Nutrition - Free Fitness Library
The truth about dieting and metabolic rate
By Anita Bean BSc
Does dieting reduce your resting metabolic rate (RMR) and make it harder to lose weight? Its easy to blame dieting failure on a slow metabolic rate. Since RMR accounts for 60 - 75% of your total daily energy expenditure, a drop in your RMR can slow down weight loss, reduce motivation and make it more difficult to achieve your goal.
The resting metabolic rate is the energy expended by your body at rest (lying down) to maintain essential functions such as brain function, digestion, heart beat and circulation.
Dieting and Metabolic Rate
When you restrict your food intake, your BMR falls initially (about 10%) in an attempt to conserve energy, as it assumes that food is in short supply. However, this decrease is not permanent; the body adapts to the reduced food intake and so the RMR increases within 1 -2 weeks. In any case, it returns to its original level once normal eating is resumed. So, it is a myth that dieting slows down your metabolism in the long term.
Dieting, Exercise and Metabolic Rate
It has been suggested that exercise will help prevent this drop in RMR. However, studies have reported mixed findings. Some have found that exercise prevents or minimises the reduction in RMR with dieting; others have found that exercise decreases RMR even more than dieting alone.
To settle the debate, US researchers have recently analysed the methods and results from 22 studies.
The majority of studies involved women aged between 31 and 45 years who were fed a relatively low fat high carbohydrate diet of less than 1200 kcal a day. The main prescribed exercise programme was aerobic in nature, 31 - 60 minutes duration, moderate intensity (51 - 70% of VO2 max) and performed 4 - 5 days a week.
The results of the analysis can be summarised as follows:
(1) The RMR decreases significantly when an individual embarks on a diet or on a diet plus exercise programme
(2) The drop in RMR with diet alone is significantly greater than with diet + exercise.
(3) Combining diet with a moderate intensity exercise programme prevents some of the decrease in the RMR.
(4) The size of the drop in RMR varies from one person to another. In general, the heaviest people experience the greatest drop in RMR. (This is most probably due to the fact that they experience the greatest deficit between calorie intake and output)
When the results are expressed per kg of fat free mass, the drop in RMR for diet + exercise is relatively small, and less than that observed for diet only.
What is interesting to discover is that the RMR expressed per kg body weight returns to pre-dieting values for both types of treatment. In other words, the RMR does not stay permanently lower after dieting with or without exercise.
The previous studies on metabolic rate and dieting have produced variable results. This, say the researchers who analysed the data, is due to differences in methods used and to poor reliability of RMR measurements.
The researchers suggest a number of mechanisms to explain why exercise diminishes the drop in RMR with dieting:
(1) Exercise prevents loss of lean tissue associated with dieting and therefore helps maintain RMR.
Indeed, the results of the analysis show that the diet-only groups lost a greater percentage of lean weight (25% of weight loss) than the diet + exercise group (17% of weight loss).
(2) Exercise has a ‘carry-over’ effect on RMR, sometimes called the ‘afterburn’, causing it to remain elevated for a while after exercise.
Your Weight and Metabolic Rate
It is a myth that overweight people have a slow metabolic rate (except in rare cases e.g. hypothyroidism, Cushing’s Syndrome),
Studies have shown that RMR is proportional to total body weight. In other words, the heavier you are, the higher your RMR. The proportion of lean mass to fat mass also has a small effect on RMR. Lean tissue is active tissue and burns more energy than fat. Thus, if two people weigh the same but one has a higher amount of lean tissue and less fat, he will have a slightly higher RMR than the other person. However, this makes only a relatively small difference to RMR since fat mass also has an additional energy cost i.e. extra energy is used to support the fat mass.
RMR can be predicted from total body weight using standard equations developed by the World Health Organisation (Fig 1).
