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NutrInsight • About the value of controlling appetite
Lipids: little effects on satiation and satiety
Lipids are the macronutrients with the highest energy content at 9 kcal/g. As a result, for a given energy content, the volume of a lipid food is much smaller. In contrast, for the same volume, the amount of energy ingested will be much greater for a lipid food than for a food containing mainly any other type of macronutrients. This is why, with no precise compensation mechanisms, the studies carried out in Man and in animals have shown that a high-lipid diet generally results in an increase in energy intake and, in consequence, in bodyweight (69). In contrast, a recent study has shown that reducing the daily energy intake, without altering the volume, by reducing the quantity of lipids in foods does not reduce the feelings of hunger, but does reduce the quantity of energy ingested (82). Even though they have less effect on satiation and satiety than proteins or carbohydrates (63), lipids (or the products of their hydrolysis and in particular the long-chain, polyunsaturated fatty acids) do play a role in regulating eating by stimulating the secretion of CCK and PYY, hormones involved in satiation and satiety (63;83;84;85). As a result, the quality of lipids has all the same been investigated. However, the evidence is still very limited. For example, replacing some of the triglycerides by diacylglycerols, could improve the short-term subjective sensations of satiety (86). Some other candidates could also be interesting to investigate, such as the medium-chain fatty acids (87). One of the possible mechanisms that have been suggested is the degree of oxidation of the fatty acids: the more unsaturated (and therefore oxidisable) the fatty acid, the greater its satiating effect (88).
Conclusion
Although a fairly generally accepted classification lists proteins as being more satiating than carbohydrates, which in turn are more satiating than lipids, their mechanism of action has yet to be clearly demonstrated. Furthermore, foods, which are complex matrices, are characterised by various parameters such as the energy density, the volume, the palatability and a mix of various macronutrients. The effect of a food on food intake will therefore be the result of all these factors. To simplify, the mix that would seem to be the most satiating would be a diet containing high levels of proteins, fibre (with a preference for soluble, viscous fibre), slowly digestible carbohydrates (and therefore those with a low GI), with a low lipid content.
3.2 The effects of some of the physico-chemical characteristics of foods
The potential of food on satiation and satiety also appears to depend on its energy density, volume, weight, viscosity, texture, physical form (solid/liquid), palatability (how enjoyable it is) and its diversity. Some of these parameters are interconnected or associated, such as energy density, volume and weight, texture and physical form, or indeed smell and palatability.
Energy density
The energy density of a food depends on the concentration and nature of the nutrients that it contains. Proteins and carbohydrates contribute 4 kcal/g and lipids 9 kcal/g. Furthermore, the energy density is inversely related to the water content of the food. A positive association between energy density, increased food intake and bodyweight has been detected in laboratory studies and in transverse epidemiological studies. However, longitudinal data are required to confirm the link between the energy density of food and an increase in the risk of obesity (89).
Volume
There is a close positive correlation between the volume of a meal in the stomach and the sensations of fullness and satiety (90). However, in most of the studies investigating the effect of the volume of food, it is difficult to distinguish between the effect of volume and that of the weight of the food or its energy density. Increasing the volume by diluting the food in fact increases its weight and reduces its energy density. A study carried out by Rolls, in which the volume of milk-shakes was altered by incorporating air, has however demonstrated the effect of volume alone achieved by incorporating air, and confirmed that a significant increase in volume alone (> 150 mL) did transiently reduce the subjective appetite, and the food intake at a meal served 30 minutes later (90;91).
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