Slow versus fast weight loss – does it really matter how you lose weight?

Dr Nick Fuller
Leading Obesity Expert at the University of Sydney and founder of Interval Weight Loss.

When people decide it’s time to lose weight, they’re usually keen to see quick results. Maybe they have an event coming up or want relief from health problems and discomfort. 

But expert guidelines typically recommend slower weight loss for the treatment of obesity. This tallies with a widely held opinion that fast weight loss is more quickly regained. Slow weight loss is generally perceived as better for your health and more sustainable. Many programs offering “the fastest way to lose weight” are considered fad diets that severely restrict calories or eliminate some foods. 

But does slow and steady really win the weight-loss race? Or is fast weight loss just as effective and safe?

What’s the difference between slow and fast weight loss?

Governing bodies typically recommend a weight loss of 0.5 to 1 kilogram each week, which would be defined as slow weight loss. 

So fast weight loss – also termed “rapid weight loss” – is losing more than 1 kilo a week over several weeks.

What does the research say about fast weight loss?

There are several well-conducted studies examining 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% 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.

Some 50% of the slow weight loss group and 81% of the fast weight loss group achieved 12.5% or more weight loss during this time. 

After this initial phase, those who had lost 12.5% or more were then placed on a weight maintenance diet for approximately 2.75 years. 

By the three-year mark, 76% of those in the slow weight loss and the same percentage of those in the fast weight loss group had regained their lost weight.

So, it didn’t matter if they had lost it slow or fast, they still regained the weight. 

However, another study on 101 postmenopausal women found 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, aside from weight loss, when it comes to the differing ways of losing weight – such as changes in body composition and bone mineral density.

This is best highlighted by a large meta-analysis. These type of studies combine the results of all previous well-conducted studies on the topic. 

While this analysis found the magnitude of weight loss was similar for both approaches, slow weight loss resulted in better outcomes than fast weight loss with respect to metabolism or how many calories we burn at rest. 

There were no differences in the amount of fat-free mass or muscle mass lost between the slow and fast weight loss groups. But slow weight loss resulted in greater reductions in fat mass and therefore a better fat-to-muscle ratio.

Slow weight loss also seems better for bone density, because rapid weight loss results in a twice as much bone loss and puts a person at increased risk of brittle bones or osteoporosis.

What about other diet approaches?

Research shows it doesn’t matter what type of macronutrient diet you follow – moderate or high-protein diet, low or high-carbodyrate diet, low or high-fat diet. All diet approaches achieve similar weight loss outcomes

The same can be said for fashionable ways of cutting calories from the diet, such as intermittent fasting. Research has shown such diets don’t result in any better weight loss results than any of its predecessors. This is because our body is extremely good at protecting against weight loss

When you want to lose weight consider…

Your metabolism When you lose large amounts of weight, you 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.

However, research has confirmed slow weight loss preserves your resting metabolic rate compared with rapid weight loss. As does a weight loss program that includes exercise rather than one that focuses on diet alone.

Side effects 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. 

Sustainability Many fast weight loss diets restrict or exclude foods required for long-term health. Carbohydrates are often banned, yet wholegrain carbohydrates are an essential source of nutrition, helping with weight loss and prevention of disease. Including meal replacements as part of a restrictive diet is also not sustainable for long. 

The bottom line?

Regardless of how you lose the weight, it’s very difficult to maintain losses. Our bodies work to keep our weight around a set point by adjusting our biological systems and imposing a series of physiological changes within the body to ensure we regain weight we lose. This stems from our hunter-gatherer ancestors, whose bodies developed this survival response to adapt to periods of deprivation when food was scarce. 

Successful long-term weight loss comes down to: 

1. following evidence-based programs based on what we know about the science of obesity

2. losing weight under the supervision of qualified health-care professionals

3. making gradual changes to your lifestyle – diet, exercise and sleep – to ensure you form health habits that last a lifetime.

This article was originally published in The Conversation.

About Dr Nick Fuller

Dr Nick Fuller is the founder of Interval Weight Loss and is a leading obesity expert at the University of Sydney with a Ph.D. in Obesity Treatment. Dr Fuller is also the author of three best-selling books and his work been published in top ranked journals in the medical field, including JAMA, Lancet and American Journal of Clinical Nutrition.