Tuesday February 21 2012
Researchers say weight loss slows as the body adapts to diets
Losing is weight “twice as hard as previously thought”, according to media reports. Government weight-loss guidelines “mislead” overweight people about the effort needed to lose weight and dieting rules “don’t take into account changes in metabolism that occur when you lose weight”, several newspapers said.
The reports are based on a recent conference presentation in which obesity researchers reportedly said that current weight-loss guidelines fail to take into account the changes in metabolism the researchers identified. They said that this means weight loss takes twice the time predicted by current guidelines.
However, there is as yet no officially sanctioned way to work out a bespoke weight-loss programme for yourself, which is why if you’re worried about your weight it’s a good idea to have a chat with your GP.
The Independent said that there was a “slim chance” of the weight falling off with a reduction in calories. While this pun may have been hard to resist, it’s not particularly accurate; nor is the Financial Times’ assertion that “obesity experts [have] change[d] fat-loss guidance”.
What is the news based on?
The news reports stem from a presentation by Dr Kevin Hall, a researcher with the National Institutes of Health (NIH) in the US, at the annual meeting of the American Association for the Advancement of Science (AAAS) in Vancouver, Canada. Dr Hall reportedly said that dietary changes spark complex processes that alter the body’s metabolism and body composition and that this has made it difficult to assess the relationship between diet and weight changes. In a nutshell, he suggests that weight loss is not a straight line towards one’s target weight, but a downward curve that plateaus the closer you come to your goal.
Dr Hall’s presentation appears to be based on earlier work from the NIH in which a mathematical model was used to predict what happens when people of varying weights, diets and exercise habits try to change their weight. This model suggests that a reduction in energy intake of 100 kilojoules (about 24 calories) a day for each person would eventually lead to a reduction in body weight of about 1kg. Half the weight loss would be achieved in about a year and 95% in about three years. The NIH says this is only half the weight loss claimed for calorie reduction in current guidelines. This is important because it “leads to unrealistically large weight-loss expectations”, Dr Hall is reported to have told the AAAS conference.
The NIH has developed an online tool designed to calculate the level of dieting needed to achieve a weight loss target, taking numerous factors into account including metabolism. It allows users to adjust their calorie intake as well as activity levels and see how much their weight, body fat and other measures are predicted to change over time if they stick to the tool’s plan.
Is there evidence for this model?
In a paper published in the Lancet in August 2011, NIH researchers, including Dr Hall, described a mathematical model for what happens when people of varying weights, diets and exercise habits try to change their weight. The NIH also developed an accompanying weight-simulation tool. The paper concluded that:
- Body-weight response to a change of energy intake is generally slow and depends on individual body composition and individual metabolic changes.
- Generally, an average overweight adult who reduced their energy intake by 100 kilojoules a day would eventually lose about 1kg. Half the weight change would be achieved in about a year and 95% of the weight change in about three years.
- Adults who are heavier have a larger expected weight loss for the same change of energy intake, although reaching a stable body weight will still take them longer.
It should be noted that this is not a clinical trial looking at real weight loss in people. It is a mathematical model that has been validated by comparing it to some actual changes in people. An NIH clinical trial comparing the effects of reducing fats and carbohydrates in obese adults is currently recruiting participants in the US.
What do current guidelines say?
In the UK, general guidance on losing weight for overweight or obese adults does not make detailed calculations of the relationship between calories in diet and weight change. Dietitians can help someone calculate the calorie loss needed to achieve a target weight over a certain period, but they would tailor this to the individual.
Overweight and obese adults are usually advised to follow a low-fat diet or one where the food eaten each day provides about 600 fewer calories than the body needs to stay the same weight. This is recommended by the National Institute for Health and Clinical Excellence (NICE) based on thorough appraisal of the evidence available, rather than being a “rule of thumb” as suggested in several newspapers. Very low-calorie diets are sometimes recommended by professionals for short periods of time in specific circumstances.
Getting enough physical activity (150 minutes a week for adults) is also usually crucial for most people who need to lose weight.
Do I need to change the way I diet?
This paper uses a mathematical model to advance the theory that for most people weight loss through dieting takes far longer to achieve than has previously been calculated. It doesn’t provide any evidence on the topic of specific diets and weight loss or advise specific changes to how people diet.
However, the news stories serve to remind us to set achievable weight loss goals, be realistic about our chances of reaching them and be determined in our efforts to do so.
Can I try the weight simulator?
The NIH online weight simulator tool allows people to input their age, sex and body weight and then calculate how many calories they’d need to cut from their diet in order to achieve their target weight loss. However, it is primarily a research tool. It includes no advice on diet or exercise and is not a substitute for personal medical advice on diet and weight loss. While the general principles of weight loss apply to all humans, this has been developed in the US, presumably for a US audience where the incidence of obesity is greater than that of the UK. As yet, it is not known whether a UK version will become available.
Where can I get weight loss advice?
If you think you need to lose a bit of weight, there’s a lot that you can do to help yourself. The key things you might consider are:
If your weight is becoming a real health problem for you, it’s best to see a healthcare professional such as your GP or a nurse at your GP surgery. He or she can measure your BMI and provide information and support about losing weight, the benefits of a healthier diet and doing more physical activity. They can also tell you about local organisations that can help people to lose weight.
For most overweight people, losing just 5% of your body weight can have health benefits such as lowering blood pressure and reducing the risk of diabetes.
Analysis by Bazian.