Whether it's 'calorie-burning' foods or supplements, or workouts that are marketed to 'torch calories', one thing remains constant: most of us think we need to, and are able to, burn a large amount of calories through things we can control. However the amount of calorie burning that we can control may actually be much smaller than we realize.
Recall from my post on calorie balance for weight loss that a calorie is a unit of energy that is required by the body. We take in calories through food (in the form of the macronutrients: protein, carbohydrates, and fat) and we expend (or 'burn') calories everyday through:
Our resting energy expenditure (REE). This makes up the majority of our calories being burned to just keep us alive and functioning. This can change based on age, size, thyroid hormones, protein turnover and body composition (like the amount of fat-free/lean body mass). (McArdle, et al. 2015)
The thermic effect of food. This is the energy it takes to actually digest and metabolize foods (protein for example has the highest thermic effect of the macronutrients, so it requires more energy to digest.)
Physical activity. This is the most variable mode of calorie burning as it can change based on the individual and the activity.
We have little control over our REE and the thermic effect of food but we can control our level of physical activity, which can equate to 15-30% of our total daily energy expenditure. (McArdle, et al. 2015) Factors like body size, gender and age can also play a role in increasing daily energy expenditure. *It should be noted that we therefore REQUIRE a certain amount of calories everyday to meet our needs. With increased physical activity and increased fat-free mass (think lean muscle mass), REE increases as well, i.e. having more lean body mass means you burn more calories at rest and thus require more calories taken in to meet these increased needs. (McArdle, et al. 2015)
So if physical activity can contribute 15-30% of calories being burned, that would mean that exercising more would burn a larger portion of calories right?
Not quite.
What is largely assumed is that because of our ability to control our level of physical activity, that we can also control the amount of calories being burned from physical activity (the mentality that more exercise, or higher intensity, will equal greater calorie burning). However this is not exactly true: calories are a precious commodity to the body (they are your fuel source) and so when you take in calories from eating food, you either USE them or STORE them (you do not simply 'get rid of them'). This same principle applies to exercise where you will burn calories due to increased requirements (you need to use energy from somewhere to fuel your activity) but your body is not simply going to 'rid itself of excess calories'. In fact, the amount of calories you are able to burn during exercise is going to depend on your body composition, the activity, your current fitness level (the difficulty of the task for you personally) and most importantly, your REE. If your REE is increased, you will likely lose comparatively LESS calories through exercise (as compensation so that you are not drastically losing that precious energy that you need to maintain your REE).
For example:
thermic effect of food= 10% of daily energy expenditure
*REE=60-75% of daily energy expenditure
*Physical activity= 15-30% of daily energy expenditure
If an individual has a higher REE (like 75% of calories being burned everyday are to maintain functions and body tissues) then they can only 'afford' to contribute 15% of calories being burned through physical activity. Whereas an individual with a lower REE like 60% could 'afford' to contribute 30% of calories being burned through physical activity. **This essentially means that the higher REE you have (like from having a high amount of fat-free/lean body mass), the more calories you are burning at rest, and the LESS likely you will burn high amounts of calories through physical activity.
When it comes to exercise...
There are some findings to suggest that exercise can increase your REE in the short-term (approximately 22 hours following exercise) likely due to increased energy demands for protein repair and synthesis, and increased sympathetic tone. (Hunter, et al. 2017) [It should be noted that resistance exercise can be associated with increasing your REE in the long-run due to increases in fat-free mass.] According to this study, participants exhibited an acute increase in REE over the 22 hours following exercise by an average of over 100 calories for high intensity interval training, and an average of over 60 calories for moderate intensity continuous aerobic exercise. (Hunter, et al. 2017) It has also been suggested that exercise can increase REE due to changes in fat-free mass where there is likely to be increased energy expenditure with increased amounts of fat-free mass. **By exercising, you can increase your calorie burning for that day by up to 100 calories while at rest in addition to the calories burned during the exercise itself. However over time, your ability to burn a high amount of calories from exercise will likely decrease as you gain lean body mass (as this change in body composition will increase your REE).
So how much are you likely burning during exercise?
You may see claims of certain workouts burning 500-1000 calories or devices that track calorie burning during exercise. These claims or devices should not necessarily be taken as truth however as there is a wide variability in personal calorie burning potential. In actuality, calorie burning during exercise (let's say 30-60 mins of variable intensity with variable loads) may likely be much closer to only a couple hundred calories, at most.
Most apps or devices use equations to estimate calorie burning based on a population standard. Examples of these estimates for calorie-burning potential include:
In males: 2-5 cal/min for light exercise up to 7.5-10 cal/min for heavy exercise
In females: 1.5-3.5 cal/min for light exercise up to 5.5-7.5 cal/min for heavy exercise
*These estimates are based on an average population standard for body size (120LB FEMALE OR 140LB MALE!) and fitness level.
