It’s the law of thermodynamics. Energy in versus energy out. It’s not rocket science but so many people neglect this. If you want to lose weight (or more specifically reduce body fat) you need to be in a calorie deficit. Whether that’s through exercise, nutrition or a combination of both. The opposite is also true in that if you want your muscles to grow, you need to be in a calorie surplus. The middle point is known as homeostasis and whilst in this state, you can do both but only to a certain degree.
Say you want to lose weight (reduce body fat) and start under eating and take up exercising. If you are too far into a calorie deficit, you will initiate a starvation response. This means the body will break down muscle tissue to supplement the low calorie intake from your nutrition and your metabolic rate will slow down. Any spare calories you do have will be stored as body fat to help ensure the bodies survival (remember the body doesn’t care how you look, it cares about reaching homeostasis). All this and not to mention your thyroid and adrenal glands will be working overtime to make up for the lack of energy coming into the body. Which eventually, will get worn out and start to shut down and in some cases, causes irreversible damage. Plus the fact you feel like shit all the time. All this to lose weight. Sounds like a good deal right? I’m not so sure
The same is also true that if you want to put on muscle tissue, you shouldn’t go so far over your calorie expenditure that you start gaining a considerable amount of body fat, reduce insulin sensitivity and cause gut problems from all the extra junk food you’ve been eating.
If you go too far either way, it’s going to take so much more work to get WHERE YOU WANT to be. Take things slow, be consistent, be sensible and be responsible for yourself. No food will make you fat and unhappy. No fad will make you slim and happy. The body strives for balance so deviating too far away from balance for too long will only end in disaster.
Remember the story of the hare and the tortoise? Make sure your the tortoise.