Fast food turns fat-burning cells into fat-storing ones, study finds

The problem with ultra-processed foods goes beyond calories. They can simultaneously cause fat to accumulate in the body and impair the metabolic function of fat tissue — largely because of their high saturated fat and sugar content.

Fat tissue plays a role in energy storage, hormone signaling, temperature regulation and metabolic function. For this reason, researchers are expanding the focus of weight and health management beyond the sheer amount of body fat to include maintaining healthy fat function.

The international journal Nutrition recently published a study on the subject. Researchers at Rio de Janeiro State University in Brazil divided male mice into four groups — a standard diet, a high-fat diet, a high-fructose diet, and a combined high-fat and high-fructose diet — and observed them for 12 weeks. The group fed the combined diet showed the most severe insulin resistance and the greatest degree of brown fat whitening.

Insulin resistance is a condition in which the body's ability to regulate blood sugar no longer functions properly. Brown fat whitening occurs when brown fat loses its function and takes on the characteristics of white fat — cells that burn fat transform into cells that store it. Brown fat generates body heat and promotes energy expenditure, while white fat stores surplus energy. The researchers said these changes "negatively affect not only body weight but also metabolic health."

Ahn Jae-hyun, chief director of the Fat Stem Cell Center at Global 365mc Incheon Hospital, said that when fat's metabolic function declines, the body's capacity to expend energy falls, meaning the same food intake leads to more energy being stored as fat. He added that this "can lead to weight gain and changes in body shape such as abdominal obesity," and that it "can also interfere with maintaining weight after losing it." Frequent consumption of processed foods, in other words, can degrade fat's metabolic function and make the body prone to gaining weight more easily.

Cutting processed foods out of daily life entirely is difficult, however. Experts advise starting by reducing how often and how much you consume them. Simply dropping the habit of eating dessert after meals can significantly lower sugar and saturated fat intake. Limiting fried foods and processed meats to once or twice a week is a good starting point. High-fructose beverages can be replaced with water or unsweetened tea.

Healthy fat intake also matters. Foods rich in unsaturated fats help create a healthier environment for fat cells and support metabolic function. Almonds, avocados, mackerel and salmon are among the best examples.

As GLP-1 obesity treatments have grown in use, research linking them to fat metabolic function has also emerged. Ahn said some research (Endocrine, 2025) has suggested that GLP-1 treatments, if they continuously stimulate GLP-1 receptors in white fat in the dermal layer and in fat stem cells, could impair the metabolic function of fat tissue. He added that GLP-1 is a relatively safe obesity treatment, but should be used within the range recommended by medical professionals so as not to place undue strain on the body.

Lifestyle also plays a role. Irregular meals and insufficient sleep can accelerate fat accumulation. Ahn said regular sleep, adequate hydration and even just 30 minutes of walking a day can help manage metabolism.