Exercise and body weight

Exercise and body weight refers to the evidence that exercise is linked to a long-term reduction in body weight, and the controversy involved.

Figure 1 - Body weight homeostasis

Excessive adiposity now affects more than one billion people worldwide. Adipose tissue is essential for energy use and heat production, but individuals with excess adiposity are at risk of co-morbidities such as type II diabetes, hypertension, insulin resistance, cardiovascular diseases and many more. Consequently, effective and successful interventions are needed to help individuals achieve and maintain a healthy body weight.

Excessive adiposity results from an imbalance in energy homeostasis. Energy balance is the difference between energy expenditure and energy intake; a negative energy balance will mean that body weight is lost over time, while a positive energy balance means that weight will be gained.

The brain is constantly readjusting metabolic rate and behavior to maintain a target weight; (the "set point"; the level of body weight below which the brain triggers mechanisms to conserve energy stores, and above which the brain triggers mechanisms to consume energy). Diet and exercise may alter the set point but only very slowly, biological mechanisms and metabolic adaptations to 'defend' the set point mean that maintaining weight loss that is achieved by an acute diet or by exercise can be very difficult.

There is controversy regarding the amount of physical activity necessary to facilitate weight loss maintenance. The following article explores this debate and puts forward results supporting and opposing the positive relationship between exercise and body weight.

The Effect of exercise on the production and maintenance of long-term reduction in body weight

Exercise intensity and duration on long-term weight loss

Increased levels of physical activity can be effective in reducing body weight, but the optimal amount of exercise that will facilitate a long-term reduction in body weight is still unclear. Recent randomized, controlled trials (RCTs) demonstrate slight weight loss with physical activity intervention alone and little increase in weight loss combining exercise and dietary restriction. Topical studies show that higher levels of exercise may result in greater weight loss.

Most recent studies show that a reduction in body weight of 1-3kg can be achieved over 4-16 months using exercise prescriptions of 60-180 min per week. Jakicic et al. as well as other study groups, are interested in the effect of increased intensity (moderate vs vigorous) and duration (moderate vs. high) of exercise on long-term weight loss., Individuals achieving and sustaining a weight loss of 10% or more of their initial body weight after 24 months carried out more physical activity than those who maintained a weight loss of less than 10%. This study demonstrates a correlation between increased leisure time activity and weight loss over time, concluding that 275 min/week (1500 kcal/week) is needed to sustain weight loss. This suggests that increased exercise intensity and duration may create a large negative energy balance resulting in substantial weight loss.

However, few studies of the amount and intensity of exercise required to augment long-term reduction in body weight have followed subjects for more than 6-12 months. This may be due to the challenging problems concerning overweight individuals in completing and adhering to increased levels of physical activity. In one study, less than half the number of individuals engaging in physical activity completed the 16 month study despite being compensated.

Other factors influencing results and energy balance may include compensation mechanisms such as changes in resting metabolic rate (RMR) as well as increased energy intake due to long-term augmented energy expenditure which may attenuate weight loss. Difficulties in interpreting results may arise when calculating body weight, as adipose tissue lost due to exercise may be overshadowed by an increase in lean muscle mass. Studies that measure body composition compared to body weight solely, illustrate that exercise “reduces abdominal visceral fat” and “improves cardio metabolic risk factors”. Consequently body weight may not be the best measure to evaluate the effects of physical activity on health.

The results mentioned above as well as being equivalent to results of Schoeller et al., suggest that appropriate levels of exercise required to sustain weight loss is “approximately twice the public health recommendation,” at least 150 minutes of moderate intensity activity per week.

Maintenance of Long-Term Weight Loss

Many studies have found that, after weight loss, subjects often regain the lost weight, and sometimes even overshoot their original weight. Abderson et al. analysed a large number and variety of weight loss studies carried out in the USA, and concluded that more than 35% of lost weight is regained within the first year, and most is regained within five years. Metabolism plays a key role in this. Among other inputs, the reduced plasma concentrations of leptin and insulin that accompany a reduced fat mass tell the brain that the body is energy-deficient, resulting in a drive to eat along with suppressed energy expenditure. The body’s choice of fuel also changes within the diurnal cycle and is affected by lifestyle. Therefore, to maintain their new weight, individuals need to limit their food intake to the same extent that expenditure is suppressed.

