Have you ever wondered why weight loss is so difficult to maintain? Perhaps you’ve tried many diets and successfully lost weight but within months the weight is creeping back on and before you know it you are back where you started – or even heavier. Losing weight by restricting calories inevitably leads to a drop in your resting metabolic rate (RMR), the more you restrict calories the more your RMR will fall and the more you will need to restrict calories further – it becomes a vicious circle. So how do you stop this problem from occurring?
What is resting metabolic rate (RMR)?
RMR is the amount of energy (calories) your body needs to maintain its functions when you are at rest. Your RMR is determined to some extent by your sex, size and your body composition, i.e., men and large or more muscular people tend to have a higher RMR than women and small or less muscular people. However, the main control of your RMR is determined by your thyroid hormones – the more thyroid hormones you produce, the higher your RMR and vice-versa. If your RMR is falling, this means you need less calories to maintain your body's functions and therefore need to consume even less calories to lose weight.
Why does dieting lead to a fall in RMR?
Weight loss requires that you burn more calories than you consume, and most types of diets use calorie restriction to induce weight loss. However, this is a simplistic way of looking at weight loss. Many studies show that when we simply reduce our calorie intake our body responds by reducing our RMR. So, though you consume less calories, over a short period of time (a few weeks or months), your body adapts by reducing secretion of thyroid hormones and your RMR drops to match your new reduced calorie intake, this is called metabolic adaptation (1). Weight loss then stalls. If you respond to this stall by reducing your calorie intake more, your body will respond by reducing your RMR further. So, it gets harder and harder to lose weight as time goes on (and easier to put it back on).
Why do you regain weight so quickly?
When you try to stop your diet, your RMR may have fallen so much that even if you increase your calorie intake by just 100 calories a day you may exceed your new low RMR, i.e., you will be consuming more calories than you are burning, and the weight will pile on. An extreme example of this occurred in contestants on the US show ‘The biggest loser’, a reality TV show where morbidly obese contestants competed to lose the most weight through extreme calorie restriction and excessive exercising. Though they lost impressive amounts of weight, most of them regained much of the weight over the next 6 years, continued to have a lowered RMR and were unable to eat normal amounts of food – they became trapped on their low-calorie diets and felt constantly hungry (2).
Low fat, calorie restricted diets can cause RMR to fall
Successful weight loss and weight maintenance happens when your RMR is maintained at near normal levels for the duration of your weight loss period. More traditional low-fat, calorie-restricted diets do not suppress the fat storage hormone, insulin, because they tend to restrict the amount of fat eaten (no effect on insulin levels) but don’t restrict the amount of carbohydrates eaten (which increase blood insulin levels). Since you can’t burn your own fat at the same time you are storing it, your body responds by lowering its calorie requirements, i.e., by reducing RMR. This can lead you to feel tired and lethargic but also very hungry as the hunger hormone, ghrelin, is increased in this type of dieting (3).
Low-carbohydrate, high fat diets (LCHF) have a different effect.
The low carbohydrate/sugar content of this diet suppresses insulin and allows the body to go into a fat burning mode. The higher protein content of the diet suppresses appetite, making it easier to naturally reduce the amount of food eaten. With this diet, calories needed to maintain the RMR come from a combination of food calories and body fat calories so metabolic adaptation is minimised. Indeed, several studies have shown that low-carb diets have less impact on RMR than low-fat diets (4). Low-carb diets also reduce the levels of the hunger hormone, ghrelin, which itself can reduce RMR; and increase leptin sensitivity, a key hormone for body weight regulation (5). However, that does not mean that low-carb diets have no impact on RMR.
So, what else can you do to maintain your RMR?
Maintaining muscle mass is key to maintaining RMR whilst losing weight. Muscle requires far more calories to maintain it than other body tissues so the more muscle you have the higher your RMR will be. However, most types of diet can lead to muscle breakdown to some degree unless something is done to prevent it. Studies show that low-carb diets are better than low-fat diets for sparing muscle breakdown, probably due to their higher protein content (6 ). However, protein does not build or maintain muscle effectively unless the muscles are exercised (7).
Studies show that the most effective type of exercise for building and maintaining muscle is resistance training (weightlifting), which has been shown to preserve muscle during a weight loss program compared to just dieting alone (8). You do not need to go to the gym for hours to do resistance training, though you can if you want. Investing in some hand weights or booty bands (resistance bands) at home is sufficient to exercise your arm muscles and squats and lunges will improve muscle strength in your legs. Aim for at least 15 minutes of moderate resistance training three times a week. Other benefits of doing resistance training include reduction in blood pressure, reduction in HbA1c in type 2 diabetics and pre-diabetics (9) and reduction in body inflammation (10)
Most weight loss strategies can lead to a reduction in RMR which may make sustainability of long-term weight loss difficult to achieve. Low-carbohydrate diets may minimise the reduction in RMR compared to low-fat calorie restricted diets due to their higher protein content, improved body-fat burning, improvements in hormone balance, and increase in energy expenditure. However, this effect is enhanced when resistance training is adopted to maintain or build muscle mass.
So, if you have hit a weight loss stall with your low-carb diet, try hitting the resistance training to build up your RMR and kick start your weight loss again!