Weight loss is a very sensitive issue among people who want to be fit. Before answering the question “How long would it take to lose 5 kg?” and “How can I do it?”, we need to clarify some ideas.
The balance between energetic intake (e.g. food and drinks) and energetic expenditure (e.g. physical activity, doing sports) influences body weight. When the energetic intake is higher than the energetic expenditure, we put on weight. Consequently, in order to lose weight we simply need to reduce energetic intake and to increase physical activity.
People often tend to adopt excessive hypo-caloric diets and to do a lot of physical activity to lose weight quickly. The problem is that the decrease of body weight is not only in fat mass. To lose fat mass only, diet and physical activity have to be balanced in the best way. However, losing fat is difficult because the body considers the fat tissue as an energy reserve and tends to preserve it.
A balanced diet and an adapted level of physical activity permit a body weight reduction where 75-80% of it is fat mass. On the contrary, an unbalanced diet combined with no adapted physical activity provoke body weight reduction with a decrease in muscle tissue. Moreover, when the body loses 1 kg of muscle mass, the body metabolism decreases of 40 kcal for every kg. In fact, after the first period of weight loss, when people return to a “normal diet” their weight tends to increase because of the “low metabolism”.
For this reason, the secret to lose fat is to take advantage of the different properties of fat and muscle tissues. Fat is an inactive tissue aimed to be an energy reserve, whereas muscle tissue is an active tissue that uses energy. So, maintaining and increasing muscle mass is important to increase the body metabolism and burn fat.
“What do I have to do to increase my muscle mass and burn fat?”: many researchers tested different types of physical activity and diets, alone or in combination, to find the best method to reduce fat mass and retain muscle mass. Physical activity can be classified in endurance training (e.g. jogging, walking, cycling, etc.) and resistance training (e.g. exercise with free weights, elastic bands, resistance machines, etc.), and both can be performed in low, moderate or high intensity (alone or in combination).
Despite the traditional recommendation of doing moderate endurance activity all day long, the more effective training to reduce body weight is the high intensity training. Indeed, endurance training performed with a heart rate > 75% of maximal heart rate induces positive effects in weight loss. However, the most efficient way is the resistance training (intensity of 75-80% of maximal muscle force) thanks to its positive effects on weight loss and preservation of muscle mass. The disadvantage of high intensity training is the necessity for physical conditioning with a progressive increase of intensity.