Fatmax: fat fact or fat fiction?

The loss of excess fat isn’t just aesthetic – it almost always produces enhanced performance

In most sports, the loss of excess fat isn’t just aesthetic – it almost always produces enhanced performance. Not only is power-to-weight ratio and agility improved, a lower body weight is also associated with reduced injury risk through reduced impact loading, especially in weight bearing sports such as running. The perennial question, of course, is how best to shed those excess pounds without resorting to ridiculous diets or training schedules, both of which can lead to muscle tissue (and therefore strength and power) loss, injury and even illness.

Before getting into the nitty-gritty of fat burning, it helps to understand a few basics that underpin it. And one of the most fundamental is the simple concept of energy balance. Body fat can be thought of as a form of stored chemical energy.

To lose body fat, you have to consume less energy than your body expends. Since each pound of body fat contains approximately 3,500kcals of energy, to lose a pound of body fat, you have to burn 3,500kcals more energy than is contained in the food and drink you consume. This can be achieved either by increasing your energy burn rate (i.e. by exercising more), or reducing your calorie intake (i.e. by following a calorie-restricted diet), or by a combination of the two.

These laws of chemistry and physics are immutable and they reveal just why the fat loss claims of some diets out there are complete nonsense. For example, consider an eating plan claiming to produce half a stone of weight loss per week. To achieve that amount of fat loss, you would need to take in 7 x 3,500kcals (= 24,500kcals) less energy per week than you currently consume. Given the average active male consumes around 3,000kcals per day (and women even less), this means that even if our mythical male dieter stopped eating totally for a week, he could only lose 21,000kcals of energy – i.e. 6lbs of fat. To lose 7lbs in a week would mean eating antimatter – something that’s in short supply here on planet Earth!

Of course, athletes are involved in training, which burns up energy, so are able to shift the basic energy equation more easily towards fat loss. There are also other "fat loss" benefits that flow from regular exercise (see Box 1). However, a natural question to ask is whether there are any proven nutritional and training approaches, which can make it even easier to burn more fat calories during exercise, making it easier to shed surplus fat.

Box 1: Exercise and fat burning

Although nutrition is important, as we shall see later, the number one weapon against fat is not a low-calorie diet, but aerobic–type exercise (cycling, running, swimming) and anaerobic exercise such as resistance training. This is for several reasons:

  • When you exercise, you burn up more calories and this helps you tip the basic fundamental laws of chemical thermodynamics in your favour (see above).
  • The right intensity of exercise not only increases the amount of calories you burn, but also shifts metabolic pathways in the body so that a greater proportion of calories burned are derived from fat.
  • Exercise helps regulate appetite, which in turn helps maintain the appropriate balance between "energy in" and "energy out". Without regular, vigorous exercise, many people lose the ability to naturally regulate their energy intake. This leads to food cravings, overeating and ending up on the "wrong" side of the energy balance equation!
  • Exercise increases the amount of lean muscle mass in the body. This is a good thing because the more lean muscle an athlete has the more calories are needed simply to tick over at rest (basal metabolic rate),which means more calories burned even when sitting or sleeping, in turn further tipping the energy balance equation in favour of fat loss. This also explains why some intense resistance exercise (e.g. weight training) can be beneficial for endurance athletes seeking to stay lean.

Optimum fat-burning intensity

The issue of optimum training intensity for burning fat as efficiently as possible is perhaps one of the most misunderstood concepts around. The confusion over the "best" intensity for optimum fat burning arises for two main reasons: firstly, the exercise intensity at which you burn the biggest proportion of calories as fat is not the same as the exercise intensity that burns the biggest number of fat calories. 

To illustrate this, suppose a moderately fit runner runs quite gently, burning around 400kcals per hour. At this intensity, he or she might derive 75% of their energy from fat – i.e. burn 300kcals of fat. But now supposing our runner ups the intensity to a calorie burn rate of 750kcals per hour. At this higher intensity, the proportion of calories derived from fat might drop to 60%. 

