Pros and Cons of Rapid Weight Loss

Sprint versus marathon diets: A science-based look at fasting and very-low-calorie diets.

Chana Davis, PhD
7 min readMar 4, 2020


Photo by Steven Lelham on Unsplash

Rapid weight loss is a horrible idea, they say. “It wrecks your metabolism”. “You’ll only lose muscle”.

Is it really that bad? The surge of interest in fasting for weight loss had me wondering what really happens when we lose weight quickly.

It turns out that the differences in how sprint and marathon dieting impact your body are more grey than black and white. Sprint dieting even has some potential upsides. Let’s dig into the science so that you can weigh the pros and cons of sprint dieting for yourself.

Upsides of Sprint Dieting

Morgan Mitchell. Amazing, lean, green, sprinter. Image Source

The clearest potential benefit of sprint dieting is that you get it over with quickly. Some people perform better when faced with a short, but intense challenge, rather than a sustained moderate challenge.

Studies comparing sprinters and marathoners on the same diet but with different daily calories can shed light on the tradeoff. In this 2016 study, the target loss of 7 kg (15 lbs) total mass was attained in five weeks for sprint dieters, compared to twelve weeks for marathon dieters. In this 2017 study, sprint dieters took about 5 weeks to lose 5% of total body mass (~5 kg or 12 lbs), whereas marathon dieters took about 15 weeks to achieve the same loss.

In addition, very low calorie diets may offer some of the emerging health benefits of fasting, though it’s still early days. There are many outstanding questions, such as who is likely to benefit, and what “dose” of fasting is needed. Studies such as this 2017 randomized human trial showing potential metabolic benefits of a “fasting mimicking diet” are tantalizing.

Caveat: General vs Specific Weight Loss Benefits? It can be difficult to separate the metabolic benefits of a specific weight loss method from the general health benefits of losing weight. In obese adults, weight loss often improves health biomarkers, such as blood pressure, blood lipids (e.g. trigylcerides and LDL…



Chana Davis, PhD

Scientist (PhD Genetics @Stanford) * Mother * Passionate about science-based healthy choices * Lifelong learner * Founder: Fueled by Science

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