In a new paper published in Cell Metabolism, scientists say they have demonstrated that mice genetically altered to burn fats in preference to carbohydrates, will convert the unburned carbohydrates into stored fat anyway, and their ultimate weight and body composition will be the same as normal mice.
It all comes down to an enzyme known as ACC2 (acetyl-CoA carboxylase), which controls whether cells burn fats or carbohydrates. When it was shown that 'blocking' ACC2 will force cells to burn fats in preference to carbohydrates, many assumed that such 'fat burning' could make fat stores evaporate, and make people thin without changing food intake or energy expenditure.
"Our data urges a correction in people's concept of a magic bullet - something that will miraculously make them thin while they sit on the couch watching television," said Associate Professor Greg Cooney, from Sydney's Garvan Institute of Medical Research.
"While none of the large pharmaceutical companies have marketed ACC2 inhibitors, there are many kinds of so-called 'fat-burning pills' available in the health food, body building and alternative medicine markets, where limited clinical effectiveness data are required."
"Many such products can also contain potentially harmful stimulants or come with a recommendation to follow a calorie-controlled diet and do more exercise while taking them. If you follow those recommendations, then of course you'll lose weight - but you'd lose it anyway."
"The energy you use in your home can come from a coal-fired power station, hydroelectric power, or a wind turbine. You won't know which because the end result is electricity."
"The energy that fuels your body can come from fats, proteins or carbohydrates. You won't know which because the end result is ATP, or cellular energy."
"Your body will use the energy it needs and store the leftover fats, proteins or carbohydrates as fat. When you do the sums, it's ultimately a matter of calories in and calories out."
"It's important to stress that the focus of our study was limited to an analysis of the impact of fat oxidation on overall fatness. We didn't investigate all impacts of fat oxidation – and so we don't rule out benefits of burning off fats in specific tissues."
"For example, manipulating fat metabolism may - or may not - lead to better insulin action in muscles or in the liver. Should insulin action be improved, that would obviously benefit obese people with Type 2 diabetes. But we can't comment either way until we do the experiments."
And the take-home message? Follow a healthy, balanced diet and get plenty of exercise.
Citation: Hoehn, K. et al., 'Acute or Chronic Upregulation of Mitochondrial Fatty Acid Oxidation Has No Net Effect on Whole-Body Energy Expenditure or Adiposity', Cell Metabolism, Janurary 2010, 11(1), 70-76; doi: 10.1016/j.cmet.2009.11.008