Why is it hard to reverse the rise in obesity?

By John Harris

Obesity is a recognized public health hazard.  This  puts a strain on the body, as well as on health care systems. It arises because the intake of energy exceeds that needed for exercise and maintenance of the body’s integrity, and is therefore converted to fat. In an effort to reduce obesity in the population, governments can increase the tax on foods and drinks seen as undesirable, but is it likely to be enough?

Unlike alcohol and tobacco, important drains on public health, food is essential for life. We can give up smoking, but not give up eating. It may be more difficult to modify the essential behaviour of eating than to abolish non-essential ingestion of alcohol or nicotine. We all experience cravings to eat, and these are often stronger when we are bored or become emotional (1).  In human evolutionary history, supplies of food were often scarce or intermittent. It could therefore be adaptive to eat as much as possible when times were good, and build up layers of fat, as an insurance against  bad times to come. However, in modern developed societies, the bad times never come, but the now maladaptive behaviour may persist. So, how might it be changed?

We are beginning to understand how obese people differ from those who are healthily weighted. In a study of a large group, it was found that the ratings of the sweetness of a standard piece of candy by obese participants tended to be lower than the ratings of the more healthily weighted. Also, obese participants rated their liking for the candy as higher than did the more healthily weighted (2).  These findings suggest that obese people are likely to add more sugar to their food to obtain an acceptable level of sweetness, and a similar pattern may apply to other types of food and drink.

The brains of obese people appear to differ from those of the healthy weighted. In a study using brain imaging, activity was compared in regions known to process taste and those forming part of the reward circuitry, in which activity corresponds to feelings of pleasure. Activity was measured during fasting and after a standard meal. In obese participants, interaction between these regions increased after the meal, an effect which rose with how appetising the meal was rated to be. In the healthy weighted, the interaction decreased, to an extent which also depended on how appetising the meal was rated to be (3). The authors suggested that hunger and eating have a different status in the two groups. For those who are obese, hunger is an absence of reward, whereas, for the healthy weighted, it is more a desire for energy.

These differences suggest that obesity cannot simply be tackled by relatively small changes in the price of certain food and drinks.  They may, in fact, be genetically determined, since some individual differences in taste mechanisms certainly are. For example, there are differences between ethnic groups, and between males and females (4). Alternatively, they may be learned, at least in part, so that repeated ingestion of sugars leads to adaptation to sweet tastes and the need to keep increasing amounts of sugar in the diet to maintain a satisfactory level of sweetness.

To reduce cigarette smoking, the government has adopted a two-pronged approach. In addition to price increases, there has been an education campaign, with pictures of diseased lungs featured on cigarette packets. Perhaps a similar approach is needed to tackle obesity?

1.       Vallis M (2019) Sustained behaviour change in healthy eating to improve obesity outcomes: it is time to abandon willpower to appreciate wanting. Clinical Obesity 9: e12299.  https://doi.org/10.1111/cob.122991.

2.       Bartoshuk LM, Duffy VB, Hayes JE, Moskowitz HR, Snyder DJ (2006) Psychophysics of sweet and fat perception in obesity: problems, solutions and new perspectives. Philosophical Transactions of the Royal Society B 361: 1137–1148.

3.       Avery JA, Powell JN, Breslin FJ, Lepping RJ, Martin LE, Patrician TM, Donnelly JE, Savage CR, Simmons WK (2017) Obesity is associated with altered mid-insula functional connectivity to limbic regions underlying appetitive responses to foods. Psychopharmacology 31(11): 1475–1484. doi:10.1177/026988111772842.

4.       Harris J, Smith JG (2022) Sensation and Perception (2nd edition). Sage Publications. Chapter 7: Taste and Smell.

John Harris is co-author of SAGE Publishing’s recently published Sensation and Perception 2e. Find out more and request an inspection copy.