My BMR (Basal Metabolic Rate) is 1910 calories. That means if I sleep for 24h, that is what my body burns. (Note! This is different for everyone, and your muscles determine how much you burn, so don't use my number to calculate your own intake) My trainer says I should never go below 1910 calories in a day, no matter what. I was told to eat roughly 2000. With my exercise regime, this will enable me to lose fat, and if not gain, at least keep muscles. This contradicts so many of the articles you read about losing weight, all the bloggers averaging from 1100-1300 calories in a day and working out like mad (mostly cardio??)... But the clue here is that I want to not only lose weight as fast as possible, but in a healthy way - keeping as much muscle mass as possible. If my body is going to do that, it needs building bricks (protein), and enough energy to leave the muscles I already have alone. Under-eating will lead your body to burn off not only your fat, but definitely your muscle mass as well. Huge wake up call for me! Please share any thoughts you have on this topic!
This was also tied to maintenance issues. If you have a low percentage of muscle mass, your basal metabolic rate will be low. When at goal weight, the number of calories you need to keep your body going (without gaining fat) is low - and this makes it hard for people to find a balance. There's not enough room calorie wise. With a higher percentage of muscle mass, you can maintain your weight with a higher calorie intake, because your body needs more fuel. Makes sense?
In summary: To lose weight (fat mass), train with weights. To maintain muscles, eat at least your BMR. To maintain at your goal weight with less struggle, have a good/high percentage of muscle mass.
Edit: I posted this as a reply to a comment below: I think it's important to remember that seeing a smaller number isn't solely what is important. If you lose 2lbs one week, and those lbs are muscles.. that is a bad thing. I am mentally prepared for more stand still weeks with this change in diet, but I am confident that my body scan will show a decrease in fat mass, and a gain in muscle mass! There is a definite mental challenge to this, but it's more of a health oriented diet, rather than purely weight loss oriented diet. One would also have to measure, and accept that as a fantastic token of progress!