The Tsimane' people in Bolivia have remained relatively untouched by the outside world for generations, thriving off of the land by practicing traditional forms of hunting, fishing, farming, and gathering food. For years, the tropical forest forager-farmers have been known to have remarkable cardiovascular health and low blood pressure – that is, until they added one ingredient.

Cooking oil. New research published in the journal Obesity suggests that in less than a decade, the Tsimane’ have experienced changes in their body mass index (BMI), including obesity and weight gain, after being introduced to cooking oil.

Between 2002 and 2010, researchers analyzed data from nearly 700 men and women who were older than 20 living in the Amazonian lowlands. Over the course of the study, the researchers noticed that the Tsimane’ people had growing access to market foods in the face of a population boom, nearly tripling their numbers to 16,000 in less than two decades. Encroachment on their land from development paired with new access to school and wages mean that more modernized methods of cooking and eating may be the culprit.

In nine years, the prevalence of women being overweight or obese increased by more than 6 percent. Men, on the other hand, saw an increase in being overweight by nearly 9 percent and just 1.5 percent for obesity. More use of cooking oil was associated with higher BMI in females, and the consumption of domesticated animal products, like pigs and chickens, was associated with a higher BMI in women and a wider average waist circumference in men. During this time frame, household calories did not increase and homegrown crops still accounted for a majority of the food eaten at home. However, the odds of cooking oil used increased by 24 percent every year, doubling the number of households using it so that by 2010, almost one-third of households incorporated it into their diet.

“But that's only one part of their story," said study author Alan Schultz, from Baylor University, in a statement, noting that the findings present more questions than they do answers. “Our findings of a trend in weight gain and increased consumption of high-calorie foods suggest that the benefits of change are not so clear-cut."

That’s because shifts in subsistence diets often mirror other lifestyle changes and it can be difficult to separate the effects of the two. Regardless, the study authors say that even small increases of market-based foods can make a difference in a population that once thrived only on subsistence eating.

"Cooking oil adds so much flavor – we use it for a reason – but at 120 calories/14 fat grams per tablespoon, few foods can so easily alter your diet,’ said Shultz.

But there are other factors as well. Shorter stature and greater muscle mass can lead to higher BMI values, the authors note. Therefore, it could be a measure that means different things for active, forest-dwelling populations than it does for sedentary individuals in wealthier nations. It is also difficult to measure the caloric intake of foods that are harvested from the wild, which could have limited the statistical findings of the study. To that end, the study authors note that future research should focus on the consequences of similar lifestyle changes and how they impact diet, physical activity, infectious diseases, and how people live their lives.

“Does market-based living condemn most of us to obesity, stress and poor heart-health? Will it have this effect on Tsimane'?" asked Shultz. "If Tsimane' livelihoods are changing, we want to understand the causes and ultimate effects of that change for their health – and for ours."