Conventional weight-loss wisdom claims that to lose one pound (0.45kg) of body weight, you need to create a 3,500-calorie deficit.
This theory, expounded by health and fitness professionals for some five decades, first gained traction in 1958 when Dr Max Washnofsky wrote a paper in the American Journal of Clinical Nutrition concluding that “3,500 calories is the caloric value of one pound of body weight lost.”
This figure also gave rise to the 500-calorie-a-day-deficit guideline as it neatly broke that ‘magical’ figure of 3,500 into seven – seven days in a week to deliver sustainable, healthy weight loss of up to 0.45kg a week.
While it’s not entirely wrong, the real-world application of this theory is often far removed from the controlled lab environment where a pound of body mass will liberate 3,500 calories of energy in accordance with the first law of thermodynamics.
The problem is that this theory is predicated on the over-simplified theory of energy balance; a theory where all calories are considered equal.
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Complex process
The truth is that the environment within the human body is far more complex. There are multiple factors at play, all of which influence your ability to metabolise ingested and stored energy for fuel, the primary source of that energy, and the rate of metabolism of that fuel.
Factors such as your weight, hormonal balance, enzyme function, the composition of your gut bacteria, your current nutritional status, your daily activity levels and the intensity of that activity, and the composition of your diet all affect your metabolism and, therefore, influence your ability to effectively lose weight and keep it off.
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Survival response
When a person restricts calorie intake and increases energy expenditure to create a 500-calorie deficit, their body perceives this reduced energy availability as a threat due to possible starvation and acts to ensure their survival.
The various hormonally-driven biological and the numerous physiological process that ensue effectively lower your daily metabolic rate through a process known as metabolic adaptation or adaptive thermogenesis.
These steps include an increase in mitochondrial efficiency, which effectively means we’re able to burn fewer calories to meet our energy requirements, with a concomitant increase in hormones that promote catabolism and hunger, and a decrease in hormones that promote anabolism (tissue growth), energy expenditure, and satiety.
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Diminishing returns
As a result, a daily deficit of 500 calories produces slightly less of an effect on each subsequent day of your diet.
While the impact of this diminishing effect is negligible at first, the more weight we lose the greater this response becomes. It’s the reason why many people lose a significant amount of weight initially, especially obese individuals, but then plateau.
The impact of this metabolic adaptation is so profound that over a 12-month period you can only expect to lose 50% of the weight that the 3,500-calorie rule estimates.
This effectively means that you would then need to create a 7,000-calorie deficit to lose the same amount of weight you achieved in the initial stages of a diet, which is extreme and unsustainable.
While the math can become quite complicated and the law of individual difference (and the resultant individualised response) dictates that this won’t be the same for everyone, the general consensus is that the longer you diet, the greater the your calorie deficit will need to be to keep seeing the same results.
Author: Pedro van Gaalen
When he’s not writing about sport or health and fitness, Pedro is probably out training for his next marathon or ultra-marathon. He’s worked as a fitness professional and as a marketing and comms expert. He now combines his passions in his role as managing editor at Fitness magazine.
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