I wrote about a friend a few weeks ago who wants to get his weight down to a certain number, despite already losing over 100 pounds. Recently, he told me he wants to cut his calories in half. To me, this is dangerous, not just because of the potential for developing an eating disorder, but because cutting calories will not result in sustainable weight loss.
Too many people think that by not having enough calories, they will lose weight. Well, all that will happen is that the metabolism will slow down to a crawl. Do you want to feel tired and have no energy? That's what happens with calorie restriction.
Calories matter when people take in more calories than they need. But you also have to figure in the amount of activity someone does each day. If someone is highly active, they need the extra calories. If someone wants to lose weight, use an online food journal to keep track of the calorie intake. The online tools will recommend the amount of calories you need based on your activity level. Sometimes the amount they recommend is too low, which is why it's best to check with your doctor first.
The calories are just part of the equation. If you are not eating the right kinds of food, you will not be helping your situation either. Yes, someone could theoretically lose weight by eating junk food, as long as the calories are less than the required amount each day. But, remember, you need nutritious foods to keep your body running efficiently.