According to these estimates, a 120lb female performing 30 minutes of heavy exercise or 60 minutes of moderate exercise could potentially burn between 180-240 calories. (McArdle, et al. 2015)
HOWEVER...
An issue with estimating calorie burning based on a population standard: it does not take into account differences in body size and composition, nor efficiency of movement patterns. For example, an individual who is highly efficient in a particular exercise or movement pattern is likely going to expend less energy compared to an individual who is new to the task (and thus introducing a new stress to the body). When it comes to a population standard, there is no quantitative way to match YOUR abilities (and efficiency of movement) with the standard. In this case, an app or program that calculates your energy expenditure based solely on WHAT you did (for example, a 30 minute run) is using a population standard (one body size and athletic ability based on the population average) to derive an estimated amount of calories burned. This is NOT the same as the actual amount that you would have burned as could have been calculated by your oxygen consumption, for example. (McArdle, et al. 2015) What's the big takeaway here: unless you are directly measuring your energy expenditure (like your oxygen consumption) during physical activity, chances are the app you are using to estimate your calorie burning is just that: an estimation, specifically based on a population standard and not you personally. **If you don't fall into the average population size and athletic ability, then those estimates from devices and apps may not apply to you.
What about the type of exercise?
You may have heard the statement that it does not matter how fast you run a distance, you burn the same amount of calories based on the total distance (so walking 2 miles burns the same amount as running 2 miles). Well, yes and no: if calculating calorie burning with an equation (based on a population standard) you would find similar results. For example, a 160lb adult walking 2 miles in one hour could burn 192 calories but if they walked 2 miles in 30 minutes (twice the speed) they could burn 174 calories.
However, as speed increases (going from walking to running) walking economy decreases (having to breath heavier and harder= greater oxygen consumption) which leads to greater energy expenditure. (McArdle, et al. 2015) The caveat to this is that maintaining that higher output is more difficult, and thus will likely lead to a shorter workout duration, but it is argued that the real benefit comes from the potential for increased energy expenditure after exercise. To this point, it's not the distance that is dictating the energy expenditure, but the effort. (This effort comes from not just level of intensity, but also resistance, and experience like in trained vs untrained individuals.)
Additionally, there are study findings to suggest that circuit resistance training with moderate weights for higher reps equates to greater overall energy expenditure compared to higher loads (heavier weights) used for fewer reps. (Roberson, et al. 2017) In this case, while using heavier weights will utilize more energy during the exercise itself (compared to the energy required to lift moderate weights) it is the overall amount of time spent lifting (higher reps with moderate weight= more time under tension vs lower reps at heavier weight) which ultimately was associated with higher overall energy expenditure. (Roberson, et al. 2017) In this study, the subjects showed an average of 142 calories burned for the heavy load exercise compared to 152 calories for moderate load exercise (for a circuit weight training workout that lasted ~16-20 minutes). (Roberson, et al. 2017)
What about activity level throughout the day, other than deliberate exercise?
It has been noted that non-exercise physical activity (meaning non-deliberate exercise like household chores, locomotive transportation, fidgeting, etc) may contribute a greater amount to total daily energy expenditure than just exercise alone. (Drenowatz, et al. 2015) [This means that making small lifestyle changes like taking the stairs, biking to work, using a standing work station, etc can ultimately benefit weight loss and weight maintenance in the long-run, more-so than the relatively short amount of time spent completing a deliberate workout.] This study looked at overall physical activity where undergoing aerobic exercise was associated with greater energy expenditure (an average of 440 calories for 1 hour) during exercise but less non-exercise physical activity throughout the day compared to resistance exercise (an average of 240 calories). (Drenowatz, et al. 2015) Resistance exercise however, despite being associated with comparatively lower energy expenditure during exercise, was associated with greater non-exercise physical activity on non-exercise days (an average of 200 calories), which may be due to an increase in fitness (i.e. less feelings of fatigue). (Drenowatz, et al. 2015) **This means that in those who engaged in aerobic exercise, they exhibited higher calorie burning DURING exercise compared to resistance training, but LESS overall physical activity (and further calorie burning) for the rest of the day. Conversely, the resistance training showed higher calorie burning through non-exercise physical activity, particularly on non-exercise days ('rest days').
If we add everything up so far, that's still an average of a couple hundred calories (~200-400) being burned from BOTH exercise AND activity throughout the day (as a side effect of exercise)... not quite the 500-1000 calorie range like some health and fitness claims.
What about exercising in the 'fat-burning zone'?
Let's touch for a bit on where your energy (calories) are coming from when you are exercising:
Your available energy sources can be classified as immediate, short-term, and long-term.
