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It happens every year. Despite our best efforts – and promises to ourselves – healthy eating habits seem to fall apart in December. In between the family lunches and work parties, it is almost expected that we will eat more than we should, or more than we want to. 

But why do we overeat, and why is it so hard to resist during the holidays?

The answer, in part, is that we’re wired that way.

“We’re built to eat food, so it’s not a surprise that people overeat,” explains Professor Lenny Vartanian, psychologist from UNSW Sydney. Professor Vartanian researches the psychology of eating and weight, including body image, self-regulation and social influences.

“Essentially, if the food is good, people will keep eating until something tells them to stop.”

These ‘stop signals’ could be when a person feels full, when the food stops tasting good, when the food is all gone, or when social cues tell them to slow down. 

The holidays just happen to be when those signals disappear.

“It really is the perfect storm of all of these things driving us to overeat,” says Professor Vartanian, pinpointing the main culprits: the variety of food, bigger portion sizes and increased social pressure. These cues make it harder for people to recognise when they feel full. 

But despite the extra temptation to overeat during the holidays, healthy habits don’t need to be written off for the year – there are strategies you can implement to minimise your chance of overeating. 

“The holiday season doesn’t need to be structured as it currently is. We can change those parameters,” Professor Vartanian says.

“We can make fewer food options and smaller portions available. We don't have to have this shared understanding that Christmas lunch means we're all going to indulge and there's nothing we can do about it.”

Here are the three main factors to look out for, and tips for how to manage them. 

A smorgasbord of choice 

Our appetites can be easily manipulated by the variety of food available. Too few options, and we will get bored; too many options, and we will overindulge. In both cases, variety changes how good the food tastes – a phenomenon called sensory-specific satiety.

“When a person eats something with a specific sensory property (such as a taste, texture or colour), they will get bored and stop eating it after a point – even if they are still hungry. The same is true if the opposite happens. If that sensory property is changed, they will have a renewed appetite.”

The variety of foods available also tends to increase over the holidays.

“When people gather, they usually provide a variety of choice for starters, mains and desserts. This variety prompts people to eat more than they otherwise would.”

To manage the urge to try everything on offer at Christmas lunch, Professor Vartanian suggests creating a self-imposed limit per course.

“Set yourself a limit of how many different dishes to try. Tell yourself, ‘I know there will be a lot of desserts, which will make me want to have a bit of everything, but maybe I'll just take one or two.’” 

The portion size trap

As party planners know, the rule of hosting is to overcater instead of offering too little or just enough food. However, with more food available, humans lose an important signal telling them to stop eating – running out of food. 

“When people get together in groups, they tend to provide more food than needed. The more food that’s available, the more people will eat.”

To avoid piling excess food onto your plate, Professor Vartanian suggests pre-planning what you intend to eat in any given meal. This will help with impulse control.

“Decide at the start of the meal what and how much you're going to have, even if it's letting yourself have more than you typically would. Once you choose the food, put it on your plate and don’t add any more, and don’t go for a second round.” 

If food wastage is a main concern, save any leftovers in food storage containers. 

Backyard lunch

People tend to eat more when they're in groups - and the larger the group, the more they will eat. Image: Shutterstock.

The power of social pressure

Our eating habits are heavily influenced by the habits of those around us.  

Not only do we eat more in groups than when we're alone, but we also mimic the eating habits of those around us. The more people around us are indulging, the higher the chance of us overeating, too.

“People expect that they're going to overeat during the holidays,” says Professor Vartanian. “If everybody has that shared expectation, then there's a common understanding that it's okay to do that.”

Sometimes, the social pressure to keep eating is less subtle.

“Many of us have a family member that encourages us to eat more – and will make you feel guilty if you don’t.” 

To avoid caving in, think about where your external pressures might come from these holidays – do you have a grandparent who always piles food on your plate? Are you made to feel guilty if the food needs to be finished? Prepare yourself mentally for this pressure and consider how you might respond. Planning in advance can help you navigate potentially awkward and uncomfortable situations.

Remember also that people match the eating habits of those around them, so you might want to carefully pick who you sit with at the table.  

Is overeating really that big of a deal?

While it’s unhealthy to make a habit of overeating, the occasional indulgence may not be as bad as you think.

“People stress out way more than they need to about having overeaten and gaining a kilo or two,” says Professor Vartanian. “This level of stress is probably worse than the overeating itself.”

When it comes to deciding whether your overeating is a problem, ask yourself how frequent the habit is.

“If it happens once – if it’s just Christmas lunch – and it brings joy to people or family unity, then my view is to just enjoy the holiday. However, if it's no longer just overindulging at Christmas lunch but something that drags on across the whole two-week period, then you may need to rethink your approach.”