If you’re trying to lose weight, you’ll no doubt have come across the less than helpful advice to ‘just eat less’. It seems obvious on the face of it that if you downsize the amount you eat at mealtimes, you’ll consume fewer calories and your weight will drop.
But being humans, nothing is as simple as that.
Why?
One reason is because our minds and bodies compute what we’ve eaten and we consciously (“I skipped dessert earlier so it’s fine for me to eat this chocolate now”) or sub-consciously adjust our eating later in the day.
This is known as compensation.
All the same, research shows that when we eat smaller meals, overall we consume less despite this compensation, and addressing meal size is a potentially helpful and healthful habit change.
If you’re thinking of changing your portion sizes to help you drop the pounds, understanding the psychology of why this approach works – and where it can fail – can be useful. I’ve reviewed some of the latest research into downsizing portions, and added some learnings from my clinical practice to help you focus on positive habit change.
Portion sizes have increased over the past few decades
Food portion sizes have increased over time, and our current food environment is characterised by a wide availability of energy-dense food products sold in larger portion sizes. Average body weight in many countries has increased over the past few decades.
It seems that environmental factors that promote increased energy intake are largely uncompensated for and therefore can shift body weight upwards over time.
There may be a link between portion size increasing and weight gain
Psychologists and behavioural scientists are interested in the extent to which the amount we are served (or serve ourselves) determines what we consume.
Professor Eric Robinson and his colleagues at Liverpool University conducted a systematic review* of studies on portion size and energy intake. A total of fourteen studies were included in the review.
The aims were to systematically review the impact that experimentally manipulating portion size has on total daily energy intake and subsequent changes in body weight.
Across the fourteen eligible studies, smaller food portions resulted in lower daily energy intake and this effect was consistent across males and females.
The findings might be directly helpful to you if you want to reduce the size of your meals if you are currently eating more than you need.
What happens if we start reducing the size of our meal portions?
Studies measuring energy intake in the laboratory have found that serving smaller portions relative to larger portion sizes did result in lower daily energy intake over several days.
Robinson calculated that reducing portion size by 50% – in other words halving the amount served – was associated with an 8% decrease in energy intake over the course of the day. Whilst this might seem disappointing, an 8% overall calorie reduction day-in, day-out will add up.
As well as reducing your calorie intake, it’s likely that a positive side-effect will be an increase in your sense of flexible control around food.
Do we need to reduce the size of every meal?
If you’re trying to lose weight, should you reduce the size of every meal? Like everything else about changing eating habits, it depends on you as an individual and what your current habits are.
In Robinson’s review, reductions to daily energy intake were larger in studies that manipulated the portion size of foods at most meals as opposed to studies that only manipulated portion size at one or two meals.
This is interesting but in my clinical practice I find that people often report having small to medium portions at breakfast and lunch, but a large evening meal. If that is true for you, trying to reduce the portions at every meal could backfire. I’d suggest that you just focus on reducing the size of the one meal each day that is larger than you need.
We have a tendency to compensate for eating less now by eating a bit more later
Robinson found evidence that there is some energy intake compensation in response to manipulations of portion size (e.g. eating more/less after having been served a smaller/larger portion size), but this compensation was only partial and this compensation does not become larger over time.
Changes to energy intake at meals caused by serving smaller portion sizes were partly compensated for later on. Approximately 42% of the reduction in energy intake observed at manipulated portion size meals was ‘compensated for’ through additional energy intake at other meals later that day.
But that still means that by focusing on the one specific eating habit change of reducing the size of overly large meals reduced overall daily intake.
Robinson and his colleagues concluded that smaller food portion sizes decrease daily energy intake and there is evidence that over time this may result in lower body weight.
So how might you do this in practice?
What I suggest is to reduce the portion size of your meal by a quarter. That is a significant amount, but it’s do-able. Halving your meal size (as mentioned in the study) is likely to be too big a step for most of us. You can always reduce down again when you’ve got used to the new (three-quarter) size meal.
The psychology of reducing your meal size
Changing how you eat – even a single eating habit – involves mental effort and focus, both of which require energy. If you want to change the size of your regular meals, there are specific points at which you’ll need mental effort and focus, and you may find it helpful to think in terms of the key moments. The key moments are when you are serving the food and when you stop eating.
Here I’ll use the example of reducing the size of your midday meal:
When you’re serving the food
- When you choose how much you’re going to have for lunch, you’ll need your wits about you. Your brain’s autopilot will go for the amount you’ve become used to as a habit. So you’ll have to switch off your autopilot and take over the mental controls by asking yourself what your new portion size will be.
- When your brain answers this question, you may not like the answer – eating less than normal may make you feel uneasy, even anxious. Mental chatter might start up, like “no way that will be enough” or “I love that food and don’t want to miss out on all that pleasure”.
- My advice to you is to gently remind yourself to hold your nerve with the new portion size. You are learning something new and it’s an experiment to see what your body makes of it. In fact I’d go so far as to say that it’s more than an experiment – it’s an adventure.
When you’re eating the food
- Now start eating your meal and make every mouthful count – focus on the food and savour it. You’ll get much more pleasure that way, you’ll register fullness signals better and you’ll be laying down a richer memory of this meal which will help you with eating less later on. Here’s a link to a blog on this.
When you stop eating
- When you have finished what’s on your plate, you need your wits about you again because the automatic part of your brain is going to be registering that this is less than usual. So this is the make-or-break point in the day, where you either do the new thing of stopping eating now – a new Behavioural Satiety Sequence if you like (I’ve written a blog about that here) – or revert to seeking out more food.
When you think about snacking between meals
- There may be times between meals when you think of having a snack. This is where you risk compensating for the smaller meal earlier. There isn’t a simple one-sized solution here, but if you do think of snacking, ask yourself whether you are actually hungry right now or not. If not, reminding yourself that avoiding compensating for the smaller meal earlier will help you achieve your goals more quickly.
If you have any questions, please feel free to drop me an email directly to drhelenmccarthy@gmail.com.
Reference
* Robinson et al (2022) Downsizing food: a systematic review and meta-analysis examining the effect of reducing served food portion sizes on daily energy intake and body weight. British Journal of Nutrition https://doi.org/10.1017/s0007114522000903