The immediate energy available is from ATP and CP (creatine phosphate) that are already present in the cell, ready to be utilized. This is primarily for initial movement and maximal effort, for about 6 seconds. (This is also where creatine supplementation can play a role in increasing the available amount that is present for sprint or explosive movements.)
However, this is only a small amount to get you going, so you quickly switch over to your short-term energy (muscle glycogen, like from carbohydrates) which produces ATP quickly (before oxygen is even present in the cell). This is called anaerobic, or 'without oxygen', and it occurs during effort that is largely 10 seconds to 2 minutes in length.
Then comes the long-term energy source which uses oxygen to convert intermediates from both glycogen and fatty acids into ATP for energy. (This is termed aerobic respiration and it steadily picks up after 1-2 minutes of work, onward.) **Fatty acids are predominantly oxidized for fuel during rest and at low to moderate intensity exercise, but fatty acid oxidation decreases with increasing intensity. (Watt and Cheng, 2017)
The sources of fatty acids used for energy production can come from:
adipose tissue (what we typically think of as fat tissue)
triglycerides in circulating lipoproteins
intra-muscular triglycerides. (fats stored IN the muscles)*
Now you may be utilizing fatty acids for energy during physical activity, but this does NOT necessarily mean that these fatty acids are coming from your adipose tissue (like excess body fat on your abdomen or other 'trouble spots'). What IS largely contributing to these fatty acids being oxidized during physical activity is intra-muscular triglycerides (fats that are stored IN the muscle cells so that they are present in the location where they are needed). (Watt and Cheng, 2017) As an adaptation to training, an individual (especially women) may store fatty acids preferentially as intra-muscular triglycerides. (Watt and CHeng, 2017) It is therefore hypothesized that over time from exercise, you may preferentially store fatty acids in your muscles where you can use them for exercise (before storing them in adipose tissue). (Watt and Cheng, 2017) But this also means that working out in a 'fat-burning zone' is just that: you are burning fatty acids (primarily from your muscles) during exercise: this is NOT synonymous with burning off belly fat or any other stored adipose tissue.
What about 'calorie-burning' foods and supplements?
We've all seen those 'health articles' with lists of 'fat-burning foods' or supplements marketed as 'fat burners'. Maybe you have heard about grapefruit, capsaicin from peppers, or drinking lemon water or apple cider vinegar to help with weight loss. Here is the main point to know about these claims: the thermogenic effects of these foods or supplements are NOT easily defined NOR SUPPORTED BY EVIDENCE. As there is a general lack of evidence for most of these claims, this post will only focus on the two dietary thermogenic aids that do have substantial evidence (caffeine and EGCG/green tea).
Most dietary supplements marketed for weight loss include caffeine and 'green tea extract' (or EGCG, cathechins) due to their thermogenic effects. There are some theories to suggest that caffeine may increase REE by up to 5% (which may equate to burning 100 calories more for the day) and possibly have effects on decreasing energy intake (like appetite suppression) and increasing non-exercise energy expenditure. (Schubert, et al. 2014) In this trial, individuals were given either a placebo or 3mg caffeine/kg body weight (for a 160lb adult, this would be roughly 220mg caffeine, or the equivalent of 2 cans of Red Bull) in doses prior to and after exercise. (Schubert, et al. 2014) According to this trial, exercise with caffeine was associated with greater energy expenditure (about 100 calories more) and fat oxidation (likely intra-muscular triglycerides) than just exercise alone while both the placebo+exercise group and the caffeine+exercise groups experienced decreased appetite. (Schubert, et al. 2014) Another study that looked at various doses of pre-exercise caffeine consumption found that only the highest dose of 4.5mg/kg body weight was associated with a significant increase in caloric expenditure during exercise (of an additional 30 CALORIES) compared to placebo. (Fernandez-Elias, et al. 2015) Yes you read that correct: only the highest dose of caffeine was significant (by burning an extra 30 calories). In this study, caffeine intakes of 0mg (placebo), 0.5, 1.5, 3.0, and 4.5mg/kg were tested but only the highest dose showed a significant increase in energy expenditure (this means statistically significant, as many of us would not deem 30 calories to be significant!). (Fernandez-Elias, et al. 2015) This dose is the equivalent of roughly 350mg caffeine in a 160lb adult, or 3.5 cups of coffee. Another study found similar results where a high dose (5mg/kg of fat free mass) was associated with an increased energy expenditure during exercise of approximately 1kcal/minute (meaning 30 calories for a 30 minute workout). (Turkozu and Tek, 2017) This study review also found that thermogenic effects were slowed with increasing age and obesity. (Turkozu and Tek, 2017)
Green tea contains catechins, specifically EGCG, that is boasted as being a thermogenic aid with some theories that green tea extract can increase energy expenditure and oxidation of brown adipose tissues. (Turkozu and Tek, 2017) (It should be noted that brown adipose tissue is considered more metabolically active compared to white adipose tissue, but it is poor prevalent in children with adults typically having more white than brown.) There are mixed results however as to whether EGCG specifically is associated with increasing thermogenesis or whether it is due to the caffeine content, possibly in conjunction with the EGCG. As with the studies on caffeine intake, high doses of caffeine+EGCG were associated with increases in energy expenditure (approximately 100 calories) for a 24 hour period. (Turkozu and Tek, 2017) However, there are contradictory theories that green tea (and green tea extract used in many weight loss supplements) may not be increasing the burning of fat calories but rather just promoting lipolysis (the release of fatty acids from storage into the circulation). To this theory, whether or not you actually BURN those fatty acids (B-oxidation) is dependent on the individual (as in engaging in physical activity). **Notice how these dietary thermogenic aids would need to be taken in HIGH DOSES to exhibit these effects (of increasing calorie burning by about 100 calories/day). This is where supplement claims try to take over as it is likely unrealistic to ingest this much coffee or tea to reach these doses. HOWEVER, one giant red flag with supplements: THEY ARE NOT REGULATED. This means that there is ABSOLUTELY NO assurance that what is written on the label is actually present in the product you are taking. Add to this the fact that most supplements may list a 'proprietary blend' that includes several, even dozens, of ingredients with no specified amounts, and now you really have no idea what, or how much, you're taking.
