Food macronutrients and satiety
Classic satiety research has typically looked at the physiological effects of food ingredients in isolation while holding all other contributors to satiety constant. This important work has highlighted that two foods of equal energy may have distinct effects on satiety if their macronutrient compositions differ. For example, women whose diet was modified to be high in protein and carbohydrate for a day reported higher levels of satiety compared to another day when the principle energy source of their diet was fat, despite the diets being matched for energy content (Westerterp-Plantenga, Rolland, Wilson, & Westerterp, 1999). The idea of a hierarchy of satiating effects of macronutrients in the order of protein > carbohydrate > fat (Blundell & Macdiarmid, 1997) goes some way to explain why not all calories will have the same impact on satiety, and has been hugely influential in the development of enhanced satiety foods. Nowadays, for many people, “high protein” is synonymous with feeling full and is central to most satiety claims in the appetite management food market. Protein has taken centre stage as the high satiety food constitute because of considerable experimental and real-world research indicating that increasing the protein composition of the diet without changing net energy can lead to enhanced feelings of satiety (Paddon-Jones et al., 2008). Possible physiological mechanisms underlying this effect include diet induced thermogenesis (Halton & Hu, 2004) and gastrointestinal hormonal signalling (Veldhorst et al., 2008), while two recent studies indicated that the sensory experience of ingesting protein is also important (Bertenshaw et al., 2013, Masic and Yeomans, 2013). Randomized trials of high protein diets on weight management provide evidence that these types of eating plans can support longer-term weight loss (e.g.Leidy et al., 2007, Skov et al., 1999, Weigle et al., 2005) and potentially aid future weight maintenance (Due et al., 2004, Westerterp-Plantenga et al., 2004). In the laboratory the satiating effects of high protein foods or meals have been compared to iso-energetic lower protein counterparts, typically using “preload” methodology where the measure of satiety is post-consumption subjective ratings of appetite and/or food intake. The majority of these types of studies indicate that high protein foods deliver better satiety than energy matched foods with lower levels of protein (e.g.Astbury et al., 2010, Bertenshaw et al., 2009, Booth et al., 1970, Fischer et al., 2004, Hill and Blundell, 1986, Rolls et al., 1988, Teff et al., 1989), though this not always reported (de Graaf et al., 1992, Vozzo et al., 2003). Overall this body of literature indicates that increasing the protein content of a food is an effective way to deliver enhanced satiety to the consumer, but manipulating the macronutrient content of a food while keeping energy constant means it is difficult to be certain whether these effects are due to the superior satiating effect of protein, the reduction of less satiating nutrients carbohydrate and fat, or a combination of both of these. Moreover, it is not known whether these effects are maintained after repeat experience; for these reasons EFSA are yet to approve claims based on a general protein effect (European Commission, 2007, European Commission, 2012).
High protein food products invariably contain other energy-yielding nutrients, usually both carbohydrate and fat. Therefore, in order to optimise high satiety products the carbohydrate-to-fat ratio should also be considered. Protein's position at the top of the satiety hierarchy is fairly well accepted but the order of carbohydrate and fat is often disputed, with this debate further complicated by variability in glycaemic responses to carbohydrate ingestion which can influence satiety signalling (Brand-Miller, Holt, Pawlak, & McMillan, 2002). With regard to satiety, the low-fat rhetoric of recent years seems justified: consuming more energy from carbohydrate than fat has been linked to reduced risk of being overweight or obese (Astrup et al., 2000, Gaesser, 2007), the implication being that high carbohydrate foods are more satiating than those that are high in fat. In free-feeding experiments when people are offered a range of high fat foods they tend to consume more energy than when they are offered high carbohydrate foods (Blundell, Green, & Burley, 1994), a phenomenon termed high fat hyperphagia or passive over consumption (Blundell & Tremblay, 1995). Importantly, this fat-related increased intake of energy does not lead to increased sensations of satiety (Blundell & Macdiarmid, 1997). In the laboratory, studies have found that high fat preloads are less satiating than energy matched high carbohydrate versions (e.g.Cotton et al., 1994, Holt, 1999, Robinson et al., 2005), though not in every case (e.g. de Graaf et al., 1992, Rolls et al., 1994). These mixed findings might be due to between study differences in participants characteristics (Chambers & Yeomans, 2011), and preload ingredients (Rolls & Bell, 1999). One particularly important property of fat is that per gram it delivers more than double the energy of carbohydrate and protein. The prevailing view is that fat's high energy density per unit weight largely accounts for its low satiety value (Blundell and Macdiarmid, 1997, Rolls and Bell, 1999). A high fat food will often be smaller in weight (and volume) than a high carbohydrate food of similar energy and this difference may affect the timing of the processing of the nutrients in the gut (Karhunen, Juvonen, Huotari, Purhonen, & Herzig, 2008) and also consumer beliefs about the likely consequence of consuming that food. That is, people tend to believe a small serving of food will not be enough to satisfy their hunger regardless of the energy it contains (Rolls, Drewnowski, & Ledikwe, 2005) and these satiety expectations are thought to play a key role in eating behaviour (Brunstrom, Shakeshaft, & Scott-Samuel, 2008).
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