Search This Site
QUICK LINKS
- Patient Leaflets
- Press Releases
- Who We Are
- Carb Facts
- Links
FITNESS ZONE
- Physical Activity
- Energy Fuel
- Carbohydrates
- Protein
- Supplements
Consumer Articles
Born not to be fat - May 2009
While experts all over the world try to explain why some people are heavier than others, a major advance in our understanding of how the brain reacts to food intake may help us to understand an even more perplexing problem. Why doesn’t everyone get grossly fat?
It may be worrying that so many adults and especially, children are now overweight. But why doesn’t this happen automatically. After all, the explanation given for the current “epidemic” of obesity is that too much tasty food is available to everyone at knock down prices. If the issue were a simple as that, then everyone would be overweight: this is clearly not the case.
In fact, the remarkable accuracy with which the body controls energy intake has puzzled scientists for as long as they have been able to work out how much energy is taken in as food and how much is stored in the body’s tissues. Consider some basic, and surprising, facts. If an average man weighing 65 Kg were to consume as little as 1% more food energy (calories) than he needed throughout his adult life he would end up at double his body weight (130 Kg) and nearly half of this would be fat (1)! The total amount of energy stored in a typical adult human body (again 65Kg), all of which has come, at one time or another, from the food and drink that has been consumed, is less that 16% of the amount of energy taken in within a single year. So the overwhelming majority of energy intake is expended in maintaining the body in a working condition and in physical and mental work (both of which need food energy). All the while, the balance between intake and output is maintained well within 1% accuracy without, for most people, any conscious thought or effort at all.
How is this achieved? Up until now, scientists have thought that the body has two conflicting systems one “good” and one “bad”. The “good” system is termed the “homeostatic” system. This has been seen as the way we control intake of energy to match what we need. But it is recognised that this system is complex. How little it is understood has only recently become apparent. The homeostatic system is more a philosophy than a scientific theory. It is reasoned that since the body succeeds in regulating energy intake (from food and drink) to match requirement so precisely, there must be a biochemical explanation. To date research has suggested that the mechanism involves a number of hormonal signals influencing both the desire to eat (hunger) and the feeling of having had enough (satiety). But exactly how many of these hormones there are, and how they balance each other, is not known.
In contrast, the “bad” system was thought to be quite well understood. This system was considered to be the means by which homeostatic control of body weight was undermined. In a word, the guilty party was thought to be pleasure. The brain’s response to pleasure was thought to be distinct from the “homeostatic” control mechanism (whatever that might actually turn out to be). Worse, the pleasure response could overwhelm the diligent and puritanical homeostatic system and trigger over-indulgence. Thus the age old fight between good and evil was seen going on in our very brains, with food as the weapon.
The pleasure response involved the stimulation of nerve signals in particular part of the brain. The same part of the brain is stimulated by cocaine. So the “hedonic response” to pleasant tasting food was seen by some to indicate the potential for food to be addictive, at least for some people. A similarly perceptive leap in logic would indicate that everything that is green is grass. Nonetheless, some researchers diligently pursued the “food is addictive” line of thought with complete disregard to the obvious fact that eating food must be pleasurable, otherwise no one would do it and we would all starve to death. One might as well argue that breathing is addictive. It gives pleasure, we suffer withdrawal symptoms if we stop, even for a few seconds, and doing it too much is damaging (we become anxious and even faint).
But are there really two separate controls, fighting each other for supremacy? It might be suspected that this intellectually tidy theory is flawed, since it does not adequately explain how we survive at all. The primary nutritional requirement to survive is to take in food energy. We need calories and we need quite a lot of them. It would be surprising, therefore, if the body had not evolved a mechanism to encourage us to eat energy. It now looks as though this is the case. And more importantly, the brain mechanism that responds to calories seems to be the same “hedonic” system that was thought to be responsible for undermining strict calorie intake control! So, perhaps, there are not two systems fighting each other, but only one (2).
All these developments in the research arena have implications for currently received wisdom on how to control the obesity epidemic. If an error in the habitual balance between food energy intake and energy requirement of as little 1% would lead to gross obesity, then the measurements of energy intake needed to establish which foods or drinks are responsible for any overconsumption must be more accurate than this. In fact, current methods struggle to estimate total energy intake in free living subjects to better than 20%. Individual measurements may be 40% in error. Simply measuring intake inaccurately in large numbers of subjects fails to solve the problem, since the errors are not random (3).
It is therefore impossible to be certain whether people who put on weight do so because they have a perfect diet except for one component, or whether they are simply over consuming generally. Blaming sugar, or soft drinks for the obesity epidemic might make good tabloid headlines but facts are rather more difficult to acquire than unsubstantiated opinions such as these. One test that should be applied to any biological theory is plausibility. In this case, is it really likely that everyone has a perfect diet except for one critical thing? Unfortunately, implausible as this idea may be in the cold light of dawn, many people are only too willing to believe it to be true. It is convenient to claim that everyone’s weight problems would be solved if only they bought this or that product: no further effort required. Just spend some money and feel guilty if you won’t.
It is assumed that if only we could discourage consumption of a few key foods, then all our problems would go away. The available facts speak loudly against such simplicity and indeed, such evidence as there is suggests the reverse. For example, low sugar consumers are observed to be heavier, in general, than those with normal intakes (4). And including some sugar seems to actually help calorie reduced weight loss diets (5).
Of course, none of this explains how we succeed in controlling energy intake so precisely, or why this balancing act goes slightly awry for some of us. But at least we may now start to look for an explanation in the right place!
References
- Elia M, Cummings JH (2007) Physiological aspects of energy metabolism and gastrointestinal effects of carbohydrates. European Journal of Clinical Nutrition 61(suppl.) S40-S74.
- Andrews ZB, Horvath TL (2008) Tasteless food reward. Neuron 57: 806-808.
- Westerterp KR, Goris AHC (2002) Validity of the assessment of dietary intake: problems of reporting. Current Opinion in Clinical and Metabolic Care 5:489-493.
- Ruxton CHS, Garceau FJS, Cottrell RC (1999) Guidelines for sugar consumption in Europe: Is a quantitative approach justified? European Journal of Clinical Nutrition 53:503-513.
- West JA, de Looy AE (2001) Weight loss in overweight subjects following low-sucrose or sucrose-containing diets. International Journal of Obesity 25:1122-1128.
