormone that allows cells throughout the body to take up glucose from the blood. Insulin resistance occurs when cells don’t respond adequately to insulin, and it involves elevated levels of blood glucose. There’s a widely held belief that elevated insulin (hyperinsulinemia) and insulin resistance cause excess fat storage, but insulins role in overweight and obesity is still debated among obesity researchers (Guyenet, 2011a, 2011b; Masterjohn, 2011). Insulin resistance seems to correlate with body mass index, but this doesn’t necessarily mean that insulin sensitivity is as as important in regards to fat storage as previously thought (Esteghamati et al., 2008).
Leptin is produced by the bodys fat cells and allows for the brain to monitor the size of the fat stores and regulate them by varying appetite and energy expenditure. Leptin resistance is a condition where leptin produces a smaller response at the receptors in the brain. Leptin resistance is thought to trigger hunger and reduced energy expenditure, which is the bodys attempt to increase leptin production and overcome leptin resistance (Spreadbury, 2012). Leptin resistance is often seen in overweight and obese individuals and essentially involves having a malfunctioning energy homeostasis (Maffei et al., 1995; Rosenbaum et al., 2005; Spreadbury, 2012).
Obesity is characterized by low-level chronic inflammation and oxidative stress. Inflammation sets the stage for insulin resistance, diabetes and obesity, and reducing inflammation can be helpful when trying to lose weight (Cani et al., 2007; de Heredia, Gómez-Martínez, & Marcos, 2012; Gregor & Hotamisligil, 2011; Lumeng & Saltiel, 2011; Sankhla et al., 2012).
Physical activity, increased appetite and food intake
According to the first law of thermodynamics, changes in the body’s fat mass can only happen if there is a difference between the energy consumed and the energy expended. In other words, to lose weight it’s necessary to either increase energy expenditure, decrease energy intake or both in order to create an imbalanced energy equation (Novella, 2008).
The common belief is that physical activity promotes weight loss by increasing energy expenditure and thereby creating an imbalanced energy equation. However, sceptics argue that energy intake is adjusted according to energy expenditure and that more physical activity results in hunger and increased food consumption. In other words, “building up an appetite” from exercise might lead to more foods eaten the following hours or days after training. It’s also possible that individuals who exercise get “fatigued” and end up being less active during the day.
A newly published review by King et al. (2012) looked at how physical activity influences eating behaviour and food intake. Studies show varying results, but the general message is that acute exercise-induced energy deficit results in no or only partial calorie compensation. Individuals respond differently, and while some people respond to exercise by eating more; others actually eat less.
There are several physiological, neural and psychological factors influencing the compensatory mechanisms to exercise. The type of exercise performed is also important, and aerobic exercise seems to suppress hunger more than resistance exercise (Broom, Batterham, King, & Stensel1, 2008; Laan, Leidy, Lim, & Campbell, 2012).
Some individuals feel that they are allowed to eat unhealthy foods and increase their calorie intake because they exercise more. This approach easily leads to overeating since most people tend to overestimate the amount of calories burned during exercise (Willbond, Laviolette, Duval, & Doucet, 2010). However, this can also happen the other way around when the trainee tries to change his or her lifestyle completely by exercising more and eating healthier.
While most previous studies have focused on appetite hormones and exercise, new research shows that physical activity alters the food-reward network in the brain; thereby influencing how we respond to food. Some individuals experience increased reward value from food after exercise, while others display less interest in food following physical activity (Evero, Hackett, Clark, Phelan, & Hagobian, 2012; Finlayson et al., 2011).
While a certain amount of physical activity is necessary to produce statistically significant changes, one newly published study shows that a moderate amount of aerobic exercise (300 kcal/day) results in the same fat loss as a high amount of aerobic exercise (600 kcal/day). The monitor sensors showed that the group who exercised for 60 minutes each day, compared to 30, had less energy during the rest of the day; thereby expending fewer calories outside the gym. The food diaries were also self-reported, and the group who exercised for 60 minutes each day could have eaten more than they wrote down. More studies are needed to show if these results are reproducable and if they are mantained long-term (Rosenkilde et al., 2012).
More long-term studies on the activity in the food reward system, hormones and appetite in the hours and days after exercise are needed to get a better understanding of compensatory mechanisms.
