I've been struggling with this topic for quite awhile now, trying to find a way to describe why starving yourself to a fitter you is just not logical. I hope this time, my attempt makes sense. This time, let's play with some numbers and facts. Let's take a scenario where we'll assume our typical dieter is a guy who weighs 200 pounds and has 18% body fat and has set a goal to lose 20-25 lbs.

Pre-diet Stats:
Bodyweight: 200 lbs
18% body fat
36 lbs. fat
164 lbs. lean body mass

Now, our poor victim immediately assumes that starving himself is the best way to lose bodyfat, so he restricts his calorie intake to a mere 1500 calorie per day diet and factors in some exercise.

Week 1
He loses 5 lbs and is of course, thrilled with the results

Week 2
He loses 4 lbs

Week 3
He loses 3 lbs

Week 4
He loses 3 lbs

Week 5
He loses 3 lbs

Week 6
He loses 3 lbs

After 6 weeks, he has lost a total of 21 lbs of weight. Well done! Is this really possible? Of course it is... Our victim now weighs 179 lbs and he lost weight steadily without hitting a wall/plateau, although there was a slowdown in weightloss towards the end, which is okay and normal.

So, he did succeed in his goal. Or didn't he..? Let's take a closer look.

Post-diet Stats
Bodyweight: 179 lbs
14.8% body fat
26.5 lbs. fat
Lean body mass 152.5 lbs.

Total weight loss: 21 lbs.
Fat lost 9.5 lbs.
Lean body mass lost: 11.5

If you judge his success from the angle of fat/muscle composition and not his weight, then he has certainly FAILED because 55% of his weight loss came from lean body mass. This drop decreased his metabolic rate and his body is now utilising fewer calories everyday than when he started... and this is what sets him up for his downfall... this is why you should never starve! Read on..

Now that he's off his diet, even if he were to simply go back to his normal eating again, his body won't burn calories as efficiently as before. The number of calories that used to maintain his weight previously will now cause him to GAIN weight instead! What a slap in the face! As time passes by, the weight gradually comes back on until he finally gains back all the fat he lost. This will happen.. trust me.

6 weeks after the diet ended:
Bodyweight: 200 lbs
20.5% body fat
41.1 lbs. fat
Lean body mass 158.9 lbs.

Now he is right back at 200 pounds where he started, with only one difference: He has LESS MUSCLE, MORE FAT, and a SLOWER METABOLISM than when he began. He has damaged his metabolism and it will now be harder than ever to lose weight.

Don't say you weren't warned.