There are two separate issues when dealing with calories: weight management and overall health. The number of calories you eat is the sole factor in determining whether your weight goes up, down or stays the same. The types of calories you eat however, will have an impact on your overall health.
A calorie is the measurement of energy found in food. The three macronutrients that provide the body with calories are: fats, carbohydrates and proteins. Fats are the most energy dense macronutrient, providing the body with 9 calories per gram. Carbs and proteins each contain 4 calories per gram. The relationship between how many calories you eat and how many you burn will determine your weight.
Your calorie balance is the relationship between how many calories you eat and how many you burn. Your calorie balance is the only way to control your weight. As long as you eat the appropriate number of calories, the types of food you eat are irrelevant to whether you gain or lose weight.
Losing weight requires a calorie deficit. A calorie deficit is a state in which you burn more calories than you eat. When you burn more calories than you take in, your body looks for that energy shortfall in sources other than food (body fat and muscle tissue) which causes weight loss.
Gaining weight requires a calorie surplus. A calorie surplus is a state in which you eat more calories than you burn. When combined with strength training, the body uses the extra energy to build muscle causing weight gain. While the number of calories you eat and burn impacts your weight, the types of calories you eat will impact your overall health.
Your body needs two main types of nutrients: macronutrients and micronutrients. Macronutrients (fats, carbs, and proteins) provide your body with energy while micronutrients provide your body with vitamins, minerals and other substances that help regulate physiological functions (ex: enzyme/hormone production, energy metabolism and proper immune protection). Without micronutrients, your body would not be able to function.
All foods provide calories, but not all of them provide the micronutrients that are also essential for life. Foods that are high in calories but low in other nutrients are referred to as "empty calories" because they contain energy but little in the way of essential nutrients. Your body requires a certain amount of every vitamin and mineral each day. Eating a high amount of foods that are high in empty calories will cause a nutrient deficiency which can result in a wide range of problems.
Sources of Empty Calories
Foods high in empty calories mainly come from highly processed junk food. Processing typically strips a food of its nutrients to make it taste better. Examples of empty calorie foods include: white bread, fast food, sweets (cookies, brownies, cakes, doughnuts), sodas, fruit juices and sweet tea. These foods are also known as energy dense foods because they only contain calories.
The opposite of energy dense is nutrient dense. Nutrient dense foods are those that are very high in nutrients such as vitamins, minerals, fiber and antioxidants. They are typically found in foods that aren't processed and include: fruits, vegetables, 100% whole wheat flour (whole wheat bread/pasta), brown rice, beans, nuts, seeds and seafood. A healthy diet should be high in nutrient dense foods.
The Bottom Line
Unhealthy foods (empty calories) have a place within a balanced diet. The problem is over indulging in unhealthy foods. Occasionally allowing yourself to enjoy foods that you love is not a problem unless a majority of your diet is made up of those unhealthy foods.