I got this question the other day, so I thought I'd field it here: what's the best form of exercise for losing weight?
The question itself is a little misleading because, despite the popular wisdom, you can't really lose weight via the "burning off of calories by exercising as much as you possibly can." This is because the vast majority of people simply put the calories back on after long workouts. It's not their fault or a sign of a lack of willpower – the body is simply a smart system that is very efficient at retaining and replacing stored calories.
Let's do the math. You're a hardcore runner who jogs 30 miles a week – five six-mile runs on average. You finish a six mile run in a little less than an hour, less than 10 minutes per mile.
Based on my vast experience as a runner geek, I'd say you'd expend roughly 500 calories during this training session (running all-out for an hour will expend about 700 kcal).
However, this calorie amount includes your basal metabolic rate (BMR) – what you would have expended by remaining stationary for that period. For me, the BMR for a 55-minute period is about 65 calories. So the run actually only burned off an extra 435 calories. This still sounds like a lot, huh?
But you're hungry afterwards, right? Running for almost an hour? Possibly not at the moment but certainly an hour from then. So you slam a banana (a healthy treat to replace the lost potassium, but fructose and calorie rich) and an energy bar.
One medium banana (105 cals) and the bar (about 220 cals) means you've just replaced 75 percent of the expended calories, and that doesn't include anything eaten before or during the run. You also might leap onto the weight scale after the run to ogle the pounds you just lost, but the scale is probably sending more of a message about dehydration than anything else.
There are other things going on – most of what you burned off during the run was probably glycogen, a form of starch that's stored in the liver and skeletal muscles, and the body preferentially replaces that with carbs or glucose eaten after endurance-type exercise. You might have also tapped into the fat stores inside the muscle itself. These are two places – glycogen and the fats the muscles use for energy – where you don't really mind having energy depots. In fact, they represent essential energy sources for the body.
Getting back to the original question, how does exercise contribute to weight loss? By improving your metabolism in the long run. A person won't lose weight until they move into a healthy metabolic realm. This means they want to retain sensitivity to their own insulin, and not develop insulin resistance.
When you embark on high-intensity type exercise session, such as sprinting and lifting heavy weights, you use the more powerful Type II muscles (e.g., the quads and the hamstrings). The glycogen in those muscles cells is expended (as it generally isn't completely by jogging) and the muscles retain their insulin sensitivity at the same time as they pull glucose out of the bloodstream to replace the lost glycogen.
That's a simplified description for a very complex and efficient mechanism in our bodies. With better insulin sensitivity you will develop lower fasting insulin levels, and your body is less likely to be in fat-storage mode all the time. The actual calories you burn off during the sprint or weight-lift are almost beside the point.
As a person with healthy low fasting insulin and glucose levels, you will also not experience the constant hunger pangs throughout the day, which are so familiar to many of us. You will only eat when you are experiencing actual hunger (admittedly, an elusive concept) and are in need of calories to fuel your brain, for instance. Intermittent fasting (covered in Chapter 6 of Fitness For Geeks) also helps promote a fitter metabolism.
The book goes into greater depth on these issues and I will expand upon them here in the near future. However, a sprint session once a week and a high-intensity weight bout once or twice a week represents more than a good beginning.
Imagine that you want to optimize the gas mileage you're getting in your car. The typical strategy that you would use is maintaining the efficiency of the engine and the physics of burning as little gasoline as possible, and this is an apt metaphor for helping optimize your body's metabolism.