The Psychology of Food Intake and Portion Control

Overweight, obesity, and non-communicable diseases (NCD) associated with excess body weight continue to increase across the world [1,2]. Obesity is linked with cancer, coronary heart disease, diabetes and hypertension. Excess body fatness, which is the defining feature of obesity, is caused by energy intake exceeding energy expenditure over time [3]. However, there is evidence of a strong gene-environment interaction in susceptibility to excess adiposity, since in developed countries all consumers are exposed to relatively affordable, palatable foods but not everyone develops obesity [4].  In addition, obesity is socially graded with greater levels of obesity among the least affluent in developed countries and with greater levels of obesity among the more affluent in developing countries [5]. 
In affluent countries, such as the UK, high socioeconomic status (SES), wealth and education protect against obesity, but lower SES, disposable income and education contribute to vulnerability to food insecurity and the risk of overweight and obesity. 
Consumption that stems from hunger and thirst is a direct response to a physiological requirement for fuel and/or fluids [6]. However, most eating episodes are not directly need-related [7,8]. 
Much eating is a direct result of habits, hedonics and how consumers feel – for example,  ‘it’s lunchtime, I must eat now while I have the opportunity’, ‘I’ll have a second helping, because that cake is just so tasty’, and ‘I’ll have something nice for lunch to cheer me up’. Purchase decisions are also a direct response to the commercial environment, for example ‘buy one get one free’ offers (BOGOF) – when these initiatives are applied to foods that are ‘high in fat, sugar and salt’ (HFSS), consumers tend to eat more, but if they are applied to items such as shampoo or soap, usage is not affected by such deals. [9].  Taken together, everyday habits, cognitions, the pleasure of eating (hedonics), emotions and our response to the environment are more psychological in origin, than entirely need-based.

The hedonic characteristics of food refer to the rewarding qualities of foods as they are eaten – typically measured in relation to both the sensory characteristics of the food (look, smell, taste, texture) and how pleasant (rewarding) these characteristics are.  Foods which are considered high in reward value may be overeaten. Deliberate manipulations of flavour and texture to increase liking have resulted in increased consumption [24,25]. Greater familiarity with flavours and foods [26,27], and a greater variety of flavours and foods may also result in increased consumption [28,29].

Cognitive, hedonic and emotion-driven eating behaviour is very much located within the individual, but of significant interest to research is the extent to which individuals interact with the environment, to influence how much and which foods are eaten.  For example, the environment shapes consumer food intake, sometimes without awareness.  Consumers may be ‘nudged’ towards healthy eating choices, or towards HFSS items.  Another of these hidden determinants of eating and portion size is that of consumption norms - our understanding of the amount of consumption considered to be ‘normal’ for the situation in which we find ourselves. An example of a social norm is eating at certain times of the day [36]; certain foods are typically eaten in the morning and others in the evening, and portion sizes of some foods tend to be small or large, depending on circumstance, culture and habit [37]. As a useful illustration of this, Kramer et al [37] provided individuals with breakfast food items and lunch food items for consumption at both breakfast and lunch. Breakfast foods were more liked than lunch foods at breakfast time, and lunch foods were more liked at lunchtime than breakfast foods. Individuals also consumed more of the lunch foods, regardless of whether they were consuming them at breakfast or lunchtime, and consumed more for lunch, regardless of whether they were consuming breakfast or lunch foods. These effects can be explained as a result of individual or social norms applied to consumption patterns - breakfast is normally a small meal, composed of foods which are associated with breakfast and which are generally served in small amounts. 

Portion size manipulations are especially noteworthy since cleaning the plate is a social norm in many countries.  However, if larger portions than normal are offered, what does the consumer do then?   Do they eat their ‘typical’ amount, or does their intake shift upwards to match the larger portion size offered? The portion size effect, where consumers shift intake upwards in response to the larger amount offered, is a robust and reliable effect observed across different countries and contexts (see [38] for a review).  For example, adults were given 100, 125 or 150% of a meal they previously self-selected typically consume all of the portions provided [39].

For children, the portion size effect is not only robust but observed over many days [40] and in adults, serving larger portion sizes over a period of 11 days results in a mean cumulative increase in intake of more than 4000 kcal [41].

Environmental factors such as packaging size, functionality and on-pack prompts can influence how much is eaten.  A systematic review of the literature revealed that experimental studies which manipulated packaging features influenced consumption, with the largest effect observed for packaging which guided consumers either by on-pack cues or structural features [42].  For example, images on the front of the pack, packaging size, as well as partitioning and resealability all helped to reduce food intake. However, individual differences and attentional focus mediated packaging effects. Thus, some consumers are more responsive to packaging prompts to limit intake than others. Packaging has been used successfully to limit the intake of HFSS items and to increase the intake of low-energy density, healthy foods, e.g. carrots.

Food intake is also susceptible to the social and physical eating context, who is with us, and where we are eating.  Independent of all other factors, individuals consume more in a restaurant than in a café, cafeteria, at home or in a non-eating environment [43]. Individuals consume more with usual dining partners, than with strangers or when alone [44]. Individuals consume more when paired with a confederate who eats 40 crackers compared to one who eats only 10 [45], and consume more when paired with a confederate who eats 20 crackers, less when paired with someone who eats just one cracker, and an intermediate amount when alone [46].
Two other strong environmental determinants of eating are the effort required for consumption and the amount of distraction simultaneously provided. The more effort required for consumption, the less is likely to be consumed. In a cafeteria setting, where crisps were available at the main check-out, potato crisps were selected by 70% of consumers, on average. When such snacks were placed at the far end of the dining hall and required queuing at a separate check-out, solely for the purpose of purchasing them, crisps were selected by an average of only 10% of consumers [47]. In similar studies, consumers:
i)    chose ice cream less often (5% of the time) from a chiller with a lid on, compared to a chiller without (16% of the time)
ii)    consumed less nuts if they are shelled, as opposed to being unshelled
iii)    consumed less sweets if they were wrapped, compared with those that were not [48]. 

Some of the effects mentioned above, as a result of portion and serving size, are in fact not only effects of consumption norms but also of effort. Distraction, furthermore, can interfere with normal satiation processes and can extend the meal, producing greater food intake. We tend to consume more with friends and family, not just because consumption here is normal, but also because these individuals are likely to provide greater distraction from the food we are eating – conversation is more likely, and often more engaging [44]. Distraction also explains an effect, where independent of all other factors, the more people that are present, the more we consume [44]. Distraction also increases food intake across a number of contexts - more is eaten in the presence of music, compared to without [49], and with a radio or television on, compared to when off [50].