Life for humans has changed a lot over the last hundred thousand years, but our bodies have not. Humans evolved and adapted to life in the African savannah, but what worked for us then is not necessarily ideal in modern society.
All of us are engines, powered by calories from food, and expending energy through basic metabolism (staying alive) and movement. When our prehistoric ancestors were fighting for survival, being an efficient engine was an advantage. When food was plentiful, their bodies stored any excess calories as fat, and only burned off the fat if really necessary. Muscles are a big drain on calories, so when not required, muscles would be burned away to save energy. These adaptations are great for survival in the wild savannahs, but in the modern world they make it hard to lose weight while maintaining muscle.
Basal Metabolic Rate (BMR): these are the calories your body burns to keep you alive. Everyone has a slightly different basal metabolic rate, which is dependent on your weight, muscle mass and genetics. A larger person with more muscle mass will burn more calories than a smaller person, or someone with less muscle.
Non-Exercise Activity Thermogenesis (NEAT): These are the calories you burn by moving around, but not through deliberate exercise. If you are generally active, always on your feet, or fidget a lot, you will burn a lot of calories this way.
Exercise Activity Thermogenesis (EAT): The calories burned through deliberate exercise, like running, cycling and swimming. The more you do, the more calories you burn.
Thermic Effect of Food (TEF): The energy your body uses to process the food you eat. The less processed food you eat, the more work your body has to do to digest it.
If through exercise and diet you can achieve a negative energy balance, consuming slightly fewer calories than are being expended through metabolism and movement, your body will burn off its fat stores. This is why having more muscle helps you burn more calories, as increased muscle increases your BMR. Exercising more increases your EAT, burning more calories, which is one of the many advantages of exercise. And eating good wholesome unprocessed foods instead of junk food also increases your energy consumption, pushing you more into a negative energy balance.
Our early ancestors did not have access to the sweet, fatty, calorific food that we do today. The only sweet or fatty treats likely to be part of their diet were wild fruit and nuts, and the occasional meat feast after a successful hunt. It was of benefit to crave calorific foods as these have a high energy reward, for little effort. When these treats were available, it was advantageous to eat as much as possible, as it could be a long time before the next big meal. When food was scarce, for example if it was winter, it was better to be lazy, limiting calorie consumption to last until the spring.
Fast forward to today, and the very adaptations which helped us survive are working against our health and fitness goals. Food is plentiful and very little effort is required to gain it. But we all still favour sweet and fatty foods, and it's all too easy to be lazy instead of heading out for some exercise.
However, evolution has provided us with a powerful tool for combatting this problem: our brains. We can collect a large variety of knowledge to help us achieve our fitness goals, such as exercise regimes and knowledge about the health benefits of the food we eat. We also all have the potential for an enormous amount of willpower, a strength to turn off our lazy urges, allowing you to be fit and healthy despite the trappings of modern society.
As well as being smart, in order to succeed in the wild our ancestors had to pick up new skills and adapt their bodies to them. Skills like throwing a spear or firing an arrow. Humans are excellent at learning new motor skills such as these through repetition, which trains your muscles to perform movements efficiently and skillfully. Our bodies are also excellent adapters, with muscles growing in response to the demands placed on them. Without this trait, we would not have survived in the wild. And it is this trait that allows you to build muscle and get stronger through training.
If you can calculate the number of calories you burn in a day, and tailor your diet to provide a slight negative energy balance, your body fat will reduce as your body burns off your fat for energy. If this diet is combined with a fitness regime that includes strength training, you body will retain and grow our muscles, as it will believe the muscles being trained are required for survival.