Calories measure the energy in your food. You may have heard that you need 2000 or 2500 calories per day, but in reality, many people have no idea how many they need, nor do they realise the different calorie densities of the macronutrients - carbohydrates, protein and fat.
Let's look at macronutrients first. Carbohydrates and protein contain four calories per gram, while fat contains approximately nine calories per gram (and sometimes up to eleven). In case you're wondering, alcohol is a separate nutrient entirely and contains seven calories per gram.
Can you use this to calculate the macronutrient makeup of your diet? Well, no you can't. 100 grams of rice do not contain 400 calories - due to water content - so you'll need to read the nutrition box on the food that you buy, or get a nutrition calculator like the MyFitnesssPal app.
Some people also take these figures to mean that fat is so calorie dense that it is unhealthy. It's not. In fact, fat is essential to your diet. That's why you've probably heard the term 'essential fatty acids'.
Protein is also a critical part of your diet. Protein makes up muscle fibres and is an energy source for the body too. In fact, carbohydrates are the only non-essential macronutrient. Your body can produce carbohydrates by converting the protein and fat that you consume.
Still, carbohydrates can be useful. Your body can process (high-glycaemic) carbohydrates into energy very quickly, which is useful during endurance events or when you need a quick boost. High-glycaemic carbs can also spike your blood sugar levels. This is usually a bad thing but after a workout, such a spike can help to drive nutrients into your muscles, which helps recovery and growth.
Sometimes a low carbohydrate diet can reduce your leptin levels, which then has a negative effect on metabolism (i.e. the rate at which you burn calories). High carbohydrate 'cheat days' can prevent this, while keeping you sane. Let's face it, carbohydrates can be so damn tasty.