Why Restricting Calories Doesn’t Work

And why you tend to regain any weight that’s lost

Jonathan Adrian, MD


Photo by Toa Heftiba

Calories Out > Calories In = Weight Loss

It seems quite simple, doesn't it? Considering the above equation is true, there are two strategies to losing weight. One could plan a rigorous exercise regimen to keep caloric expenditure constantly above the intake, or one could just simply eat less.

While the idea appears convenient, our biology does not conform to those premises. Homeostasis seeks to maintain nearly constant conditions in the internal environment. Any biological function will therefore oscillate around a predetermined mean, within a range that is considered normal.

The concept of homeostasis is rarely talked about outside medical lecture halls, but it is a prerequisite for life. It regulates our body’s hormone levels, keeps our electrolyte concentration in check, and maintains other vital processes in homeostatic equilibrium. Homeostasis is what keeps our heart rates relatively stable when we’re sitting and resting, but high during periods of strenuous exercise, to match the metabolic demand of our persevering muscles.

Diets work along the same tropes. We have an internal set weight that the body utilises as a homeostatic checkpoint. Like a thermostat, our internal sensors will be ticked off by parameters that exceed the homeostatic threshold (e.g. a Thanksgiving feast), and respond by providing calculated instructions to counteract such changes (increase metabolic activity). Once these corrections take place, our weight returns to equilibrium. Order is preserved.

With diets, the body achieves this equilibrium by altering our energy expenditure or resting metabolic rate.

The problem with our initial assumption is that the equation shifts as we reduce caloric intake. Our body reduces caloric expenditure to maintain homeostasis.

This feedback system makes evolutionary sense. During times when food was considered a scarce resource, starvation would have lead to certain doom if our biology hadn't solidified a system to somehow curb caloric expenditure. However, under the same intended benefit, we tend to stock up on additional energy when food did come around.

If the hunters were successful in the day’s pursuit and a feast ensued, the thermostat wouldn't attempt to reduce weight back to baseline. Instead, it keeps it a little above the threshold. The reason? To mitigate emaciation in events of starvation. Other biological kingdoms approve of this principle. The black bear, for instance, can gain up to 30 pounds a week before its winter-long hibernation.

Several studies seem to agree with this theory. One study followed 48 participants for 6 months and divided subjects into a control group receiving diets that matched their caloric expenditure and test groups receiving 12.5–25% below theirs. Total daily energy expenditure (TDEE) was significantly reduced at 3 months and 6 months. Another study followed overweight and obese women, utilising similar methods to the previously described study, and found reduced resting energy expenditure (REE) after 3 months. The first study also mentioned that behavioural adaptation (e.g. decreasing physical activity) may play a role in reducing TDEE.

So if we know that caloric restriction alone isn’t effective at weight loss, then what are our options? A growing body of evidence suggests that intermittent fasting may overrule this homeostatic notion.

Intermittent fasting is a dietary approach that is based on meal timing. There are many variants of the intermittent fast, the most popular one being the 16:8, where a person has an 8-hour eating window during the day and fasts during the remaining 16 hours. Water and other calorie-free drinks such as unsweetened coffee and tea are generally unrestricted.

The proposed benefits from intermittent fasting come primarily from the hormonal changes that follow certain meal schedules. Studies have shown that fat deposition or lipogenesis is stimulated by insulin, an anabolic hormone. The conventional 3-meals-a-day schedule with sporadic snacking causes random blood glucose and insulin surges throughout the day, in contrast with one (albeit tall) surge for people who fast intermittently. The concentration of other supplementary hormones, such as ghrelin (the hunger hormone) also declines the longer a person fasts.

Additionally, intermittent fasting may increase fat loss as the body starts burning fat for fuel in place of its usual substrate — sugar.

While this pathway may seem counterintuitive to the idea of preventing malnourishment in periods of lengthened starvation, it makes a full circle. Evidence has shown that with intermittent fasting, more fat is burnt and less muscle is wasted. This ensures a person stays lean and fit.

Being lean of course meant improved mobility and athletic prowess, which for our hunter-gatherer ancestors could have made all the difference.



Jonathan Adrian, MD

Doctor, writer, photographer, and part-time social media strategist.