Old rule: Always choose fat-free foods.
These days, nearly every full-fat food, from cookies to ice cream, has a fat-free counterpart. It's the first instinct of many people to simply eat these reduced fat foods to control their weight; however, this tactic could actually derail your good intentions. Why? Because most of the fat-free foods you can buy are things you shouldn't be eating anyway: empty-calorie junk food and heavily processed sweets, crackers and cookies. These items have been available for over a decade, but people aren't getting any thinner by eating them. Remember, dietary fat isn't the sole culprit that has made us overweight, excess calories are. Sure most of us could stand to cut back on our fat intake to a more reasonable level, but calories count when it comes to weight loss. All the reduced-fat foods in the world will not help you lose weight if you're making poor food choices or eating too many calories in general.
New rule: Include a moderate amount of heart-healthy fats in your weight loss plan.
Your body needs dietary fat for day-to-day organ protection, vitamin absorption, hormone production and more, so you won’t be doing yourself any favors by completely depriving yourself of this macronutrient. A sensible amount of fat can also aid in satiety, making you feel fuller longer. Try adding healthful fats such as nuts, avocado, or olive oil to your diet. Choose low-fat or fat-free products when it comes to dairy and meats to limit your intake of unhealthy fats and control calories, but leave the other fat-free foods on the supermarket shelf.
Old rule: You should burn every calorie you eat through exercise.
To lose weight, it's true that you need to burn more calories than you consume. But some people misinterpret this weight-loss equation, thinking they must burn off every calorie they eat—and then some—by exercising. Besides being inaccurate, this practice can be unsafe and lead to exhaustion, overuse injuries, and stalled weight-loss among other problems. Remember, your body is constantly burning calories throughout the day, even when you're not physically active. This is known as a basal metabolic rate (BMR), and it accounts for more than 1,200-1,500 calories per day (on average). Add to that all the calories you burn by moving, walking, standing, and yes, exercising, and you can see how easy it is to "burn more calories than you consume" without spending your life in the gym.