Fat burning: train your metabolism to use up those calories

Endurance-type training usually enhances your muscles’ abilities to metabolise fat

Endurance-type training usually enhances your muscles’ abilities to metabolise fat, but fat remains a bit snobbish about exactly when it is willing to provide you with the energy you need to exercise. When you run a 10k race, put forth your best possible effort in a 40k bike competition, or swim as well as you can for 2000 metres, for example, fat disdains from furnishing more than a pittance of the large quantity of energy you need to finish your event.

Fat’s willingness to be metabolised at high rates during exercise in some situations and its unwillingness to take part in energy provision in others has concerned many athletes and regular exercisers, especially those who would like to lose body fat and improve their overall body composition. Such people believe that if they could identify the exercise intensity which evokes the highest rate of fat oxidation, they could selectively carry out workouts at this intensity and make dramatic improvements to their bodies.

Recently, researchers from Birmingham University’s Human Performance Laboratory attempted to pinpoint the exercise intensities at which fat metabolism is maximised in a study of 18 male endurance cyclists with a training background of at least three years. The average age of the participants was 28, and their mean VO2max was 58.4 ml.min-1.kg-1(1).

All the subjects performed a graded exercise test to exhaustion on a cycle ergometer, starting at 95 watts and increasing their work rate by 35 watts every five minutes. Once the respiratory exchange ratio (R) reached 1.0, the work rate was increased by 35 watts every two minutes until exhaustion.

The respiratory exchange ratio (R) is simply the ratio of the carbon dioxide released by the body to the volume of oxygen consumed (VCO2/VO2), and it can be used to estimate the degrees to which fat and carbohydrate are being used for fuel. R works as an indicator of fat and carbohydrate breakdown because fat and carbohydrate differ strikingly in terms of the amount of oxygen used and carbon dioxide produced during oxidation.

Fat oxidation requires much more oxygen than carbohydrate metabolism. Thus, the denominator of our VCO2/VO2 term will tend to be a big number when fat is the major fuel, while R will tend to be smaller. Correspondingly, the denominator will be a more modest number when carbohydrate is the key fuel, while R will be bigger, since less oxygen is required to break down carbs than fat. In fact, when physiologists measure R at about 0.70, they estimate that 100% of the energy for exercise is coming from fat; when R settles at 1.00, carbohydrate is almost the sole source of fuel. And when R is at 0.85, metabolism is equally divided between fat and carbs. As you can see, R measured during exercise can tell physiologists where an athlete is with regard to fuel usage.

For each athlete in the Birmingham study, the data collected during the graded exercise test were used to construct a graphical ‘curve’ of fat oxidation rate versus exercise intensity. This curve was used to determine two key variables for each subject:
1. Fatmax – the exercise intensity at which the highest rate of fat oxidation occurred, and
2. Fatmax zone – the range of exercise intensities in which the fat oxidation rates remained within 10% of Fatmax.

As exercise intensities increased above 40% VO2max, fat-oxidation rates gradually increased too, reaching a maximum of 0.6 grams per minute at an average intensity of 64% VO2max. There was an incredible range in this Fatmax intensity, however, with some individuals reaching Fatmax at 42% VO2max, and others not hitting it until they soared to 84% VO2max! Fatmax corresponded with an average heart rate of 74% of maximum, but again there was considerable variation, with some athletes attaining Fatmax at 54% of max heart rate and others not reaching it until 92% of max.
The Fatmax zone was located between 55 and 72% VO2max, or between 68 and 79% of max heart rate. As indicated above, rates of fat burning dropped precipitously once athletes moved above the Fatmax zone. In fact, fat oxidation dropped to zero above an average of 89% VO2max (just below the intensity of a 10k road race or a 40k bike race). Again, there was wide variation between individuals, however, with some athletes burning no fat once they had reached 71% VO2max (84% of max heart rate) and others continuing to burn it until they reached an amazing 99% VO2max (98% of max heart rate)!

Fat is released during leisurely efforts
As you can see from this study, fat prefers to release its generous energy stores during fairly leisurely efforts. In fact, other research has suggested that when you cycle, swim, row or run at a modest intensity of only 50% VO2max (about 65% of your maximal heart rate), fat provides about half of the calories you need to keep pottering along for the first hour or so. If you keep going after that, fat becomes even more generous, chipping in around 70% of total energy after two hours of such light exertion and 80% or more if your work duration exceeds three hours.