What is the big takeaway here?
Let's try to add everything up:
IF you were to burn ~100 calories from an increase in REE from exercise
IF you were to burn ~200-400 calories from the workout itself
IF you were to burn ~100 calories from taking a high dose caffeine + EGCG 'fat burning' supplements
We are looking at a proposed ~400-600 calories being burned for that day. *And this all depends on how high or low your REE is (to allow for higher calorie burning during exercise).
If weight loss, weight maintenance, or altering body composition is your goal, here is the big message: trying to rationalize that you 'earned' that pizza/beer/doughnut/etc because you worked out, or were planning on 'working off' that high calorie day or weekend, the chances are your efforts will NOT balance out for your high calorie intake. (Let's say a typical slice of pizza is about 250-300 calories, plus another 100-150 calories for 1 beer, and now you've equaled roughly what you worked hard to expend.) If you are focused on weight loss, weight maintenance, or altering body composition, focus NOT on how many calories you can potentially burn through exercise, but rather what you CAN control through your dietary intake, duration and intensity of exercise, and physical activity throughout the day. *The all too famous phrase: you can't out-work a bad diet!
References:
Drenowatz C, Grieve G, DeMello M. Change in energy expenditure and physical activity in response to aerobic and resistance exercise programs. SpringerPlus. 2015; 4(1): 798. https://www-ncbi-nlm-nih-gov.proxy.lib.fsu.edu/pmc/articles/PMC4688292/
Fernandez-Elias V, Del Coso J, Hamouti N, et al. Ingestion of a moderately high caffeine dose before exercise increases postexercise energy expenditure. International Journal of Sports Nutrition, Exercise, and Metabolism. 2015; 25(1):46-53. https://www-ncbi-nlm-nih-gov.proxy.lib.fsu.edu/pubmed/24901809
Hunter G, Moellering D, Carter S, et al. Potential Causes of Elevated REE following High-Intensity Exercise. Medicine and Science in Sports and Exercise. 2017; https://insights-ovid-com.proxy.lib.fsu.edu/pubmed?pmid=28737531
McArdle W, Katch F, Katch V. Exercise Physiology. 8th edition. Lippincott Williams and Wilkins. Philadelphia, PA. 2015.
Roberson K, Jacobs K, White M, et al. Loads and movement speed affect energy expenditure during circuit resistance exercise. Applied Physiology, Nutrition, and Metabolism. 2017; 42(6): 637-646. http://www.nrcresearchpress.com.proxy.lib.fsu.edu/doi/full/10.1139/apnm-2016-0552#.WXtryYjyt1s
Schubert M, Hall S, Leveritt M, et al. Caffeine consumption around an exercise bout: effects on energy expenditure, energy intake, and exercise enjoyment. Journal of Applied Physiology. 2014; 117(7): 745-754. http://jap.physiology.org.proxy.lib.fsu.edu/content/117/7/745.long
Turkozu D, Tek N. A minireview of effects of green tea on energy expenditure. Critical Reviews in Food Science and Nutrition. 2017; 57(2): 254-258. http://www.tandfonline.com.proxy.lib.fsu.edu/doi/full/10.1080/10408398.2014.986672
Watt M, Cheng Y. Triglyceride metabolism in exercising muscle. Biochimica et Biophysica Acta: Molecular and Cell Biology of Lipids. 2017; http://www.sciencedirect.com.proxy.lib.fsu.edu/science/article/pii/S1388198117301191