Exercise is important for health, but study suggests that activity alone does not necessarily burn extra calories, and that diet should be the focus of weight loss
In the UK, health advice includes a recommendation for 150 minutes of moderate activity a week for adults.
Exercise alone is not enough to lose weight because our bodies reach a plateau where working out more does not necessarily burn extra calories, researchers have found.
The team are the latest to challenge obesity prevention strategies that recommend increasing daily physical activity as a way to shed the pounds.
In a study, published in Current Biology on Thursday, they suggest that there might be a physical activity “sweet spot”, whereby too little can make one unhealthy but too much drives the body to make big adjustments to adapt, thus constraining total energy expenditure.
If true, it would go some way to explaining an apparent contradiction between two types of study carried out by researchers. On the one hand, there are studies which show that increasing exercise levels tends to lead to people expending more energy and on the other, there are ecological studies in humans and animals showing that more active populations (for example hunter-gatherers in Africa) do not have higher total energy expenditure.
Prof Herman Pontzer of City University of New York (CUNY), one of the new study’s authors, said: “Exercise is really important for your health. That’s the first thing I mention to anyone asking about the implications of this work for exercise. There is tons of evidence that exercise is important for keeping our bodies and minds healthy, and this work does nothing to change that message. What our work adds is that we also need to focus on diet, particularly when it comes to managing our weight and preventing or reversing unhealthy weight gain.”
The researchers measured the daily energy expenditure and activity levels of more than 332 adults drawn from five countries across Africa and North America over the course of a week.
They found that physical activity does have a weak influence daily energy expenditure, but only among subjects on the lower half of the physical activity spectrum. People with moderate activity levels had higher daily energy expenditures – about 200 calories more – than the most sedentary people. However, people who did more than moderate activity had nothing to show for it in terms of increasing the amount of energy they expended.
The results could help explain why people who start exercise programmes with the aim of shedding pounds often see a decline in weight loss – or even a reversal – after a few months.
In light of their findings, the authors suggest revision of World Health Organisation guidance on how to prevent weight gain and obesity, which suggests 150 minutes of activity a week for adults (although it also includes dietary advice). They say it should “better reflect the constrained nature of total energy expenditure and the complex effects on physical activity on metabolic physiology”.
In the UK, the advice is 150 minutes of moderate activity a week, although maintaining a healthy weight is just one of the listed benefits.
The well established contribution exercise makes to reducing risk of a range of diseases such as coronary heart disease, a range of cancers, stroke and type 2 diabetes as well as reducing symptoms of depression and anxiety is also highlighted.
Dr Alison Tedstone, chief nutritionist at Public Health England said: “Being physically active is good for your physical and mental health and also helps to maintain a healthy weight. However, the evidence shows the most effective way of losing weight is to reduce calorie intake through a healthy balanced diet.”
Dr Asseem Malhotra, cardiologist adviser to the National Obesity Forum, went further: “We know exercising in the right way has many health benefits but weight loss isn’t one of them,” he said. “We need to disassociate obesity with exercise altogether. If we’re going to combat obesity, it’s going to happen purely from changing the food environment.”
But Dr Frankie Phillips, a dietician, and spokeswoman for the British Dietetic Association, expressed concern about the message the study might send out. “It is an interesting study and there is a possibility that if we are very, very active there may be some adaptation,” she said. “But for most people even moderate activity isn’t what they are achieving at the moment and that’s crucial. Let’s not put people off before they have even got to a stage where they are moderately active.”
The next step planned by Pontzer and his colleagues is to study how the body responds to changes in activity level, in a bid to explain how it adapts to greater physical demands without consuming extra calories.