Professor Eric Robinson, Reader in Psychology from the University of Liverpool and the British Nutrition Foundation blog on portion sizes
Getting portion sizes right is an important part of a healthy and balanced diet.
The causes of obesity are complex, but overconsumption of food and sugary drinks is a key direct determinant, driven in part by large portion sizes. Recent research conducted at the University of Liverpool suggests that portion sizes may not only contribute to overweight and obesity but also may be part of the possible solution.
Survey data estimates that between 1980 and 2019, the prevalence of obesity in England increased from 6% to 27% in men and 9% to 29% in women. At the same time portion sizes of many foods served at home and out of the home have increased, pointing to the possibility that larger portions may have played a contributory role in rising obesity.
To tease apart what kind of impact this is likely to have had on diets several studies have examined the relationship between portion size and the amount people eat at meals. A 2016 review concluded that portion sizes that are twice as large as usual increase meal consumption by more than a third (35%). Whilst these studies show a clear effect of portion size on intake at a single eating occasion, what they don’t tell us is whether the impact portion size has is maintained over time and how it influences body weight. For example, although a larger portion at lunch may increase calorie intake for lunch, maybe later in the day your body naturally compensates for this by requiring less food to feel full. If this was the case, then it could also mean that reducing portion sizes may not be useful for weight management because any reductions in portion size would be made up for later in the day. In our current food environment where energy dense foods are easily accessible, it is also important to consider what happens when large portions are continually available. Will people keep overeating, or will compensatory mechanisms sense the accumulating excess energy and limit intake?
The only way to answer this question is to look at experiments in which participants have been randomly allocated to be served smaller vs. larger portions over time and then their daily energy intake or body weight has been measured.
This is what researchers from the department of psychology at the University of Liverpool did in a new piece of research. In the systematic review, they looked at studies that had manipulated portion sizes and measured energy intake across the course of a minimum of one day. What’s important to note about these studies is that if participants did want to make up for or ‘compensate’ for the smaller portions provided, they had opportunities to do so throughout the study day/s. The researchers analysed data from 14 studies and found that portion size does have a prolonged effect on what people eat – smaller portions at breakfast, lunch or evening meals result in people eating fewer calories (approximately 235 kcal on average) daily. The other key finding was from a smaller subset of studies which looked at changes in body weight. Participants served smaller portions gained 0.6kg less weight than those served larger portions which suggests that getting portion sizes right could help with preventing weight gain and managing weight loss.
But is it as simple as reducing your portion sizes of all foods and meals? We think not! The amount (volume) of food we eat does impact how full we feel and therefore the likely best portion size orientated solution for weight management is to be tactical – decrease the portion size of more energy dense foods and serve generous portion sizes of much lower calorie foods (e.g. vegetables). This approach could be particularly valuable for weight management as it should help people manage the number of calories being consumed at meals without feeling constantly hungry. The British Nutrition Foundation looks at this concept in their information on the energy density approach. It’s also in keeping with the British Nutrition Foundation's recently revised Get portion wise! resource which has lots of practical tips and advice on getting portion sizes right.
Professor Eric Robinson, University of Liverpool
Ayela Spiro, British Nutrition Foundation
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