What Happens to your Metabolism when you Restrict Calories?

Wouldn’t it be great if losing weight was simply a function of decreasing how many calories you ate?  Everyone wants to manipulate calories to control their weight just like dollars determine their bank account balance.  It’s just not that simple, though.  When dealing with body weight, we’re talking about an infinitely complex human body, not a cold steel bank vault.

Here’s an inconvenient problem with the Calories In Calories Out model: When we eat more calories, our metabolism increases.  When we eat fewer calories, our metabolism decreases.  That’s right…our body actually adapts to how much we’re eating.

That’s a smart design, if you think about it.  When our ancestors were faced with states of famine, it would have been dangerous if their bodies still demanded the same amount of energy as when a bountiful harvest was available.  That would be one way to ensure our own demise.  Instead, the human body adapts to a state of famine by reducing its energy requirement.

The Biggest Loser Study

An excellent demonstration of this phenomenon is available from the reality TV show, The Biggest Loser.  The contestants were individuals with obesity who worked with personal trainers to lose weight during a 30-week competition, by means of intensive Calorie restriction and intensive exercise.

Despite a significant increase in physical activity while maintaining lean-body mass, participants experienced a significant decrease in their daily Total Energy Expenditure (TEE).  The major component of TEE, the Resting Metabolic Rate (RMR), decreased from 2607 kcal/d at baseline to 1996 kcal/d at the end of the 30-week competition.

At a follow-up evaluation 6 years after the competition, participants had experienced significant weight regain, and their RMR’s remained low, at 1903 kcal/d.  Even after accounting for body composition changes and the increased age of the participants, the RMR after 6 years remained ~500 kcal/d lower than expected.

This phenomenon of reduced metabolic rate in the setting of significant caloric restriction has been demonstrated before, most notably in the Minnesota Starvation Experiment.

The takeaway message from these and related studies is that significant calorie restriction is a recipe for easy weight regain.  There is also potentially significant harm to one’s metabolism in the long-term, as seen in the Biggest Loser follow-up study.