Almost anyone who has tried to lose weight knows how frustrating it can be when the weight loss slows and eventually reaches a plateau. There is a great deal of misunderstanding as to why this happens, most of which stems from a confused idea of what the term “metabolic rate” means. People often talk about having a “slow metabolism” or that their metabolism is “slowing down” as a result of weight loss. So let’s try and clear up the confusion.
The term “resting metabolic rate” (RMR) simply refers to the number of calories per day you need to maintain your current body weight. Various factors can influence RMR including age and gender but body weight is the most important. Put simply, the heavier you are, the more calories you need to maintain your weight. So, contrary to what is widely believed, overweight individuals have higher metabolic rates, not lower ones than their normal weight peers. Incidentally, the terms “slow” or “fast” metabolism are confusing and shouldn’t be used.
Now we can understand the plateau effect. Suppose that at your present weight you have an RMR of 2500 calories (scales that give printouts will have the RMR on them) and you then start a 1000 cal/day diet. This means you save 1500 calories each day – which is what we call the energy deficit. A daily energy deficit of 1500 calories equates to a calorie saving of 10,500 in a week (7 x 1500) and because 1lb of fat equates to 3,500 calories, it results in a weekly weight loss of 3lbs.
However, it’s important to remember that each time you lose weight there will be a corresponding reduction in your RMR. Assume that as a result of your weight loss your RMR has fallen to 1500 cals/day. If you stay on the 1000 cals/day diet, you now have an energy deficit of only 500 cals/day (or 3500 per week) which equates to a weekly loss of 1lb. Then, if you lose even more weight and your RMR falls to 1250 cals/day, your energy deficit is just 250 cals/day, or half-a-pound of fat weight per week. So you can see that the initial weight loss of 3lbs per week has fallen to half-a-pound per week – this is the plateau effect.
How do you get over the plateau? The trick is to increase your physical activity as your weight comes down. The additional calorie expenditure will compensate for the narrowing of the energy deficit from diet alone. It’s the bit that most people forget, but it’s crucial to getting rid of those last few – stubborn – pounds and keeping them off.
Dr David Ashton
29th November 2011