Cut calories slowly
If you normally eat 2500 kcal/ day, cut down to between 2000 kcal and 2250 kcal and increase your energy expenditure by around 250 kcal per day. This will result in a calorie deficit of 3500 - 4750 per week, equivalent to a fat loss of 1 - 1.5 lb per week. If your weight loss slows, you can gradually cut down to 1750 or increase your energy output. That way you will avoid or at least minimise a drop in your BMR.
Yo-yo dieting
While dieting usually produces short term weight loss, most people regain that lost weight within a year. So they begin another diet and so the cycle continues. According to research at Nottingham University, 40% of women embark on a diet two or more times a year. Is this ‘yo yo’ dieting harmful to health?
It has been suggested that yo yo dieting leads to a drop in the metabolic rate, increase in % body fat and an increased difficulty in achieving weight loss in the future. But what exactly are these suggestions based on?
The research to prove these negative effects of yo yo dieting is a little inconsistent at present and the exact mechanism is unclear. In an attempt to clarify the issues relating to yo yo dieting, the National Institute of Health (NIH) in the United States commissioned a panel of experts to review the evidence as part of the National Task Force on the prevention and treatment of obesity. They examined the methods and results of 43 studies. Their findings are summarised below.
Yo Yo Dieting and Metabolic Rate
Early studies with lab animals reported that repeated dieting caused a slow down in the body’s resting metabolic rate, making the body more efficient in storing food energy and reducing the amount of energy used in heat production. The theory was that once you stopped dieting, you would regain weight (fat) more readily because your body needed fewer calories. Future attempts to lose weight would be harder because you would need to cut calories even further.
However, this theory is not supported by scientific evidence in humans. Most human studies show no relationship between number of dieting attempts and ability to lose weight. A study in 1990 found that yo yo dieting produced no change in metabolic rate - just the opposite of popular belief!
The NIH concluded that there is no evidence that fluctuations in body weight produce a decrease in the metabolic rate.
Yo yo dieting and body fat
It has been suggested that yo yo dieting causes an increase in body fat stores , especially around the abdominal region which, in turn increases the risk of heart disease. However, most studies have fond no difference in body fat distribution between yo yo dieters and non yo yo dieters. Other researchers have suggested that repeated dieting causes a loss of lean body mass (muscle and organ tissue) but, again this has not been proved to be significant.
The NIH panel concluded that there is no evidence that yo yo dieting causes an increased storage of body fat around the abdomen. They also said that there is no proof that yo yo dieting has an adverse effect on body composition or future attempts to lose weight successfully.
Yo yo Dieting and Heart Disease
Repeated weight fluctuations have been linked with an increased risk of heart disease, secondary diabetes, gall bladder disease and premature death. Studies carried out at Baltimore University in the United States concluded in 1991 that the risks of repeated weight fluctuations are equivalent to the risks of being severely overweight.
A study on Harvard graduates found that those individuals whose weight had fluctuated by 10 kg or more over 30 years had a significantly greater risk of death than those whose weight had remained relatively stable.
This, propose researchers may be due to increases in blood pressure, blood glucose levels, insulin levels, reduced glucose tolerance, increased risk of secondary (no insulin dependent ) diabetes, increased blood fats or blood cholesterol.
However, the NIH panel have carefully examined the methods used in these studies and found that there is no clear risk of heart disease or premature death associated with yo yo dieting in the majority of studies. However, they did concede that some studies have shown that yo yo dieting increases risk. They made no firm conclusions on this area.
Yo yo Dieting and Self esteem
Yo-yo dieting can be bad for your morale and psychological health. Each time you regain weight, you experience a sense of failure which can lead to lowered confidence and self esteem. Indeed, many people have found that repeated dieting and weight gain causes a negative psychological state, depression and a sense of failure. Unfortunately, very few studies have been carried out on the psychological effects of yo yo dieting and the NIH recommend that more research is devoted to this area.
Conclusions:
In conclusion, studies to date show that moderate intensity exercise (51 - 70% VO2 max) performed 31 - 60 minutes a day for 4 - 5 days a week can reduce some of the decrease in RMR with dieting. However, more research is needed to ascertain the ideal type, intensity and duration of exercise or the ideal calorie intake.
