The digestive process in man is regulated by a hugely complex array of feedback mechanisms. One of the main feedback mechanisms that regulates digestion and subsequent metabolism is the satiety induced by certain food. Under normal circumstances, in a individual who is metabolically healthy, satiety is induced during consumption of food in order to maintain suitable body weight. Dysfunction in the appetite regulatory system is thought to contribute to weight gain and obesity, through insensitivity of the hypothalamus to the leptin feed back signal from adipose tissue. Obesity is therefore characterised by hyperphagia, which explains the association between energy intake and body weight. Some foods are particularly problematic for such individuals because they can be ingested in large quantities without activating satiety. In particular, liquid foods with low viscosities, usually typified by a lack of fibre, can be problematic.
Because of the weak appetite suppressing effect of some liquids, they tend to be overeaten and are associated with weight gain. Soft drinks containing calories as sugar typify the ease with which energy can be ingested without undue physiological regulation. Increases in energy consumption from liquids has been demonstrated by researchers, when compared to more solid foods. For example, one study1 fed 108 healthy subjects either a liquid meal, a semi-solid meal or a solid meal with similar macronutrient, palatability and energy density contents, in an ad libitum style in free living conditions. Subjects consuming the liquid meal ingested 15 % more food compared to the semi-solid meal group and 30 % more food compared to the solid meal group. When subjects were fed either the liquid meal or semi-solid meal in a laboratory setting, they consumed 29 % more of the liquid meal.
Interestingly, the researchers even tried to standardise the eating time, but the liquid meal group still consumed 12 % more product. Therefore the consumption of a liquid meal seems to provide a larger quantity of food, and as a result energy, when compared to a normal solid meal. In a normal metabolically functioning individual, this would not be a problem, because the hypothalamus would sense the increased energy intake and counteract appetite and energy expenditure appropriately to maintain weight. However, in those with insulin and leptin resistance, it is likely that the extra energy would not stimulate compensatory action. This may be the reason that sugar sweetened soft drinks are associated with obesity. Not only are they a source of fructose which is known to cause insulin resistance, they also result in little regulation of appetite. Eating solid food containing high amounts of fibre is therefore recommended to individuals who may be insulin resistant.
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