Energy in, energy out. The body normally burns a mix of carbohydrate, as glucose, and fat for fuel. How much of either depends on your physical activity and if, or what you have eaten recently. When you use more energy than you take in from food and drink, the body burns stored fat and carbohydrates, and then even protein, to fuel your everyday activities even if you are not exercising

That's what happens when people starve of course; the body starts to eat itself. Depending on your family history -- your genetics -- and the way you eat and exercise to create this energy deficit, your body may decide to get conservative and drop your metabolic rate to try to hold onto body weight. Some of us seem to have inherited this tendency more than others, the origins of which may be in the early periods of human evolution where 'feast or famine' was more or less the norm.

Glucose, fat and protein. Even so, starvation always works eventually and the body starts to break down its own tissue for fuel. Stored carbohydrate called glycogen is quickly used up, then goes the fat stored under the skin and around the internal organs. Protein in muscle is then broken down to create glucose to keep the brain working and you conscious.

Fat and glucose are the body's two main energy sources. Fat you know well, glucose comes mainly from carbohydrate foods like rice and bread and potatoes and protein is supplied mainly by meat and beans and dairy products. The amino acid building blocks of protein foods can be converted to glucose in emergencies. Your body always burns a mix of fat and glucose except at very high intensities, and the ratio of the fat and glucose in 'the burn' varies with intensity and time of exercise.

Fat burning zone. You may have noticed that some bikes and treadmills at the gym have a setting that says 'fat burning zone', which implies a setting for intensity or speed. The reason for this is that the body burns a greater percentage of fat at a slow pace (or after about 90 minutes of exercise). The fat burning zone, a low intensity speed zone is mainly a gimmick, and here is the reason.

Even though you burn more fat going slowly, you still burn some fat at much faster speeds or intensity. It all boils down to how much energy you expend in totality. For example, if you compare exercising at a slow rate that burns 60 percent fat and 40 percent glucose and a higher intensity or duration that burns only 30 percent fat and 70 percent glucose, you may still burn more fat at the higher intensity.

A typical example. Exercise (1) is the slower 60/40 mix and exercise (2) is the faster, 30/70 mix of fat and glucose fuel.

1. Walking on a treadmill for 30 minutes -- 180 calories used -- 108 calories of fat burned
2. Running on a treadmill for 30 minutes -- 400 calories used -- 120 calories of fat burned
You can see from this example that the bottom line really is how much energy you expend -- and that is the ultimate fat burning measure. The theoretical fat burning zone is mostly a convenient myth.
 

Muscle burns more fat. Weight training is increasingly recommended as a fat-busting tool because some experts say extra muscle burns more energy than body fat at rest, so if you develop more muscle and have a higher muscle to fat ratio than before, you must burn extra energy and more stored fat as a result. This is true and has been shown in metabolic studies. However, the differences are not that dramatic; perhaps less than a few tens of calories per day for each pound of muscle increased, for most people.

Does that mean you shouldn't worry about weight training? Certainly not, because weight training has many other benefits for health and performance, not the least of which is extra muscle. It's just that this advantage has been somewhat overstated and we need to get this fat burning thing right in order to develop the best weight loss and performance programs.

Getting the afterburn. Okay, so extra muscle does not provide that much advantage, but what about the afterburn? The 'afterburn', or the amount of energy you use after you stop exercising, has been promoted as an important slimming idea. If you can get afterburn, which is really another way of saying your metabolism increases for several hours or longer after a particular exercise, then that's a bonus because you burn fat during the exercise and after you cease as well. Will the fun ever stop!

However, this idea has recently been reconsidered as well. An article in the Journal of Sports Science reported that despite some promising early studies of this effect, the idea has not proven to be as useful as first thought.

Exercise scientists call this afterburn effect EPOC, which stands for Excess Post-exercise Oxygen Consumption. The authors of that study say that the high intensities required -- greater than about 75 percent of maximum heart rate -- are probably beyond what most people wanting to lose weight can cope with in sustained exercise. So the afterburn advantage from lifting weights or running fast is there, but you need to be able to sustain that intensity, which means a lot of hard work. No secrets there, I'm sure.

