How long does it take for body composition to change?

· Body Composition

Changing your body composition in a healthy way is going to involve both losing fat and putting on muscle. Regardless of whether your goal is to reduce belly fat or build amazing guns, because your body is made up of fat, skeletal muscle, bone mass and water, adjusting one of these measures means changing the others as well. It is a matter of prioritisation, and what you are prepared to do to reach your goals. In this post, we’ll discuss how long it can take to build muscle or lose fat depending on how you go about it. 

In previous posts we’ve talked about why knowing your body composition is important – be sure to check these out if you want to know more. 

Losing Body Fat 

If your priority is to lose fat, chances are you already know you will have to reduce the amount you eat. What you’re doing is creating a caloric deficit – that is, you are ensuring that your body burns more energy than it takes in, forcing it to consume existing stores of energy (fat) rather than new sources (food)1. The greater the caloric deficit, the more fat you will lose. 

Healthy Rate of Fat Loss 

Your body uses a certain amount of energy just to stay alive – this is your basal metabolic rate2, and it is different for everyone. A body composition machine can estimate what yours is. In general, you don’t want to eat less than this figure (we can talk about fasting in a later post!). To work out a healthy rate of weight loss, calculate your Total Daily Energy Expenditure (TDEE) from your BMR. The formula uses your BMR and activity level to estimate how much energy you need to maintain your current weight. To lose weight, you just need to eat below this figure.  

Typically, you will see recommendations to drop below your TDEE by 500 calories. This is based on the fact that 500 grams of fat is represented by 3,500 calories, and therefore if you eat 500 calories less every day for a week, you will lose about 500 grams each week. General advice is that losing more than this puts you at risk of slowing down your metabolism, losing muscle, and nutritional deficiencies3. However, every person is different and it is important that you speak to your doctor if you have any concerns about your body and what a healthy rate of weight loss is for you. 

Putting on Muscle 

If your priority is to build muscle, you need to ensure you eat more than your TDEE. Current research indicates you need to consume about 15% more than your TDEE4.  

In order to maximise skeletal muscle mass gain and minimise fat mass gain, the increase should be made up of protein rich foods and carbohydrates. It is important that you consume a balanced diet – studies have suggested that there is no benefit to consuming more than 0.9 grams of protein per half kilo of body weight5. Moreover, you still need all the good energy and minerals that come from wholegrains and plenty of vegetables.  

Muscle Growth Rate 

Building muscle is not a fast process. You don’t need to spend days in the gym, just 30 or so minutes a couple of times a week is enough, but it does require consistent effort over several months. 

Also, people who have more skeletal muscle mass to begin with tend to notice changes faster – your genetics play a large role in how quickly you can gain muscle. Muscle growth is affected by your body size, composition, and hormones. In particular, testosterone has a lot to do with muscle development6. Although men and women have similar responses to strength training, men do tend to put on muscle faster. 

Increasing skeletal muscle mass is achieved by gradually increasing the stress on your muscles by slowly increasing their workload7. The ideal rate at which you do this depends on large number of factors, from the amount of time you have to exercise and how you exercise, down to your genetics. However, most people notice changes within three to six months. 


The speed at which your body composition changes will be different for everyone. We all have different heights, shapes, hormone levels and lifestyles. However, regardless of what your goal is, changing your body composition in any way takes consistent work over several months.