Liz is a typical 50-something woman, fit, 70 kg, 30% body fat.
So what’s going on here: is it Liz, or is it the universe conspiring against her?
How do you actually ‘lose weight’?
Let’s start by considering the body as a store of energy. The body can be divided into two components. One is fat mass, and the rest of the body is called fat-free mass. It’s mostly water, but there’s also bone and muscle protein. Fat contains much more energy (and thus requires more energy to burn).
Like most of the adult population, Liz wants to lose weight. To do this, she has to go into energy deficit: energy out must be greater than energy in. The amount of weight she loses will depend on whether she is losing fat or fat-free mass.
It takes a much bigger energy deficit to lose a kilogram of fat than a kilogram of fat-free mass. We also need a bigger energy deficit per kilogram of weight loss if we are fatter to start with.
For most people, it takes an energy deficit of about 27-32 kJ to lose a gram of body weight. If Liz runs for 35 minutes at 10km/h on the treadmill, she has a deficit of about 1500kJ, so she will have lost only 50g in a session. If she does this five times a week for a year, however, she will lose over 12 kg.
Except, of course, she doesn’t. After a year she’s still stuck on 70 kg. Why?
Eating more to compensate for exercise?
The first possibility is that Liz is eating more to compensate for the extra exercise. Her 35 minutes of treadmill running will be entirely undone by a glass and a half of merlot that evening.
There is some evidence people use food to reward themselves for exercising. A recent analysis suggested women may be particularly prone to fuel up after exercise. So Liz may be unconsciously munching or drinking away that energy deficit.
Being less active elsewhere?
A second possibility is that Liz compensates for going to the gym by being less physically active elsewhere. She may flop down in front of the TV rather than busying herself with chores. She may even fidget less.
This theory is known as the “activitystat” hypothesis: the idea that we have a setpoint for energy expenditure like the setpoint on a thermostat. If we increase physical activity in one domain, then there is an automatic compensation in another.
A simple guide to not feeling bad about your body at the beach…Post continues after audio.
So is Liz undoing all the good work at the gym by couch-potatoing her way through the rest of the day? We tested this rather depressing theory. Readers will be pleased to know we found no evidence for the activitystat when sedentary adults started an exercise program. They just cranked up the exercise, drawing time from sleep and TV.
A reduction in your resting metabolic rate?
One of the unfortunate side effects of losing weight is resting metabolic rate — the rate at which you use energy when you’re sitting doing nothing — starts to fall (meaning you burn less energy).
A recent study of contestants on The Biggest Loser found their resting metabolic rate was depressed six years after having lost and regained most of the weight.