We also need to consider how fuel is used preferentially according to how your body stores are maintained. After you do a vigorous or long workout, your blood and muscle glucose will be much lower than before you started. Low glucose stores signal the body to burn fat preferentially. So after hard exercise that uses a lot of glucose, the body switches to burning fat. That's why all energy expenditure is important, not just fat burning during exercise.

Strength training has so many great things going for it that I'm a big fan -- increased strength, more muscle and body shape, better balance and bone density and improved functionality across all facets of human movement. But let’s be honest, we all need aerobic or cardio training as well. It has its own set of important functional benefits including general fitness, elastic arteries, increased heart and lung function and lower blood pressure to name a few benefits.

Lifting weights can easily move us into the high intensity exercise zone above the 75 percent effort required to get some afterburn, but it's only for short bursts. This is not consistent, steady-state effort and does not generally burn as much energy as a good run on the treadmill, cycle or row machine at moderate pace. For example, here are the energy expenditure calculations for weights versus cardio for one hour of exercise from the NAT Nutritional Analysis Tools web site. I've based this on a 150 pound person (just under 70 kilograms).

1. Running at 8 minutes a mile pace (5 min/km) -- burn 852 calories (kilocalories)
2. Weight lifting, vigorous, free weights or machines -- burn 409 calories (kilocalories)

I’ve tried to line these activities up for effort so that the comparison is worthwhile. Whenever I check these numbers it astounds me because I run and I lift weights, and sometimes I feel much fresher after a run than going for it in the gym with sub-10 RM (repetition maximum) and three sets of ten exercises. Nevertheless, the numbers always come out the same with any reputable energy calculator. Sustained aerobics always spends about twice the energy of weight training in a comparable comparison. You can see from this why cardio sessions are important for fat loss.
 

The answer is 'not necessarily', because even though you will burn more fat on an empty stomach, ultimately this will probably make little difference because your energy intake and expenditure and metabolism balances out, more or less, over the 24-hour period. What really matters is your total energy intake and expenditure, that is, how much you eat and how much you exercise and move in general.

However, stay tuned on this because until this is examined further scientifically, how much meal timing manipulation could help with fat loss is not certain. One thing that seems clear is that people who eat breakfast maintain weight better and lose fat quicker, so don’t skip breakfast.

So where are we at with our fat burning project? Here is a summary.

Increase muscle with weight training. Extra muscle helps to burn more energy at rest, even if only a little. This is called the resting metabolic rate of muscle or RMR. Extra muscle will also burn more fat in active phase, the active metabolic rate if you like, or the AMR, so having more muscle will definitely help burn more energy and fat.

Lift heavier weights. What I suggest is that the weights workout should be vigorous, with the number of repetitions kept at the low to medium end of the scale between 8 and 12 RM. To remind you, the RM is the repetition maximum, which means the most weight you can lift for this number of reps before fatigue. The 8-12 is within a range that should provide strength and bigger muscle growth, which is called hypertrophy.

If you go higher than this, say 15 to 20 repetitions to a set, or more, you are getting into the range where you would probably be better off doing cardio because the return on effort, the energy burn, is better spent jogging, cycling, stepping or rowing. At that number of repetitions you won’t build much muscle either, so very high-repetition training with weights has minimum value in my view.

Do aerobic exercise. Considering how much energy you would use in an hour of either type of exercise, weights or cardio, you must do some consistent aerobic or cardio work to burn fat.

Try high-intensity cardio. High-intensity exercise, even if only in short bursts, may rev up the metabolism and get that fat mobilized in the post-exercise period. Do some high intensity as well, but don’t overdo it, because burning the fat is a long-term project and you don’t want to get ‘burned out’. A group exercise program such as a solid cycle spin class might match this requirement. In fact, I highly recommend group cycle spin classes where you are encouraged to go fast, yet with the option to slow down if you need to.
 

Combining weights and cardio in a circuit interval session is also an excellent approach to fat burning. The weights circuits are based on the idea of mixing high and low-intensity weights and cardio in a circuit. This idea is not new, but what I've designed uses basic equipment and is easy to follow.



References

Owen O. Resting metabolic requirements of men and women. Mayo Clin Proc 1988;63:503-510.

LaForgia J, Withers RT, Gore CJ. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. J. Sports Sci. 2006 Dec;24(12):1247-64.