The role of exercise There is evidence to suggest that regular exercise prevents or counters these metabolic adaptations that lead to weight regain. MacLean et al. showed that relapsing rats that exercised not only that presented a reduction in weight in liver tissue and mesenteric fat pads compared to their sedentary counterparts, but did not succumb to the overeating normally seen after dramatic weight loss. The energy balance previously mentioned was delayed and much reduced (roughly 40%), therefore greatly reducing the rats’ desire to gorge themselves. Lean rats show a diurnal shift in fuel usage (favouring carbohydrates during the dark cycle and fat during the light). Obese rats did not present this shift. In sedentary rats, carbohydrates were favoured regardless, while fat was stored and increased lipogenesis was observed.

AMPK is a hypothalamic nutrient sensor that responds to low nutrient availability. Signals from the periphery (including the hormones leptin and insulin) were not increased by exercise regimes. The reception of these signals, however, seems to be more involved. Acute bouts of exercise were shown by MacLean et al. to lessen the response of AMPK to peripheral deprivation signals, thereby reducing the drive to overfeed. Rats that had lost weight were observed to alternate massive overeating with periods of deprivation. Daily aerobic exercise reduced the extremes in fuel consumption that are associated with this phenomenon as well as the excessive desire to eat and/or hunger pains that are the downfall of many individuals after a calorie-restricted weight loss program. The authors suggested that preventing the typical increase in adipocytes may affect the ability to store excess calories, as the peripheries are wired to process and store any fuel excess rapidly and efficiently to promote regain and return to the defended body weight. The drive to physical activity is tightly regulated and directly influences adiposity and body weight.

Contradictory evidence for the link between exercise and a reduction in body weight

In 2004 it was reported that more than half of British adults are overweight, and that obesity among school children has increased by 70% in the past generation The correlation between increasingly sedentary lifestyles and the obesity epidemic has lead to a broad assumption that physical inactivity is to blame for the worldwide crisis. However, there is compelling evidence emerging to the contrary.

Perhaps the most reliable reports have been produced by the EarlyBird study at the Universities of Exeter and Plymouth. This 12 year cohort study, to be completed in 2013, involves measuring the physical activity and body composition of 300 children from the age of 5. Unlike other trials, physical activity was measured using an impressively precise CSA accelerometer-based activity monitor. Similar previous trials have depended on unreliable qualitative reports from parents or children One of their 56 peer reviewed articles finds no association between physical activity and BMI or body fat in either sex .

Increasing evidence suggests that children are intrinsically programmed to undertake a set level of activity per day, which is determined either genetically, or as a result of an early childhood experience. Hence, it would be incorrect to implicate TV watching in any resulting daily energy surplus . Adding further credence to this, results being published by EarlyBird in the near future suggest that it is obesity that makes children decreasingly active, and not inactivity that makes children obese.

A cluster randomised controlled trial was carried out in Glasgow, in which 545 pre-school children were subjected to a realistic level of enhanced activity per week, over 24 weeks. It was similarly deduced from this study that physical activity does not reduce BMI in young children . The same conclusion was reached by Harris et al., who scrutinised 18 studies meeting inclusion criteria to explore the effect of school-based physical activity interventions on BMI in children.

Such findings have not been exclusive to children. Church randomized 411 sedentary women to either 1 of 3 exercise groups of varying energy expenditure, or a non-exercise control group. He found that the group assigned to no exercise lost a similar amount of weight to the exercisers .

The explanation may depend on the observation that exercise increases appetite through orexigenic peptides such as NPY . Caloric overcompensation is the result of our limited capacity to deal with excess calories, due to our minimal brown fat stores, the activation of orexigenic and reward pathways in the brain, and the tendency to underestimate calorie consumption. The frequent result is excessive dietary intake, resulting in an energy imbalance and net weight gain. This theory is supported by an 18-month study by Sonneville and Gortmaker involving 538 students who, on average, consumed 100 calories more than they had just expended following exercise .

These findings suggest that, in the absence of the will-power to override the desire to consume surplus calories post exercise, the key to achieving a net daily energy deficit is not to increase exercise, but to limit daily calorie consumption.