The drop in RMR resulting form dieting can be minimised by including exercise. Both aerobic and strength training should be included. Aerobic exercise will increase calorie expenditure and cause a small ‘carry over’ effect on RMR. Strength training will maintain (or increase slightly) muscle mass and therefore, increase total calorie expenditure.
Calorie intake should be reduced only modestly (e.g. 250 - 500 fewer calories than normal). You should aim to create a calorie deficit of 500 - 1000 per day to produce a fat loss of 1 - 2 lb a week. The combined result is faster and more effective fat loss and preservation of lean tissue.
Yo yo dieting does not cause a long term drop in the metabolic rate and there is no evidence that yo yo dieting increases body fat or reduces lean tissue however, these are not good reasons to repeatedly diet or to remain complacent about overweight. The solution is to attain a healthy weight once and for all.
Boost Your Metabolism
It is possible to increase your metabolic rate in a number of ways.
Strength Training:
Regular strength training, performed 2-3 times per week increases lean (muscle) mass. Since muscle tissue has a higher energy requirement than fat tissue ( i.e. it is metabolically active) you will use more calories throughout the day. Research has shown that adding 3lb of muscle increases the metabolic rate by 7% and the daily calorie requirement by 15%.
Strength training also increases the number of calories you use during exercise. The more muscle tissue you have the greater number of calories burned during rest and during activity.
Aerobic Training:
Aerobic exercise as a two fold effect on your metabolism. Firstly, during exercise your metabolism increases significantly. In other words, you burn considerably more calories - as much as 5-10 times more than at rest (depending on how hard you are exercising). Secondly, there is the 'afterburn', as your metabolic rate remains slightly elevated for a short while after exercise. Technically termed the 'excess post-exercise oxygen consumption' (EPOC), your body continues to take in more oxygen and burn more calories to replenish it's store of high-energy compounds (APT and PC). The greater the intensity and duration of your exercise session, the greater EPOC and rise in metabolism. It may only be a few minutes after gentle exercise but can last for over an hour after vigorous exercise. Therefore, the harder, longer and more frequently you workout, the greater the metabolism boost.
Dividing your exercise programme in to 2 shorter daily workouts produces two separate EPOC periods and therefore may result in a greater increase in calorie output. In other words, your metabolism is boosted even more.
Eating Frequency:
Divide your food intake into 4-6 meals or snacks a day at regular intervals. Each time you consume a meal, the metabolic rate rises approximately 10%. This rise is called the thermogenic affect of food and accounts for the extra energy needed to digest, absorb, transport and metabolise the food. Therefore, the more frequently you eat, the greater your calorie expenditure.
Carbohydrate and Protein Calories
All calories are not equal. When carbohydrate or protein are consumed, they cause an increase in protein or carbohydrate oxidation (breakdown) respectively, thus increasing your metabolic rate. So, a significant proportion of carbohydrate and protein calories are given off as heat. Latest research indicates that protein has the greatest metabolism boosting potential. However, that doesn't mean that you should eat a high protein diet - an intake corresponding to approximately 15% of your daily calories is recommended. Carbohydrates should comprise 60-70% of calories.
Ten Ways To Boost Your Metabolism
1. Include strength training in your programme at least twice a week.
2. Include aerobic activities 3-5 times a week for 20-60 minutes.
3. Divide your training sessions into 2 shorter daily workouts.
4. Gradually increase your workout intensity (this will increase the EPOC)
5. Eat more frequent meals and snacks, at least 4-6 per day.
6. Consume a high carbohydrate (> 60% calories). moderate protein (15-20%) low fat diet (less than 25% calories)
7. Gradually increase your total food/calorie intake; the body adapts by burning more calories off as heat.
8. East most of your food intake early in the day.
9. Have your main meal at lunchtime and avoid eating a large meal late at night.
10. Eat a large high-carbohydrate breakfast e.g. cereals; porridge;
toast; bread; fresh and dried fruit.