The Debate Between Fast and Slow Weight Loss: Which Method is More Effective?

Losing weight is a common goal for many individuals. However, the approach to weight loss is often debated. While some prefer to lose weight quickly, others believe slow and steady is the way to go. Expert guidelines typically recommend slower weight loss for the treatment of obesity, as fast weight loss is more likely to be regained. Slow weight loss is considered better for health and more sustainable. However, is fast weight loss equally effective and safe compared to slow weight loss?

Governing bodies suggest a weight loss of 0.5 to 1 kilogram each week, which is defined as slow weight loss. Rapid weight loss, on the other hand, is defined as losing more than 1 kilogram a week over several weeks. Several well-conducted studies have examined these differing approaches.

One study of 200 people randomly assigned them to fast or slow weight loss – 12 weeks versus 36 weeks – aimed at a 15 percent reduction in weight. The fast weight loss group was put on a very low energy diet using meal replacements, including shakes, bars, and soups, three times per day. The slow weight loss group was advised on the Australian Guide to Healthy Eating with the goal to eat 500 calories less than they used for energy (creating a calorie deficit) each day. They also used one to two meal replacements daily. The results showed that 50 percent of the slow weight loss group and 81 percent of the fast weight loss group achieved 12.5 percent or more weight loss during this time.

After this initial phase, those who had lost 12.5 percent or more were then placed on a weight maintenance diet for approximately 2.75 years. By the three-year mark, 76 percent of those in the slow weight loss and the same percentage of those in the fast weight loss group had regained their lost weight. This suggests that regardless of whether the weight was lost slowly or quickly, it was still regained.

However, another study on 101 postmenopausal women found that fast weight loss resulted in better outcomes than a slow weight loss group at the three-year mark. But there are other factors to consider, such as changes in body composition and bone mineral density.

A large meta-analysis found that while slow weight loss and rapid weight loss resulted in similar weight loss outcomes, slow weight loss resulted in better outcomes than fast weight loss with respect to metabolism or how many calories we burn at rest. Slow weight loss also resulted in greater reductions in fat mass and therefore a better fat-to-muscle ratio. Slow weight loss is also better for bone density, as rapid weight loss results in twice as much bone loss and puts a person at increased risk of brittle bones or osteoporosis.

Research shows that all diet approaches achieve similar weight loss outcomes, regardless of the type of macronutrient diet you follow or fashionable ways of cutting calories from the diet, such as intermittent fasting. This is because our body is extremely good at protecting against weight loss.

When you lose large amounts of weight, your resting metabolic rate – the energy you burn at rest – will lower. Keeping your resting metabolic rate high is essential for keeping the weight off. Unfortunately, once it slows down, your resting metabolic rate doesn’t recover to the level it was pre-dieting even after you regain weight. Research has confirmed that slow weight loss preserves your resting metabolic rate compared with rapid weight loss. A weight loss program that includes exercise rather than one that focuses on diet alone also helps preserve resting metabolic rate.

While restrictive diets can achieve rapid results, studies suggest they can come with adverse effects. This includes a higher risk of gallstones and deficiencies that can result in poor immune function, fatigue, and a decrease in bone density. Such restrictive diets can make it challenging to meet your nutritional needs. Many fast weight loss diets restrict or exclude foods required for long-term health, such as wholegrain carbohydrates, which are an essential source of nutrition for weight loss and disease prevention. Including meal replacements as part of a restrictive diet is also not sustainable for long.

Therefore, successful long-term weight loss comes down to following evidence-based programs based on what we know about the science of obesity, losing weight under the supervision of qualified health-care professionals, and making gradual changes to your lifestyle – diet, exercise, and sleep – to ensure you form health habits that last a lifetime.