Originally Posted by
BarracksSi
+1 to what umd said. If the plan is to eat 1700 per day, then you exercise 1500 calories' worth (not impossible, even on a bike), your body's gonna freak out and edge into starvation mode. The muscles you've just worn out won't have any fuel to rebuild, so they're going to suffer, too.
If you think of it as net calories, then that's cool. The LoseIt app, for example, makes a baseline assumption of daily calorie usage, figuring that just spending time at the office and doing regular stuff will burn x amount of calories. If you eat only that much, but still exercise well, you'll hit a daily calorie deficit of up to a thousand calories at a time, and that's just not sustainable. Trust me, I've tried that, and it sucks.
A 200lb man uses 2400cal per day just keeping his body functions running...his heart beating, his lungs breathing, his liver...livering. If he eats 1700cal per day he's running a daily deficit of 700calories or 4900 calories per week. He's losing about 1.5 lbs per week. If he adds 1500 calories of exercise per day (10,500 per week) he's suddenly losing 4.5 lbs per week. That's a lot, but it's not unheard of. If he's losing that much he really should be on a medically supervised program. Interestingly, 4.5 lbs per week is exactly what I averaged when I was on a program years ago before I got into biking. So it's not unrealistic (especially for people who need to lose a lot of weight), but it probably isn't the kind of thing one should attempt without a doctor's supervision.
That scenario describes the beginning of a lot of failed diets, however. Come up with a rigid, unrealistically low calorie plan, then add in an extreme amount of exercise. The diet plan stays rigid (or tries to) and doesn't allow for the desire for more food, but the body demands more food, plan breaks, diet over.
That's why it's better (we're talking
weight loss) to stick to a moderate, regular amount of exercise and focus all attention where it can do the most good...the diet. If the guy in your scenario had stuck to about 2000 to 4000 cal in exercise weekly, he'd happily be losing 2.5 lbs per week, and no his body would not be freaking out and going into starvation mode.