Is the obesity crisis linked to a common pesticide?

A recent study led by scientists at McMaster University investigated the effects of 34 different pesticides and herbicides on the activity of brown fat cells in mice. The researchers found that chlorpyrifos, an organophosphate insecticide widely used on fruits and vegetables, reduced the burning of calories in the brown fat cells and promoted obesity.

In mammals, maintenance of weight is usually accomplished through balancing caloric intake with absorption and expenditure. One of the ways in which energy is expended is by generating heat to keep warm. The process is known as thermogenesis and it involves the activation of brown adipose (fat) cells.

The study showed that chlorpyrifos suppresses thermogenesis and the activation of brown fat cells, which implies that fewer calories are used up in the generation of heat. This promotes obesity in these mice as the excess calories are stored in the body in the form of white fat.

Study senior author Gregory Steinberg is a professor of Medicine and co-director of the Centre for Metabolism, Obesity, and Diabetes Research at McMaster.

“Brown fat is the metabolic furnace in our body, burning calories, unlike normal fat that is used to store them. This generates heat and prevents calories from being deposited on our bodies as normal white fat. We know brown fat is activated during cold and when we eat,” said Professor Steinberg.

For the investigation, the researchers exposed mice to 2.0 mg/kg body weight of chlorpyrifos daily, a dose that is aligned with the real-world exposure of humans. Steinberg said that, while chlorpyrifos has been linked to rising obesity rates in both humans and animals, most of these studies have attributed weight gain to associated increases in food intake and not to the suppression of metabolism in the brown fat cells. 

“Lifestyle changes around diet and exercise rarely lead to sustained weight loss. We think part of the problem may be this intrinsic dialling back of the metabolic furnace by chlorpyrifos,” explained Professor Steinberg.

The daily burning of calories in brown fat tissue would only have to be inhibited by a small amount in adult humans (around 40 calories a day) to result in a weight gain of five pounds over a year. This could lead to a significant gain or even to obesity over time. 

Mice that developed obesity due to chlorpyrifos-induced suppression of brown fat cell activity also showed increased incidence of insulin resistance and non-alcoholic fatty liver disease.

Professor Steinberg states that, although the findings have yet to be confirmed in humans, it is important that consumers wash fresh produce thoroughly before eating it. 

The results of this study are published in the journal Nature Communications.

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By Alison Bosman, Earth.com Staff Writer