However, the actual number of fat calories burned by our cyclist rises to 60% of 750 – i.e. 450kcals per hour – 50% more than at the lower intensity.

Another confounding variable is that the exercise intensity at which you burn the biggest proportion of calories as fat may vary according to your fitness. In a series of studies conducted at the University of Birmingham, scientists sought to discover at what intensity the total amount of fat burned per hour was maximised – the so-called ‘Fatmax’ (see figure 1 by clicking the link below).

The scientists reported that exercise at moderate intensity (around 70-75% of maximum heart rate) was the optimal intensity for fat burning, whereas it was only around 60% of maximum heart rate for less trained individuals(1,2).

However, they also discovered that the inter-individual variation is very large. A trained person may have his or her Fatmax as high as 80% of max heart rate or as low as 55% of max heart rate.


Fatmax questions

Over the past decade, the concept of Fatmax – an optimum exercise intensity zone for fat burning (see above) – has become widely accepted. The Fatmax zone is assumed to be around 70-70% of maximum heart rate (MHR) for trained athletes and perhaps a bit lower (around 60-65% of MHR) for novice/less fit athletes. However, recent findings in this area suggest that it’s not quite as straightforward as we first thought. In particular, some scientists have pointed out that many of the earlier studies may be flawed because they didn’t use a wide enough range of intensities to determine Fatmax and that the typical exercise durations may have been too short.

A study published just a few months ago has looked into this more carefully. Researchers set out to find out the exercise intensity that produced the maximum amount of fat burning during longer-intensity (one-hour) rides in sixteen reasonably fit male cyclists(3). To do this, each rider first underwent an incremental test to exhaustion to find out his maximal oxygen uptake (aerobic) capacity – also known as ‘VO2max’. They then used conventional Fatmax calculations to determine the riding intensity that should have produced the highest rate of fat burning in the cyclists – around 60% of VO2max (roughly equivalent to around 70% MHR).

All the cyclists then completed three separate one-hour riding trials. One of these was at the calculated Fatmax intensity (60% VO2max), while one was at a significantly lower intensity (52% VO2max) with the third at a much higher intensity (70% VO2max). As well as collecting the expired air from the cyclists to determine how much fat they were burning in each trial, the researchers also recorded the cyclists’ heart rates and levels of blood lactate (a measure of muscular fatigue).

As expected, the heart rates and blood lactate measurements matched the riding intensities – i.e. they were lowest in the low-intensity trial and highest in the high-intensity trial. What surprised the researchers, however, was that the rate of fat burning was exactly the same at each of the three riding intensities. In other words, there was simply no evidence of an optimum ‘Fatmax intensity’ for burning fat! This study is just one and used a relatively small group of subjects. However, if these results are repeated in subsequent studies, our Fatmax assumptions could be thrown into question, with obvious implications for exercise intensity prescription. For example, given the rate of fat burning was sustained at just 52% of VO2max, perhaps it would be better to exercise for longer at a slower pace because this would maximise the amount of fat burned while keeping the total training load low. Likewise, athletes performing higher-intensity workouts can relax in the knowledge that they are still burning the same amount of fat per hour as in lower-intensity workouts.

As to why the amount of fat burned remained the same at such different riding intensities, one possible explanation is that at 52% of VO2, these riders had already reached their maximum fat burning capacity. Therefore the extra energy required to ride at the higher intensities had to be supplied from increased carbohydrate burning, rather than by increasing the rate of fat burning. 

The findings above actually tie in neatly with some 2007 research(4). In this study, ten healthy recreational athletes conducted an initial incremental cycling test to determine their VO2max (which averaged 59.2mls/min/kg – i.e. fairly fit) and their individual anaerobic thresholds (the exercise intensity at which exercise becomes anaerobic and fatiguing lactate begins to accumulate in the exercising muscles). Over the next four weeks, five constant-load tests of one hour’s duration were carried out at 55%, 65%, 75%, 85%, and 95% of the riders’ individual anaerobic thresholds. During each of these tests, the rate of fat oxidation was measured and the results compared.