Exercise and weight gain
Heavy resistance training promotes muscle growth, and weight gain will be determined by energy expenditure versus energy intake. A common practise is to consume excess calories as a means to assure optimal muscle growth and anabolic state. This “bulking”-phase results in both muscle growth and fat gain, and total bodyweight increases. It’s also possible to gain lean body mass (the mass of the body minus the fat (storage lipid)) while still losing weight, and the increase in lean body mass seems to correlate negatively with the rate of the weight reduction (Demling & DeSanti, 2000; Garthe, Raastad, Refsnes, Koivisto, & Sundgot-Borgen, 2011).
Aerobic exercise rarely promotes weight gain, but the review by King et al. (2012) shows that a small percentage of people gain weight when they are adhering to a training program with a lot of aerobic exercise. If this weight gain is due to compensatory mechanisms or some type of metabolic alteration remains unclear.
Obesity is characterized by increased intestinal permeability and different gut flora composition compared to lean individuals (Cani et al., 2007; Frazier, DiBaise, & McClain, 2011; Moreno-Navarrete, Sabater, Ortega, Ricart, & Fernández-Real, 2012). Inflammation is a main component in most chronic diseases, and it can be hypothesized that a dysfunctional gut flora and “leaky gut” are important proinflammatory conditions present also in many non-obese individuals.
The effect exercise has on the gut microbiota is largely unknown, but exercise has been shown to affect gut permeability. Although exercise has been shown to be anti-inflammatory (Abramson & Vaccarino, 2002; Szostak & Laurant, 2011), the stress and heat during prolonged and high-intensity exercise might disrupt intestinal epithelial cell tight junction protein (Lamprecht & Frauwallner, 2012; Zuhl et al., 2012). It’s likely that individuals who already have a dysfunctional gut flora (e.g., with increased numbers of LPS-containing bacteria) and decreased gut barrier function are more susceptible to the effects of prolonged, strenuous exercise.
Does exercise promote fat loss?
There are hundreds of studies done on physical activity and fat loss, and a review by Miller et al. (1997) looked at all the research done between 1969 and 1994 that met the criteria. Most of the studies in that time period were done on the moderately obese and middle aged population. The meta-analysis showed that a 15-week intervention led to an average weight loss of 7 pounds with physical activity alone, 17 pounds with diet alone and 20 pounds with both exercise and physical activity. These results show that a combined approach of both exercise and diet results in somewhat greater weight reduction than diet alone. A newer meta-analysis by Wu, Gao, Chen & van Dam (2009) also support these findings.
Studies show that some people can be labelled as “responders”, while others experience little weight loss with physical activity alone. One of the studies reviewed by King et al. (2012) showed that 12 weeks of supervised exercise designed to burn a total of 2500 calories per week led to an average weight loss of 3.2 kg. While all of the test subjects performed identical exercise routines some lost more than 10 kg’s, while others actually gained weight. Hence, individual responses to exercise and compensatory mechanisms seem to determine exercise-induced weight loss.
A systematic review by Thorogood et al. (2011) looked at the randomized controlled trials done on isolated aerobic exercise (no dietary intervention) and weight loss. The results after reviewing 14 trials involving 1847 patients suggest that isolated moderate-intensity aerobic exercise only induce a modest weight reduction. Isolated aerobic exercise only resulted in an average weight loss of 1.6 kg after six months, and an additional 6 months of aerobic exercise didn’t results in any additional weight loss. Aerobic exercise, with no dietary intervention, is for most people not an effective weight loss strategy, and it can be hypothesized that the calories burned from exercise leads to increased food intake.
High-intensity intermittent exercise protocols usually involve repeated brief sprinting at an all-out intensity followed by rest or low-intensity exercise. While regular aerobic exercise usually involves longer workouts on a steady-state intensity, HIIE consists of short work and rest periods (5 s to 4 min). Compared to regular aerobic exercise, studies show that HIEE promote marginally greater weight loss in obese and overweight individuals (Boutcher, 2011; Lee, Park, Kim, Choi, & Kim, 2012).
It’s difficult to adjust the intensity and volume of resistance training in order to accurately compare it to aerobic exercise. Heavy resistance training results in muscle growth, but usually a smaller caloric burn during exercise than aerobic training. Studies with the purpose of comparing these two types of exercise protocols show mixed results, but aerobic exercise seems to be slightly more effective at promoting fat loss (Ismail, Keating, Baker, & Johnson, 2012; Slentz et al., 2011).
Prior to the 1950s there were few health practitioners who believed that weight loss could be achieved through exercise. Studies done by the French-American nutritionist Jean Mayer showed a connection between sedentary lifestyle and higher body mass index. The notion that expending energy could lead to weight loss was easy to believe, and exercising for fat loss became popular despite the fact that few good studies had been done (Taubes, 2007a, 2007b).
