I have a dumb question and am asking it because I am one of those fortunate people that can eat quite a bit and not gain weight, although that is becoming less true as I get older. Isn't the reason most people gain weight because they eat more than they need? In other words, you can't gain 10lbs if you only eat 5lbs, right?
Portion size is important, but it's more about what you eat rather than how much you eat. Everyone is different with regards to their caloric needs. I can eat 4000 calories a day, and if I'm expending more calories than I consume, I will lose weight. Now if those 4000 calories consisted of reeses peanut butter eggs and snickers, I would still be fat even if I burned them off. Your body will use up all of its glycogen before dipping into the fat cells for energy, this is where fat burning occurs. Feeding your body processed foods that are filled with sugar will only provide fuel in the form of glycogen, which will result in little to no fat loss.
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fly2cast, there's two types of people. Those that want to lose weight and those that want to be healthier. Your weight is only a percentage of the overall effect of working out. Muscle weighs more than fat. Pretty simple. If that's your only outcome, not eating works wonders for losing weight. The problem with only losing weight is that 99% of people gain it back. I'll show you the person who stayed the same weight but changed their body composition and I guarantee, they look healthier than the person that just went out to lose weight. Calorie counting is a dead horse as well. I can eat my daily allowance of skittles and call it calorie counting. Proteins, fats and carbohydrates all have their place. It's the portions and ratios of those three things that make someone a healthy eater.
This is why I'm not a fan of the fad diets like Atkins and Paleo. They are difficult to stick with for an extended period of time, and a majority of the people will eventually gain the weight back. In my opinion, a diet is a temporary endeavor that will eventually come to an end. Try to think of it as a lifestyle change rather than a diet.
No more than 30% of your caloric intake should come from fats. Ideally, that number should be around 15-20%, and the fats should come from eggs, fish, nuts/seeds, and unsaturated fats (olive and coconut oils). Protein and carbs will account for roughly 50% and 30% of the daily caloric intake. These will be your lean meats (chicken, turkey, fish/seafood, pork, venison), and whole wheat/grains (breads, pastas, brown rice, oats, etc.). Just need to find the right balance to meet the needs of your body.