TIME 4 FAT LOSS SERIES – PART 4 – How much fat should I aim to lose and how quickly?

April 20, 2019


This is the fourth in a series of original articles we will be publishing looking at the science behind fat loss. Join our mailing list to be informed when we have added the latest article.


PART 4 – How much fat should I aim to lose and how quickly?

Regardless of how much weight you ultimately want to lose, your initial fat loss goal should be to reduce your body weight by no more than 5-15%, unless otherwise directed by a medical professional (1).

The reason for this modest goal is that setting targets greater than 5-15% often results in potentially unachievable expectations and, consequently, failure (1). As a general guideline, you should aim to lose 3-10% of initial body weight over a three to six month period at a rate of 0.5-1 kg (1lbs to 2.2 lbs) per week (2,3).

The most effective way of achieving this is to create a daily energy deficit of 500 – 1000kcal, which produces a weekly shortfall of 3500 – 7000kcal.

However, in order to minimise the loss of muscle mass, research suggests that we should limit our weekly deficit to approximately 3,500 kcal or 500 kcal per day (2). The results of a study by Mayo et al (4) showed that although energy deficits greater than 700 kcal/day, produce greater weight loss, they also result in a greater loss of muscle. Consequently, if you wish to lose 10 kg, you should allow a period of approximately 20 weeks to achieve this.


What’s the easiest way of achieving the desired rate of weight loss?

Diets that are highly restrictive and exercise programmes that are too demanding are difficult to maintain and are often unnecessary, as the desired rate of fat loss can be achieved relatively easily by reducing energy intake by approximately 200 – 300kcal per day, the equivalent of 50g of potato crisps, and expending an additional 300 kcal per day through physical activity (2). For example, an individual weighing 100kg could achieve this by jogging at 8km/hr for 24 minutes or by swimming gently for approximately 50 minutes. Even if we reduce our daily food intake by just 100 kcal per day and increase our energy expenditure by just 100 kcal, our energy deficit over the course of a year would equal the amount of energy in 9.5kg of fat (1).


How much of our weight loss come from fat?

As a general guideline, if energy intake is reduced slowly without too severe a restriction, approximately 75% of the energy shortfall will come from fat and the remaining 25% from muscle (1).

The precise composition of the bodyweight we lose when we restrict our energy intake changes over time. For example, during a 4 week period of energy restriction, the weight lost on week 1 consists of approximately 70% water, 25% fat, 5% protein. For weeks 2-3, weight loss consists of 70% fat, 20% water, 10% protein. By week 4, weight loss is approximately 85% fat and 15% protein. Consequently, it takes approximately 4 weeks to achieve the desired loss of fat for each kilo of body weight lost (1).


The science is clear that the most effective way to achieve long-term fat loss is by a modest restriction in energy intake and a modest increase in energy expenditure through increased physical activity. As these require only minor changes to our diet and lifestyle, they should be relatively easy to maintain and therefore increase the likelihood of success.