It turned out that all the intensities led to similar absolute amounts of oxidized fat over the one-hour period. What was clear, however, was that there was a significant increase in fat metabolism with increasing exercise duration – i.e. as the one-hour test progressed, increasing amounts of energy were derived from fat oxidation. The authors concluded: “It is impossible to define a theoretical ‘optimal intensity’ for fat oxidation that holds true for all individuals. To achieve this requires an individual assessment using indirect calorimetry. Moreover, each individual may also exhibit day-to-day variations in fat burning rates, further complicating the situation.”

Fatmax: duration rather than intensity

At this point, if all you want to know is what exercise intensity is going to burn the most fat, you may be feeling rather confused so let’s try and clarify. Firstly, it seems that the use of a ‘Fatmax’ intensity based on 70-75% MHR/60-65% MHR in fit/less fit athletes is far too simplistic because recent studies show that:

  • The rate of fat burning may well be constant across a wide range of exercise intensities.
  • The rate of fat burning at any given exercise intensity is varies hugely from person to person, even among those of similar fitness levels (see figure 2 by clicking the link below).
  • The rate of fat burning increases as exercise duration increases – it’s therefore meaningless to talk about Fatmax without knowing the exercise duration.

The last point is particularly relevant; a very recent (2013) study on runners measured fat oxidation while they ran on a treadmill(5). It showed that that the exercise intensity to induce Fatmax during a 10-minute test was very different to that when the test was extended to 60 minutes. 

Given all these facts, a rather better approach to fat burning might be to think in terms of duration rather than intensity. In other words, rather than worrying about training at a specific % of maximum heart rate or VO2max, perhaps athletes should perform fat-burning workouts by training for longer periods (an hour or more) at a very comfortable intensity. 

In the studies above(3,5), the rate of fat burning remained the same at both the lowest and highest exercise intensities. A long, low-intensity workout would therefore burn just as many fat calories as a long, higher-intensity workout but be far less taxing in terms of training load. This is important because we know that increased or excessive training loads can put athletes at risk of overtraining and compromised immunity. Of course, the total calorie burn in a long, higher intensity workout will be higher, but the downside is that these kinds of sessions may impair the capacity to complete high-quality workouts such as interval training.


Boosting your Fatmax

Training – It seems there’s no magic training formula to determine your optimum training intensity for fat burning. But is there anything you can do to enhance your overall capacity to burn fat during exercise regardless of training intensity? Looking at Figure 2, it’s clear that there’s a huge genetic variation in fat burning capacity, which is why it’s impossible to predict somebody’s Fatmax purely from their aerobic power measurement.

Overall, however, the literature does suggest that the fitter and more highly trained an individual is, the greater will be their Fatmax potential – also that it will be reached at a somewhat greater exercise intensity. This suggests, therefore, that although long, gentle training sessions might be best for burning fat, some high intensity work aimed at improving aerobic capacity (e.g. interval training) could also be valuable – not because these sessions will be especially effective for fat burning, but because they will improve aerobic and fat-burning capacity, enabling more fat calories per hour to be burned in the longer, slower sessions.

Nutrition – there are numerous supplements out there touted as ‘fat burners’. One of the few exceptions was thought to be green tea extract (GTE) because earlier animal studies were promising and one human study found that GTE increased fat oxidation during exercise by about 20%(7). Unfortunately, however, more recent research has drawn rather mixed conclusions about GTE and fat burning during exercise. A very recent study by British scientists and headed up by Professor Jeukendrup, one of the world’s leading authorities in this area, investigated the effects of 1 and 7 days of green tea extract (GTE) ingestion on whole body fat oxidation during moderate intensity exercise(8). Thirty-one men completed two one-hour cycling trials at 50% of their maximum sustainable power output (fairly moderate intensity). After a baseline trial day, subjects were randomly assigned to one of three conditions involving a week supplementation of the following:

  • Seven days of placebo.
  • Six days of placebo/one day of GTE.
  • Seven days of GTE.

