OBESITY
With all of these brain functions promoting food regulation, why, then, is there such a high incidence of obesity in the world today?
Many different factors can contribute to obesity, and there is only rarely a single cause (see Garrow, 1988). Occasionally, hormonal disturbances, such as hyperinsulinemia (that is, substantially elevated levels of insulin in the bloodstream), can produce overeating and obesity. Otherwise, there are a number of possible contributory factors:
It is possible that the appetite of some obese people is more strongly stimulated by external factors such as the sight and smell of food (Schachter, 1971). The palatability of food is now much greater than it was in our evolutionary past, leading to an imbalance between the reward from orosensory control signals and the gastrointestinal and post-absorptive satiety signals controlling the reward value of sensory input. In other words, the rewards from the smell, taste and texture of food are far greater than the satiety signals can control.
Animals evolved to ingest a variety of foods, and therefore nutrients. So satiety is partly specific to a food just eaten, while appetite remains for foods with a different flavour. Overeating may therefore be partially explained by the tremendous variety of modern foods, encouraging us to eat more by moving from one food to another.
Modern humans take less exercise than our ancestors due to our more sedentary lifestyles, so unless regular exercise is proactively built into our daily lives, we may be inclined to gain weight.
Human meal times tend to be fixed. Animals normally regulate their food intake by adjusting the inter-meal interval. A long interval occurs after a high energy meal, and a short interval after a low energy meal. Quite simple control mechanisms, such as slower gastric emptying (and therefore a feeling of fullness for a long time after an energy rich meal) may contribute to this. But the fixed meal times often preferred by humans deter this control mechanism from operating normally. Obese people tend to eat high energy meals and then eat again at the next scheduled mealtime, even though gastric emptying is not yet complete.
Obese people often eat relatively late in the day, when large energy intake must be converted into fat and is less easily burned off by exercise and heat loss. Regulation of heat loss is one way that animals compensate for excessive energy intake. They do this by activating brown fat metabolism, which burns fat to produce heat. Although brown fat is barely present in humans, there is nevertheless a mechanism that, when activated by the sympathetic nervous system, enables metabolism to be increased or reduced in humans, depending on energy intake (see Garrow, 1988; Trayhurn, 1986).
Obesity may be related to higher stress levels in contemporary society. Stress can regulate the sympathetic nervous system to increase energy expenditure, but at the same time it can also lead to overeating. Rats mildly stressed (e.g. with a paperclip on their tail) show overeating and obesity.
But what of water intake, and drinking?
The human body can survive without food for very much longer than it can survive without water – how does our physiological make-up help direct this vital function? Body water is contained within two main compartments, one inside the cells (intracellular) and the other outside (extracellular). Intracellular water accounts for approximately 40 per cent of total body weight; and extracellular water for about 20 per cent, divided between blood plasma (5 per cent) and interstitial fluid (15 per cent). When we are deprived of water, both the cellular and extracellular fluid compartments are significantly depleted. The depletion of the intracellular compartment as cellular dehydration, and the depletion of the extracellular compartment is known as hypovolaemia (meaning that the volume of the extracellular compartment has decreased).
With all of these brain functions promoting food regulation, why, then, is there such a high incidence of obesity in the world today?
Many different factors can contribute to obesity, and there is only rarely a single cause (see Garrow, 1988). Occasionally, hormonal disturbances, such as hyperinsulinemia (that is, substantially elevated levels of insulin in the bloodstream), can produce overeating and obesity. Otherwise, there are a number of possible contributory factors:
It is possible that the appetite of some obese people is more strongly stimulated by external factors such as the sight and smell of food (Schachter, 1971). The palatability of food is now much greater than it was in our evolutionary past, leading to an imbalance between the reward from orosensory control signals and the gastrointestinal and post-absorptive satiety signals controlling the reward value of sensory input. In other words, the rewards from the smell, taste and texture of food are far greater than the satiety signals can control.
Animals evolved to ingest a variety of foods, and therefore nutrients. So satiety is partly specific to a food just eaten, while appetite remains for foods with a different flavour. Overeating may therefore be partially explained by the tremendous variety of modern foods, encouraging us to eat more by moving from one food to another.
Modern humans take less exercise than our ancestors due to our more sedentary lifestyles, so unless regular exercise is proactively built into our daily lives, we may be inclined to gain weight.
Human meal times tend to be fixed. Animals normally regulate their food intake by adjusting the inter-meal interval. A long interval occurs after a high energy meal, and a short interval after a low energy meal. Quite simple control mechanisms, such as slower gastric emptying (and therefore a feeling of fullness for a long time after an energy rich meal) may contribute to this. But the fixed meal times often preferred by humans deter this control mechanism from operating normally. Obese people tend to eat high energy meals and then eat again at the next scheduled mealtime, even though gastric emptying is not yet complete.
Obese people often eat relatively late in the day, when large energy intake must be converted into fat and is less easily burned off by exercise and heat loss. Regulation of heat loss is one way that animals compensate for excessive energy intake. They do this by activating brown fat metabolism, which burns fat to produce heat. Although brown fat is barely present in humans, there is nevertheless a mechanism that, when activated by the sympathetic nervous system, enables metabolism to be increased or reduced in humans, depending on energy intake (see Garrow, 1988; Trayhurn, 1986).
Obesity may be related to higher stress levels in contemporary society. Stress can regulate the sympathetic nervous system to increase energy expenditure, but at the same time it can also lead to overeating. Rats mildly stressed (e.g. with a paperclip on their tail) show overeating and obesity.
But what of water intake, and drinking?
The human body can survive without food for very much longer than it can survive without water – how does our physiological make-up help direct this vital function? Body water is contained within two main compartments, one inside the cells (intracellular) and the other outside (extracellular). Intracellular water accounts for approximately 40 per cent of total body weight; and extracellular water for about 20 per cent, divided between blood plasma (5 per cent) and interstitial fluid (15 per cent). When we are deprived of water, both the cellular and extracellular fluid compartments are significantly depleted. The depletion of the intracellular compartment as cellular dehydration, and the depletion of the extracellular compartment is known as hypovolaemia (meaning that the volume of the extracellular compartment has decreased).
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