Since fat is horrified by high-intensity exercise (in most athletes) but can contribute the lion’s share of energy during easy efforts, health professionals, aerobics instructors, personal trainers and coaches often advise athletes and people who are interesting in losing body fat and improving body composition to train at fairly light intensities. The recommended exertion levels – often at around 50-65% of VO2max – are sometimes said to comprise the ‘fat-burning zone’ (FBZ) of possible exercise intensities. As you can see, FBZ is very similar to the Birmingham Fatmax zone; FBZ is slightly lower on the intensity scale but encompasses most of the intensities in Fatmax.
FBZ exercise is truly light: 50-65% of VO2max equates to just 65-76% of maximal heart rate, a level of intensity which many people can attain during a brisk walk to the post box. For an athlete with a max heart rate of 200, 65-76% of max is a range of 130-152 beats per minute, which would feel moderate and ‘conversational’ to most experienced trainers.
The beauty, then, of the fat-burning zone is that anyone with a modicum of muscle tissue in his/her legs (and possibly arms) and who has a heart which still shows a few signs of life can slip into FBZ without much trouble, trimming corpulence in the process. Out-of-shape individuals are happy to learn that they do not need to train like Olympic competitors to rid themselves of excess suet. And at the other end of the athletic spectrum, highly fit athletes who might benefit from losing a pound or two like the idea of achieving this with such easy work.
It would be great if fat metabolism and body-composition improvements were really this simple, but the truth is that recommendations to exercise in the FBZ or even the Fatmax zone are often ill-advised. In fact, you can often expend more calories – and burn just as much fat – by training at somewhat higher-than-FBZ (or Fatmax) intensities. Expending more calories can be a great thing, of course, even if those calories come from highly desirable substances like carbohydrate and protein. The extra calories burned from higher intensities of exercise can create an energetic deficit within the body, when ingested calories are inadequate to furnish the body’s caloric needs and the body begins to ‘digest itself’ by dipping into its internal energy stores. If there is a God in Heaven, the stores chosen for dipping into will be deposits of fat, and thus total fat degradation for the day will be greater than it would be on a day when a lighter-intensity, ‘FBZ workout’ failed to produce an energy deficit.
Here’s a specific illustration of how FBZ sessions can be counter-productive: let’s say you are about to complete a 30-minute workout on your bike but you’re not sure whether to exercise within your FBZ – let’s say at 50% VO2max – or to pedal along more strenuously at 75% VO2max (approximately 84% of maximal heart rate). This latter intensity is, of course, above FBZ and the Fatmax zone, but it is still fairly easy for you – and most regular exercisers – to handle. Which workout will do a better job of breaking down your unwanted body fat?
If you cycled along at 50% VO2max, fat would provide about 50% – on average – of the energy you needed to keep going, although as explained there are considerable variations between individuals. If, on the other hand, you cruised along at 75% VO2max, fat would cough up just 33% of the required calories. Thus, the slower workout sounds better from the fat breakdown perspective!

