About one-third of Canadians have become obese — with more weight gain happening during the pandemic, according to a new study. 

The research, published in the Canadian Medical Association Journal (CMAJ) Monday, found that 32.7 per cent of Canadians (10.6 million people) were obese in 2023 — an increase of about eight percentage points since 2009. 

But the researchers also saw a steeper increase in obesity during the COVID-19 pandemic.

"The pandemic had such a profound impact on our daily lives … including changes in our eating behaviours and access to healthy foods," said Laura Anderson, lead author on the study and associate professor in health, research methods, evidence and impact at McMaster University in Hamilton, Ont.

Obesity is a medical condition that puts people at higher risk of other serious health issues such as stroke, Type 2 diabetes and certain types of cancer. With frequent lock downs forcing people to stay at home during the pandemic, researchers wanted to see how this changed people's weight. 

To study this, Anderson and her team looked at the most recent self-reported body mass index (BMI) data from 746,250 Canadians who were 18 years or older between 2009 and 2023. 

The researchers followed the World Health Organization's definition of obesity, classifying someone as such if they had a BMI equal to or higher than 30. 

(BMI has its problems as a measure of health; it's not a direct measure of body fat and doesn't account for differences across ethnic groups, sex or gender, for example. But, it's still the preferred measure for researchers looking at groups and population-level trends.)

Before COVID-19, Anderson says Canada's obesity rate was increasing by about 0.5 percentage points per year. But during the pandemic, she says that rate doubled, meaning the obesity rate increased by about one percentage point per year.  

While the research didn't look into why this might have happened, Dr. Sean Wharton, an internal medicine doctor who works with people struggling with obesity, says mental health issues could have played a role. 

"There was isolation and depression," said Wharton, who did not take part in the new CMAJ study. Wharton says he's a clinical advisor to all of the weight loss drug companies. 

"People who were living with depression, their depression may have [gotten] even worse. And that's where we saw people having an increase in their weight." 

When asked whether obesity rates will drop now, Wharton says he doesn't think so. 

"Once you have that weight gain, it's hard to actually get it off." 

Over the 15-year study period and during the pandemic, the greatest increase in obesity was in young adults between the ages of 18 and 39 years old. 

And specifically, the research found that an increasing number of women became severely obese, which the authors considered to be a BMI of 40 or higher.

While the study didn't explore the reason behind this, Anderson says it could be that "women had increased stress and burden of other caregiving duties during the pandemic." 

And that was the case for Toronto resident Angela Martin.

During COVID-19, she worked as a nurse in a family doctor's office. She also separated from her partner. Between those two events, Martin says 2020 was a hard year. 

"Looking back, I was so not fine," she said. 

Even though she's struggled with obesity throughout her life, Martin said she gained a significant amount of weight in the early COVID months. 

"I mean you did the one thing you could do … you were not prevented from cooking so I tried all new recipes and then you couldn't go to the gym," she said. 

While more people became obese in general, the study's authors were most concerned about the rise in people with severe obesity. 

In 2009, about 2.4 per cent of Canadians were severely obese, but in 2023 that increased to nearly five per cent. 

"With more severe rates of obesity, we're going to see more of the complications," said Wharton. 

In particular, he said people are more at risk of illnesses like Type 2 diabetes, osteoarthritis or heart disease. 

When looking into other conditions affecting people with obesity, the research found that half of them didn't have any chronic health conditions, but five to 10 per cent had between three to six long-term health problems, such as heart disease, stroke and cancer. 

This study looked at data up to 2023, so it's not clear yet what the growing availability of weight-loss drugs like Wegovy — approved last year — may have on obesity rates post-pandemic.

The study notes that may already be changing, but the drugs weren't widely available during their study years. 

The researchers didn't dive into this in their study, and won't be able to explore it until more data comes out.