Can You Lose Weight Without Losing Muscle? UC Berkeley's New Compound Burns Fat While Keeping Muscle

Bottom line: UC Berkeley researchers report in Science Advances that an experimental compound called TOFA makes cells burn up to ~18% more energy, helping obese mice shed fat with no significant loss of lean muscle — and it works additively with GLP-1 drugs. The next race in weight-loss medicine isn't about how many kilograms you lose, but whether what you lose is fat or muscle.

Over the past five years, GLP-1 medications have transformed the treatment of obesity, diabetes, and fatty liver disease. Ozempic, Wegovy, Mounjaro, and Zepbound have become global phenomena, and their effectiveness at driving weight loss and blood sugar control is genuinely remarkable.

But the miracle comes with a catch. GLP-1s work by suppressing appetite and reducing food intake — and that path carries two known problems: gastrointestinal side effects like nausea, and muscle loss. When you eat less, protein intake drops, and the body begins breaking down muscle for energy. Over time this can lead to sarcopenia and frailty, which is especially dangerous for older adults.

The GLP-1 blind spot: weight loss at the cost of muscle

Body weight responds to two levers: taking in fewer calories, or spending more energy. GLP-1s pull almost entirely on the first lever. The UC Berkeley team decided to pull the second.

"GLP-1s work almost entirely on the first [lever], so we went after the second." — Anders Näär, professor of metabolic biology and nutrition at UC Berkeley, senior author of the study

TOFA: from "eat less" to "burn more"

In the study, published online August 21 in Science Advances, the team identified a molecular compound called TOFA (5-tetradecyloxy-2-furoic acid). TOFA is actually an "old molecule" first discovered in the 1970s. It belongs to a class called ACC inhibitors — ACC is a key enzyme the body uses to build lipids like cholesterol and triglycerides, so inhibiting it reduces lipid production.

Several ACC inhibitors previously reached mid-stage clinical trials, yet none has been approved for metabolic disease — largely because many of them raise triglycerides, posing a significant heart-health risk.

A dual mechanism: one compound, two jobs

TOFA is different. The researchers found it doesn't just act as an ACC inhibitor — it also activates PPARα and PPARδ, two cellular receptors that switch on genes enabling cells to take up fat and burn it for energy. In mice, cells burned up to 18% more energy with no change in physical activity and no rise in body temperature. The energy was simply spent.

Because of this "block synthesis + boost burning" dual mechanism, TOFA did not raise triglycerides the way other ACC inhibitors do. First author Justin Y. Lee describes it: "TOFA appears to engage a coordinated metabolic response. It is not simply blocking lipid synthesis. It is also activating energy expenditure pathways that may help the body handle excess lipid and glucose more effectively."

Mouse results: less fat, more muscle, better metabolism

In obese mice, TOFA delivered benefits across the board:

• Fat loss without muscle loss: weight dropped from fat, with no significant loss of lean mass
• Metabolic improvement: better insulin sensitivity and glucose control
• Lower blood lipids: reduced triglycerides
• Healthier liver: improved features of fatty liver disease

The team also ran a control experiment: splitting the two mechanisms into two separate compounds — one blocking lipid production, one boosting energy expenditure — worked less well than TOFA alone. The coordinated, dual action appears to be the key.

With GLP-1s: complementary, not a replacement

The most intriguing finding is the combination. In mice, pairing TOFA with semaglutide (Ozempic/Wegovy) or tirzepatide (Mounjaro/Zepbound) produced greater improvements in body weight, glucose control, insulin levels, and triglycerides than either treatment alone.

"In our combination experiments, TOFA worked additively or synergistically with the GLP-1 appetite suppressing drugs, so we view it as complementary rather than as a replacement," Näär said — hinting that future weight-loss therapy may become a two-track approach: eat less and burn more.

How far is it from humans?

The researchers stress that TOFA has only been studied in animals; its safety and efficacy in humans have yet to be tested. With support from Berkeley's life-sciences entrepreneurship ecosystem, including Nucleate and Berkeley SkyDeck, the team has founded a new company, ReRx Therapeutics, to carry the work toward patients.

From animal studies to an approved drug typically takes years of human clinical trials. But the direction is clear: the next generation of weight-loss medicine isn't about eating even less — it's about getting the body to burn more, and to burn fat, not muscle.

FAQ

Q1: Why do GLP-1 drugs cause muscle loss?

GLP-1s suppress appetite, so total calorie and protein intake drop. When energy and protein are scarce, the body breaks down muscle tissue to supply amino acids and energy. The faster and larger the weight loss, the more noticeable the muscle loss tends to be.

Q2: How far is TOFA from reaching the market?

Only animal data exists so far; human safety and efficacy are untested. New drugs must pass Phase I–III human trials, which typically takes years. The team has founded ReRx Therapeutics to advance development.

Q3: What is TOFA, and why is this called a "new use for an old drug"?

TOFA was discovered in the 1970s and was classified as an ACC inhibitor (blocking lipid synthesis). This study found it also activates PPARα/PPARδ receptors to boost energy expenditure — giving an old molecule a brand-new therapeutic angle.

Q4: Why do we need new drugs if GLP-1s already work?

GLP-1s act on only one lever — reducing intake — and come with muscle loss and gastrointestinal side effects. TOFA acts on the other lever — increasing expenditure — so the mechanisms complement each other, and the combination outperformed either alone in mice.

Q5: Can I buy or take TOFA today?

No. TOFA is a laboratory compound; no approved drug or supplement contains it. Any product claiming otherwise should be treated as unverified hype.