Total calorie expenditure is the key
However, there is just one little problem. ‘The easier session seems superior only if you neglect to reckon the total number of calories expended during the workout,’ says Jack Wilmore, a distinguished exercise physiologist from the University of Texas-Austin(2). In research carried out with renowned physiologist Dave Costill, Wilmore found that a moderately fit individual exercising at 50% VO2max generally consumes about 220 calories during a 30-minute workout. But when that same person works out at 75% VO2max, 330 calories are burned during the same period.
Of course, 50% of 220 calories and 33% of 330 calories yield an identical number of calories coming from fat – 110. In other words, the FBZ and non-FBZ workouts metabolise exactly the same quantity of fat, while the higher-intensity, non-FBZ session increases the chance of creating an energy deficit for the day, forcing the body to whittle away at fat stores to satisfy routine metabolism.
Even if you could devise a way for FBZ-type workouts to lop off larger quantities of lard, it is not certain that such sessions would make you leaner in the long run. Let’s say, for example, that you discovered a workout in which fat provided all of the energy you needed to exercise. Theoretically, each of these workouts would remove a nice chunk of fatty tissue from your body.
However, if fat was picking up the whole tab, you wouldn’t be breaking down carbohydrate (glycogen) during your workouts, and as a result your leg muscles would be amply and permanently stocked with glycogen (assuming, of course, that your diet contained a normal carbohydrate content). Each time you ate, the carbohydrate from your meal would move across the walls of your small intestine into your bloodstream, and your blood would transport the carbs to your muscles. Your muscle cells, though, would have to say, ‘No thanks, I don’t need more carbohydrate, I’m already full.’ The surplus carbohydrate from your meal would then be converted to – you guessed it – fat!
However, there is one feature of FBZ training which is attractive to some athletes: since the intensity of FBZ sessions is low, they can be sustained for very long durations; thus, even though the per-minute rate of calorie burning is puny, the total calories burned during single FBZ sessions can be monumental. While this may not appear to be a strong selling point, it is true that an athlete who could keep exercising for only one hour at an intensity of 75% VO2max might be able to complete a full two-hour workout at the lower intensity of 50% VO2max. In the former case, 660 calories would be burned, but in the latter 880 calories would sail out the window. Seen from this perspective, FBZ training has a certain attraction – for people who have two hours to spare.
But the flaw in this logic is that the athlete who can no longer continue beyond one hour at 75% VO2max has the option of simply dropping down to 50% VO2max (or even 60% VO2max) and continuing for longer. Just 30 extra minutes at 50% VO2max would give him as much of a calorie burn in 90 minutes as the FBZ athlete would achieve in 120 minutes, and anything beyond 90 minutes would give him a nice caloric edge.
In addition, most people who exercise are time-constrained to some degree and do not have hours to squander on low-intensity exercise. When workout time is limited, there is little reason to slow down to your FBZ unless your body is begging you for really easy work. If your overall goal is to get leaner, the bottom line is that calorie burning is the best way to achieve it. The most effective way to lose body fat is to burn slightly more calories than you take in, and to continue this negative energy balance over an extended period of time. A modest expansion of your training programme – by completing your regular volume of training at a slightly higher intensity or by tacking on a few additional miles or minutes at your normal pace – is a better way to accomplish that goal than feasting on FBZ training and using up calories at a lower rate.
What about those athletes who appear to be poor fat metabolisers, the ones who pile on pounds of unwanted flab quite easily and have trouble losing fatty tissue even when caloric intake is reduced? Don’t such people need special workouts which will stoke their fat fires?
This is a possibility – although there is no guarantee that a poor overall fat metaboliser will burn fat effectively during exertion. Note, too, that without the kind of testing carried out by the Birmingham scientists, it would be very difficult to prescribe a specific training intensity for someone who wanted to maximise fat loss. As we’ve seen, some athletes reach Fatmax at 84% VO2max, while others reach it at just 42% VO2max. As the Birmingham researchers themselves acknowledge, it is probably not possible to extrapolate Fatmax from research studies like theirs to individual athletes.
Very long workouts lasting more than 90 minutes do enhance the abilities of most athletes to metabolise fat, and this can lead to some cloudy thinking among coaches and athletes. For example, ironman triathletes and marathon runners, believing that fat metabolism will be a key source of energy during their competitions, will often undertake long workouts ‘cold turkey’, while drinking only water and refusing to ingest sports drinks. Such a strategy will indeed give fat metabolism a boost, but there is one little glitch in the overall thinking: these athletes usually carbo-load and take in one last carbohydrate-containing meal a few hours before their competitions begin This carbohydrate ingestion dramatically reduces fat-oxidation rates during the subsequent exercise(3)! These athletes would be better off using sports drinks during their long workouts; although such drinks would mute fat oxidation, they would boost the quality of the long exertions and also provide specific preparation for competition, when sports beverages will certainly be ingested.
There’s one final ‘wrinkle’ which takes even more lustre out of FBZ training: Fatmax depends not only on prior carbohydrate intake but also on training status. In one study, well-trained individuals reached Fatmax at 40% VO2max, while untrained people reached it at 59% VO2max(4). For a person who is getting steadily fitter, it is likely that Fatmax varies over the course of a training season, making an exact prescription even more difficult.
Ultimately, though, that’s not really a problem for the athlete who wants to lose fat. As we have pointed out, FBZ sessions can lead to total quantities of fat breakdown which are lower than those associated with more intense workouts, and FBZ training efforts will burn far fewer calories. To lose body fat, you don’t need to become a Fatmax specialist, just use up calories in a reasonable and sustainable way.

Owen Anderson

(1) Medicine and Science in Sports and Exercise, vol 34(1), pp 92-97, 2002
(2) Running Research News, vol 9(2), pp 10-11, March-April 1993
(3) American Journal of Physiology, vol 273, pp E768-E775, 1997
(4) Journal of Applied Physiology, vol 86, pp 479-487, 1999