A friend recently went vegetarian just for kicks, as an experiment. In 6 weeks, the friend lost the 8 lbs that no amount of exercise could burn off without quitting dinnertime wine. (This friend is an athletic sort, a strength and fitness buff but not a fanatic.) She asked me how that could happen since she goes light on carbs.
It turned out that the answer was easy: Too much protein.
That sounds crazy, doesn't it? It's not crazy because any excess protein (protein in excess of what you body needs to maintain or repair muscle) is converted into and stored as fat. Many people are not aware that most of their steak ends up as body fat.
So whether you are sedentary (less than 6 hrs/week of fairly intense exercise/wk, not including walking) or not, your protein needs might be less than you imagine because you can only use about 20 gms of protein every 4-6 hours.
That's why people who restrict carbs, and replace the food volume with protein, have trouble losing body fat.
People who pursue daily strength-building (weights) or daily endurance training (ie triathlete types) can need more than the basic 45 gm/day for women or 55 gm/day for men - maybe up to 100-150 gms/day. That's why they tend to go for 4-5 smaller balanced (carbs, protein, fats/oils) meals/day.
Nutrition and fitness go hand-in-hand, and it takes a little bit of thoughtfulness. A few simple recommendations:
If you do barbells/powerlifts 3+ times/week, eat a lot of everything. If in daily endurance race training, eat a lot of everything. Until "stuffed". If a regular daily exerciser (some wts, some calis, some cardio, some endurance), 45-60 gms/day is plenty in divided doses. For weight loss, cut the volume dramatically. If overweight, you do not really need hardly any food at all other than some protein and vitamins and minerals. Overweight people have no caloric needs.
Grams protein conversion to lbs for all sorts of foods are easily found online. For one example, a regular chicken breast contains around 55 gms of protein. So if you eat one in a meal, about 30 gms of that protein goes to body fat and some in excretion. Sliced into thirds or quarters over the course of a day, that breast would be plenty for a full-grown male who works out.
So, again, volume management matters for weight-lifters, and for the overweight. It's a "First World Problem." And do not imagine that protein does not become body fat. It does.
Book recommendation: 
An Ancient Four-Legged Whale
A Greek Salad seems too healthy to be tasty, but it is. 
Pete Best