Physical activity promotes weight loss by several different mechanisms. Increased energy expenditure during exercise, excess post-exercise oxygen consumption and muscle growth are some of the factors resulting in an increased basal metabolic rate and elevated energy expenditure (Borsheim & Bahr, 2003; Knab et al., 2011; LaForgia et al., 2006; Speakman & Selman, 2003; Singh et al., 2010).
Regular physical activity improves insulin sensitivity, reduce leptin levels, helps prevent oxidative stress and inflammation and promote other metabolic improvements (Abramson & Vaccarino, 2002; Balkau et al., 2004; Borghouts & Keizer, 2000; Hulver & Houmard, 2003; Jiménez-Pavón et al., 2012; Pasman, 1998; Szostak & Laurant, 2011; Venables & Jeukendrup, 2008; Westerterp-Plantenga & Saris, 1998).
It’s difficult to estimate the varying importance of the mentioned factors in regards to weight loss, but it’s clear that the actual caloric burn isn’t the only weight loss-benefit achieved from aerobic and anaerobic exercise.
Physical activity seems to only result in partial energy compensation from food. However, individual’s respond differently, and while some people increase their energy intake following exercise, others actually seem to eat less. Studies also show that some individuals who exercise end up compensating for the “caloric burn” by being less active throughout the rest of the day. Both physiological, neural and psychological factors seem to influence compensatory mechanisms, but more research is needed to establish why and how people react differently to exercise (Broom et al., 2008; Evero et al., 2012; Finlayson et al., 2011; King et al., 2012; Laan et al., 2012).
The amount of exercise performed seems to also influence compensatory mechanisms to exercise, and one newer study shows that the group who performed a high amount of exercise ended up spending less energy the rest of the day and most likely increased their food intake (Rosenkilde et al., 2012).
Heavy resistance training promotes muscle growth, and if bodyfat remains unchanged or increases, sticking to a heavy resistance training program will result in weight gain.
Aerobic exercise can also promote weight gain (King et al., 2012), but whether this increase in bodyweight is due to compensatory mechanisms or metabolic alterations remains unclear. Prolonged, high-intensity exercise potentially disrupt gut barrier function (Lamprecht & Frauwallner, 2012; Zuhl et al., 2012) and therefore initiates a vicious cycle of inflammation, leptin resistance, insulin resistance and weight gain.
Using both diet and exercise seems to be the most effective approach for individuals who want to lose weight. However, the additional weight loss from a combined approach is only marginal compared to diet alone. Both aerobic and anaerobic exercise without diet only cause a modest weight reduction. People respond differently to exercise and while some individuals lose a lot of weight by engaging in some type of regular physical activity, others experience no weight reduction (Miller et al., 1997; King et al., 2012; Thorogood et al., 2011; Wu et al., 2009).
Some studies show that high-intensity intermittent exercise results in greater weight loss than standard aerobic exercise (Boutcher, 2011; Lee et al., 2012).
Aerobic exercise seems to promote slightly better weight loss compared to resistance training, but it’s difficult to accurately measure the two (Ismail et al., 2012; Slentz et al., 2011).
Health practitioners at each end of the spectrum either claim that exercise is useless for weight loss or that exercise is great for weight loss. Studies show that they might both be right; some people respond to exercise with a substantial fat reduction while others respond poorly (King et al., 2012). Whether this discrepancy is due to actual individual variability or the difficulty controlling compensatory mechanisms in these types of studies is still unclear.
Several issues arise when the benefits of physical activity are measured over several months in human subjects. While it’s possible to give specific lifestyle instructions it’s still difficult to control additional activity during the day, compensatory mechanisms, sleep etc. These “issues”, and the fact that people respond differently to exercise results in different results depending on the study.
Epidemiological studies linking sedentary lifestyle and obesity have made exercise synonymous with a lean physique (Taubes, 2007a, 2007b). However, studies show that although regular aerobic and anaerobic exercise results in several health improvements, they only seem to induce modest weight loss in most people (Miller et al., 1997; King et al., 2012; Thorogood et al., 2011; Wu et al., 2009).
The actual energy expenditure during a regular workout is quite low compared to what is achievable through diet, and some individuals seem to compensate for the “calorie burn” by increasing their food intake (Broom et al., 2008; Evero et al., 2012; Finlayson et al., 2011; King et al., 2012; Laan et al., 2012).
The scientific literature suggests that weight loss plans should focus on diet, and exercise prescription should be based on individual response to physical activity.
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