Also available is the companion article on food-related facts and fallacies.
1. High Intensity Exercise Burns More Fat
Yes, but only if you do enough. When someone tells you in a few words that 20 minutes of high-intensity interval training on a stationary bike or an interval circuit burns more fat than 40 minutes jogging on a treadmill, then they’re not telling you the complete story. This type of training is hard work and you need to do sufficient to see results. Not everyone is up for high-intensity interval type exercise on a regular basis. This type of training, which was studied in a recent trial, alternates high-speed sprints with lower speed rest intervals. The Circuit Fit program is an example of a circuit training program with elements of high intensity.
So don’t believe the hype about '10 minutes-a-day to a great body'. That person’s trying to sell you something!
2. Low Intensity Cardio Burns More Fat
Yes, but read on. This is the mirror image of the the first item. Generally speaking, low intensity exercise in the zone where you can talk comfortably while exercising is where fat gets burned preferentially as fuel for the body. Higher intensity exercise such as weight training and interval training uses more glucose fuel from carbohydrate. Nevertheless, some fat gets burned at higher intensities so you can probably see that if you exercise hard enough for long enough, you could conceivably burn more, or a similar amount of fat at higher intensity than equal or less work at lower intensity. The post-exercise afterburn also ads to an increase in metabolism after high intensity workouts. This article on fat burning has more information. Ultimately, the body doesn’t care what fuel you burn, the result is the same : what you consume in excess of what you expend gets stored as fat even accounting for a few metabolic differences between fat, carbohydrate and protein. See the next question.
3. You Need to Burn Fat to Lose Weight
True, but you don’t need to directly target fat burning. Consider how the body processes its energy stores. Fat is not a permanent fixture in cells – it comes and goes according to your fuel requirements. When blood glucose is low, say first thing in the morning, fat will be the main fuel. After the first few pieces of toast or muesli, blood glucose will rise and insulin will start storing glucose and fat in cells and you will start burning some blood glucose. Glucose is stored in liver and muscle, and fat in fat cells as triglycerides. When your blood glucose gets low again, or when you do moderate intensity exercise, this fat can be retrieved from fat cells when hormones called lipases break down the fat molecule and send the free fatty acids to the bloodstream to be used as fuel. This is what we call ‘fat burning’. It's a dynamic process.
The key to understanding this is the 'sliding scale' between fat and glucose (carbohydrate). After you burn a lot of glucose, fat gets its turn because blood glucose is low. What happens if you always have glucose to burn? Doesn’t fat just stay stored away and you stay fat? No, because if your food intake and activity output is balanced you always have fat to burn – in the early hours of the morning before breakfast, while doing everyday things like housework, before meals, when you're active – it just balances out because glucose gets sucked up to replenish muscle and liver stores -- and then fat becomes an important fuel. So in a sense you don’t have to worry about fat burning, only food fuel burning and energy balance.
4. Weight Training Burns More Fat
Same answer as number one – only if you do enough of it. You can’t just do a few heavy lifts with a few minutes rest in between and expect to use a lot of energy and burn a lot of fat. Even a full session of very vigorous weight lifting with only minimal breaks between exercises and sets will only burn about 600 calories. An hour of vigorous walking will burn about 300 calories. An hour of strolling around in the weights room having a chat, a drink and completing 10 exercises at moderate intensity won’t burn much more than 300 calories.
5. Exercising before Breakfast Burns More Fat
Yes, but see items 3 and 4. Also, a recent study suggests that if you want to lose weight this may not be the best strategy because exercising on an empty stomach may encourage overeating after exercising. An hour of weights may burn 400 calories and a large bagel may cost you 350. That’s not all bad because you do have to replace some of that energy lost. Yet a Big Mac with large fries and Coke will put you over the top. You have to watch the post-exercise meal if you want to lose weight.
Exercising on an empty stomach will reduce blood and muscle glucose substantially. Two things will happen: 1) you will burn more fat; and 2) your brain will start sending signals that you need to eat. The brain is the ultimate survival controller. When blood glucose gets down around 70 mg/dL (4 mmol/L), it gives you the ‘hurry up’ because at less than 55 mg/dL (3 mmol/L) you can pass out.
One more thing will happen. If the brain can’t get glucose from food, the hormone cortisol will start to break down muscle for glucose. If you want to lose fat and gain muscle, this is certainly not the way to do it – which is what happens on very low-calorie and starvation diets.
6. You can Lose Weight from the Abs by Exercising Them
Not directly. The body tends to lose fat in a genetically determined way from all over the body. However, some people maintain belly fat until the end. The abs muscles may be the last to show for these individuals. See Fact or Fiction? Answers to Weight Training Questions.
7. Your Genes Can Make You Fat
Controversial but probably true. It seems that certain genes may influence the risk of you being obese or unable to lose weight because of the way your body processes food or reacts to exercise. Much of this may turn out to be a predisposition, which means that the tendency to be overweight is programmed in genes, but that an environmental factor such as overeating and under-exercising may have more effect for you than someone without such genes. Direct genetic obesity may turn out to be quite rare.