The Calorie
You've probably heard many people who are trying to lose weight mention, "count your calories". There is a legit reason for this. The scientific definition of a calorie is the energy needed to raise the temperature of 1 gram of water through 1 °C (now usually defined as 4.1868 joules). In terms of eating habits, a calorie is essentially the amount of energy stored from food into the body. In other words, how much energy does your body take in from your food? This is why calories become essentially important. Based on the intake from food per day, the amount of calories that you ingest have one of three instances: you ingest more calories than the body needs, less than the body needs, or as much as the body needs. In each case, your body will respond accordingly to this outcome.
The calorie makeup of food is determined by the total makeup of the food that is ingested. The makeup of food is largely determined by the macronutrients, mainly protein, carbohydrates, and fats. In each of these, a different number of calories is found per 1 gram of the macro. Adding the total amount of calories per these macros, one can find the total number of calories in the food. You should use the total number as the determinant for how many calories you are ingesting.
Ingesting Less-Than-Needed Calories
What happens when you ingest less calories than you need? Well, let's begin with why your body needs a certain amount of calories. Every day, you are moving around, expending energy to complete tasks. If you are individual who moves around frequently, your energy expenditure (how much energy you use per day) will be higher than someone considered a couch potato. A college student, on average, uses a moderately high level of energy per day due to walking to class, possible job employment, as well as going to the gym. When you begin to intake less calories than the body requires to use per day, the body begins to use some energy stores, draining them. As a result, you begin to lose weight, hopefully fat-related (talk about in Substantial Weight Loss). Think of it as what goes in (energy intake) does not equal what goes out (energy expenditure). As a result, energy that would normally be "reimbursed" is instead depleted. In fact, many studies (see Bibliography) have proven that calories, above all else (carbohydrate and fat intake) are the ultimate determinant for weight loss.
Ingesting As-Much-As Needed Calories
What happens when you ingest as many calories as your body requires? Essentially, nothing in actuality happens as long as your macronutrient intake (protein, carbohydrates, fats, etc.) equals the same. Your body is receiving the required amount of energy needed to complete its tasks. The baseline calorie intake fluctuates, however, on a small and big term scale. For example, you may track your calories, making sure you eat the same, baseline amount. Suddenly, due to responsibilities, you find yourself moving around each day. This will require your daily calorie intake to change, rising with the increased energy expenditure. This shows that you shouldn't expect your calorie intake minimum to be essentially the same at all times. This is especially true if you recently gained or lost weight because your body needs more or less energy to maintain its current energy levels, respectively.
Ingesting More-Than-Needed Calories
What happens when you ingest more calories than you need? Essentially, the opposite effect would happen if you ingested less calories than you need. Your body is receiving an abundance of energy from your foods. Despite this increased energy intake, your body remains at its constant energy expenditure; therefore, your body is receiving excess amounts of energy. All this energy will be furthered stored into your body, ready for future use. Individuals who eat more calories than their body demands often gain weight as a result: their body is receiving more energy than it needs, so the energy is stored.