Decreasing portion sizes could cut obesity levels

Behind the Headlines

Wednesday September 16 2015

Reducing portion sizes may also reduce the desire to eat more

One in four adults in the UK is obese

"Reducing the portion sizes … would help reverse the obesity epidemic, say researchers," BBC News reports. 

The researchers, who pooled the results of more than 70 previous studies, found a link between portion size and overeating.

Researchers found that increased portion size, packaging and the size of a plate led to people choosing larger amounts of food and eating more. It may be that the old saying "you have eyes bigger than your belly" rings true for some people. They eat what they are given, not what they need.

People also drank more when non-alcoholic drinks were provided in shorter, wider glasses and bottles than tall, thin ones. The researchers say that although the results were not surprising, they lend weight to the argument for portion sizes to be decreased to help reduce the UK's obesity epidemic.

It should be noted that the quality of the individual studies was rated as poor by the researchers, and most of the studies were conducted in the US, where portion sizes are infamously large.

The studies also did not look at whether people were able to reduce their intake over the long term through smaller portion sizes.

These limitations aside, it would seem a sensible option to choose a smaller portion if you are trying to achieve or maintain a healthy weight. Making sure you get your five portions of fruit and vegetables a day and increasing physical activity levels will also help.

 

Where did the story come from?

The study was carried out by researchers from the University of Cambridge, the University of Oxford, MRC Human Nutrition Research, the University of Plymouth and the University of Bristol. It was funded by the UK Department of Health Policy Research Programme.

The study was published in the peer-reviewed online medical resource The Cochrane Database of Systematic Reviews. As with all Cochrane studies, the research has been made available on an open-access basis, so it is free to read online.

The UK media reported the findings accurately and supported the notion that portion sizes have been increasing, which may be contributing to increasing obesity levels.

The Independent provided helpful expert commentary from one of the lead authors, Dr Gareth Hollands, that "helping people to avoid 'overserving' themselves or others with larger portions of food or drink by reducing their size, availability and appeal in shops, restaurants and in the home, is likely to be a good way of helping lots of people to reduce their risk of overeating". 

 

What kind of research was this?

This was a systematic review of studies that have looked at the effect of different portion sizes on the consumption of food, alcohol or tobacco. The researchers pooled the results together in a meta-analysis

Although this type of research brings together all of the evidence available for a topic, the results are reliant on the quality of the individual trials.

In this case, only randomised controlled trials were included, either comparing consumption between two groups or in individuals in crossover studies. However, despite this type of study design being the "gold standard", the researchers judged the studies to be at high or unclear risk of bias, so they say the overall evidence is of moderate to very low quality.

 

What did the research involve?

The study searched 12 medical databases and trial registries for relevant studies up to July 2013. Randomised controlled trials were included in the analysis if they compared the amount of food, alcohol or tobacco consumed or chosen, according to different portion:

  • size
  • shape
  • package
  • crockery dimensions

Standard Cochrane techniques were used for the search strategy in applying inclusion and exclusion criteria consistently across the identified search results, and when performing the statistical analyses.

 

What were the basic results?

There were 72 studies that met the inclusion criteria; 69 assessed food portion size and three looked at cigarette size. No studies were identified that assessed alcohol portion size.

Exposure to larger food portions, packaging or crockery size was associated with moderately increased food consumption for adults and children (standardised mean difference (SMD) 0.38, 95% confidence interval (CI) 0.29 to 0.46). 

The researchers estimated that if smaller portion sizes were used consistently across meals, the average daily calorie consumption could reduce by 144 to 228 calories per day. This would be equivalent to 4,032 to 6,384 less calories per month, which would equal a weight loss of one to two pounds (0.45kg to 0.9kg) if everything else stayed the same.

A meta-analyses of 13 studies found that increased portion or crockery size led to adults selecting a greater amount of food (SMD 0.55, 95% CI 0.35 to 0.75). This was not found in studies on children.

There was low-quality evidence from three studies that shorter, wider glasses or bottles compared to thin, tall glasses increased the amount of non-alcoholic drinks selection (SMD 1.47, 95% CI 0.52 to 2.43). 

Only one study looked at consumption of non-alcoholic drinks, which found that young adults drank more water if using shorter, wider bottles, but this was judged as very low-quality evidence (SMD 1.17, 95% CI 0.57 to 1.78).

Meta-analyses of the three studies on cigarette size found low-quality evidence that the length of cigarette did not influence the amount consumed. No studies were identified that looked at the effect of differently sized packs, such as packs of 10 cigarettes compared to packs of 20.

 

How did the researchers interpret the results?

The researchers concluded that, "people consistently consume more food and drink when offered larger-sized portions, packages or tableware than when offered smaller-sized versions". 

They say this "suggests that policies and practices that successfully reduce the size, availability and appeal of larger-sized portions, packages, individual units and tableware can contribute to meaningful reductions in the quantities of food (including non-alcoholic beverages) people select and consume in the immediate and short term". 

There was insufficient evidence for them to make recommendations for tobacco or alcohol portion sizes.

 

Conclusion

This systematic review and meta-analysis suggests that increased portion sizes, packaging and crockery influences the amount people choose to eat and actually consume. 

The methods used to produce this review are robust; however, all 72 identified studies were assessed as being at high risk of bias or of unclear risk. This reduces confidence in the results. Other limitations include:

  • the majority of studies were conducted in the US, so the results may not be directly applicable to the UK, because of the potential differences in portion sizes
  • most studies were not conducted on people who were trying to lose weight, so it is not clear how effective this strategy would be for weight loss
  • the studies included only assessed food consumption or selection at one time point, or over short time periods. This means that the studies did not look at whether eating more at one meal was compensated for at subsequent meals that day
  • the studies were performed in controlled environments such as a laboratory, so it remains unclear what effect portion size may have in "normal" environments over the long term

Overall, common sense tells us that people are likely to eat more if the portion size is bigger for a variety of potential reasons, such as:

  • social norms – someone has decided the portion size is appropriate, which may challenge internal perceptions.
  • there is a delay in the time it takes to feel full (satiety) than the time it takes to consume the food in front of you
  • people may not want to waste food and are taught from an early age to "finish your plate"

Reducing portion size or the size of the plate the food is presented on is not a new concept for weight loss – it is a strategy employed by many diet regimes. Other strategies to help maintain a healthy diet can be found in the healthy eating pages

Analysis by Bazian. Edited by NHS Choices. Follow NHS Choices on Twitter. Join the Healthy Evidence forum.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

Portion size key in tackling obesity, says study. BBC News, September 15 2015

Serving food on a larger plate 'makes people eat more'. The Independent, September 15 2015

End to supersizing could reverse obesity trend. The Daily Telegraph, September 15 2015

Links to the science

Hollands GJ, Shemilt I, Marteau TM, et al. Portion, package or tableware size for changing selection and consumption of food, alcohol and tobacco. Cochrane Database of Systematic Reviews. Published online September 14 2015

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Media last reviewed: 27/04/2015

Next review due: 27/04/2017

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