After the supplementation period, they then repeated the trial to see what effect (if any) the GTE had on fat burning during exercise. The results were disappointing; one day of GTE supplementation neither raised levels of blood lipid (a precursor to fat burning during exercise) nor increased fat burning during exercise. Seven days of GTE raised blood lipid levels but still had no effect of fat burning during exercise.

Overall, it’s fair to say that the jury is still out on GTE; some human studies have shown positive effects while others have drawn a complete blank. This may be due to differences in study designs, the bio-availability and activity of GTE used in these studies, and also variations in the measurement processes used to determine fat oxidation. To further complicate matters, we don’t really understand the precise mechanisms of GTE in the human body that could increase fat oxidation.

One supplement that is definitely worthwhile considering is caffeine. There’s little evidence that caffeine enhances fat burning directly per se (although it might help mobilise fats from body stores into the bloodstream). However, by acting as a central system stimulant, caffeine is able to help combat the onset of fatigue during longer bouts of exercise (this is how it boosts endurance performance). The one thing we’re certain of about Fatmax in all individuals is that it increases as exercise duration increases. Given caffeine can help you extend your training duration, it also follows that it will help raise your fat oxidation rate in those longer sessions. Finally, it’s worth adding that your eating patterns before and during exercise can have a significant effect on your Fatmax during that exercise – (see Box 2). 

Box 2: Diet and Fatmax

Eating carbohydrate in the hours before exercise raises levels of a hormone called insulin, which subsequently suppresses fat oxidation by up to about 35%(9). This effect of insulin on fat oxidation may last as long as 6-8 hours after a meal, which means that the highest fat-burning rates can be achieved after an overnight fast. A good way to achieve this is to train without breakfast. When Belgian scientists investigated the effect of a six-week endurance training program carried out for three days per week (each session of consisting of 1-2 hours) they found that training in the fasted state – without carbohydrate – resulted in decreased muscle glycogen use, while the activity of various enzymes involved in fat metabolism was increased(10).

If you train in the afternoons or evenings, you can’t fast all day. But studies show that by sticking to low glycaemic index foods (low GI – foods that release their calories into the bloodstream gently) you can minimise blood sugar and insulin spikes, thereby increasing fat oxidation. For example, a study looked at runners who consumed 2 grams per kilo of bodyweight of carbohydrate three hours before running at around 70% of their maximum heart rate(11). It found that when the carbohydrate consumed was of a low GI type (pasta, chickpeas, sliced apples and low fat cheese), fat oxidation rates were significantly higher during exercise and endurance was dramatically improved. 

Finally, while it sounds counter-intuitive and goes against most conventional advice, where fat burning rather than performance is the priority, carbohydrate drinks, which raise insulin and reduce fat oxidation, should be avoided. Studies have shown that although consuming carbohydrate drinks before endurance exercise boosts performance, the subsequent rise in blood sugar and insulin reduces the proportion of energy derived by burning fat(12,13). Don’t forget, though, you’ll still need to consume some energy-free fluid when training, especially in warm conditions.


Recent research suggests that the notion we can obtain Fatmax from a simple exercise-intensity formula is far too simplistic. Instead, the evidence suggests a better approach is to train gently for longer periods. Some high-intensity sessions may still be valuable, however, because by increasing fitness levels, they may raise the overall Fatmax ceiling in any one individual. Diet and supplementation may play a role too. The use of caffeine to extend training sessions and the careful control of carbohydrate intake before and during exercise can both help produce a higher Fatmax. By contrast, the use of green tea extract to enhance fat burning during exercise remains unproven and controversial.

Andrew Hamilton BSc Hons, MRSC, ACSM is a member of the Royal Society of Chemistry, the American College of Sports Medicine and a consultant to the fitness industry, specialising in sport and performance nutrition.


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