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Closing the energy gap |
How active do we need to be to manage and prevent weight gain? Here International Obesity Task Force (IOTF) senior policy officer James Erlichman offers a vigorous argument for better guidelines.
Are current physical activity guidelines now set by public health agencies adequate to prevent most people living in Western societies from unhealthy weight gain?
The short answer is yes if someone still does manual work and their appetite is modest – but arguably not if, like most people, their job is relatively sedentary and they are easily tempted by a cornucopia of affordable and accessible, energy dense food.
The problem is that current guidelines suggest moderate activity for 30 minutes on five and preferably all days of the week. Unfortunately, this ‘brisk walk’ prescription burns, at best, just 200 calories a day. Is that really enough to balance likely food and drink intake?
Heart health v healthy weight
This ‘energy gap’ appears to have arisen because the guidelines were not designed to prevent weight gain – although they were subsequently adopted for that purpose by the WHO and the US National Institutes of Health. The guidelines were originally devised to convince sedentary Americans that even a modest amount of moderate activity could significantly reduce their risk of cardiovascular disease. These guidelines were also designed for ‘heart health’ but they were discarded primarily because Americans were perceived to be ignoring them since they disliked getting ‘sweaty and breathless’. Adherence to moderate activities like brisk walking was thought to be more likely, although the evidence for this assumption is questionable.
Can moderate activity alone prevent unhealthy weight gain?
It can be argued that the switch to the new moderate guidelines has benefited already obese people who may have felt excluded from a public health target that demanded vigorous exercises that they were unwilling or unable to perform. A call to take part in moderate activities has no doubt helped some previously sedentary obese people to become more active.
However, it is important to examine what impact, if any, the switch to moderate activity guidelines is now having in the prevention of unhealthy weight gain in the rest of the population.
In simple energy terms the ‘moderate’ and ‘vigorous’ guidelines are broadly equal at around 1,000 to 1,400 kcal of expenditure per week. Studies have shown that most people who have sedentary jobs and take little or no recreational physical activity have a daily physical activity level (PAL) of around 1.55. That is the energy expenditure of their basal metabolic rate (BMR) expressed as 1.0 – plus their additional activity component (0.55 of BMR) derived from voluntary energy expenditure. However three separate teams of investigators broadly agree that the PAL of at least 1.75 is needed for most people, with typical Western eating habits, to balance expenditure with likely energy intake.
The 200 kcal a day of energy expenditure generated by the ‘brisk walk’ prescription is only enough to raise a typical person’s PAL by around 0.1 (to 1.65) even if they assiduously adhere to the 30 minutes of moderate activity every day of the week.
No doubt this increased PAL will slow down weight gain – assuming no compensatory increase in energy intake. However, it only cuts the expected ‘energy gap’ in half. To achieve energy balance at PAL 1.75 the existing exercise prescription would, therefore, have to be doubled to around 400 kcal a day to as many as 2,800 kcal a week. An individual wanting to achieve that target and close the ‘energy gap’ under current moderate guidelines would have to walk briskly (6.4km/h) for at least one hour, seven days a week – a goal that would require both considerable time and dedication. Moreover, it is unlikely that previously sedentary, unfit people would be able to sustain that pace (hence the real time would be even longer.) Several studies have also shown that people – and especially the overweight – often over-estimate their energy expenditure.
Vigorous activity: The case re-examined
Of course, it is true that the discarded vigorous guidelines yield, in theory, no greater energy expenditure. However, vigorous activity appears to have several crucial advantages in preventing unhealthy weight gain. First, it typically expends between 2-3 times more energy per minute than moderate activities (jogging at 12 km/h v brisk walk at 6.4 km/h). Hence, the frequency and duration need not be so great in achieving the 1.75 PAL target – and this could make adherence more achievable. Second, vigorous activity is much more effective in improving aerobic fitness (maximal oxygen uptake) – allowing easier execution of all activities at increased intensities including that ‘brisk walk’. Third, several studies have shown that, when total energy expenditure is kept equal, vigorous activity is much more efficient in reducing subcutaneous fat than moderate activity.
Finally, vigorous activity appears better to reduce several health risks, not least cardiovascular disease and diabetes, which are so closely associated with unhealthy weight gain.
In conclusion, existing physical activity guidelines, which were designed to reduce heart disease, appear inadequate to prevent unhealthy weight gain. New and distinct guidelines of 2,000+ kcal/wk to achieve this goal should be established. Finally, it appears unrealistic to assume, in population terms, that this higher target can be met by previously sedentary people without an element of vigorous activity being gradually introduced in their personal activity programs.
This article is reproduced with permission from the IASO Newsletter, Vol 3 Issue 1, 2001
20 Feb 2002
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