Where does fat go when we lose weight?



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As we enter the holiday season and the jingling of bells mixes with the sound of tables groaning under the weight of so much deliciousness, we might also begin to feel our waistlines expanding. But once the party’s over and you try to lose those extra pounds, you might be surprised to learn where they go.


Deep breath

Ruben Meerman of the University of New South Wales (UNSW) in Australia argues that much of the weight we lose could disappear “into thin air.” Though many believe that as we lose weight, fat converts to energy or muscle, in reality the vast majority of it is lost via the lungs as carbon dioxide.

The unused carbohydrates and protein in our diets are converted to triglycerides (a type of fat) which are made of carbon, oxygen and hydrogen atoms — the same atoms, unsurprisingly, that make up the excess carbohydrates. It was Dr. Meerman’s task to “follow” each of these molecules and trace their path out of the body after they had been subjected to the process of “oxidation.” This is essentially the action by which molecules are broken down and recombined to produce different chemical compounds that can be used or excreted by the body.

Dr. Meerman’s team found that, to dispel 10kg of fat, 29kg of oxygen must be inhaled. This is then broken down by the body and results in 28kg of carbon dioxide and 11kg of water. Far from being “burned off” by increased body heat during exercise, the elevated levels of oxygen required by the body during physical activity allows more oxidation to take place and more exhalable carbon dioxide to be created. The remaining 16 percent that is converted to water is expelled as water vapor in breath as well as sweat, urine and other bodily fluids.

According to the study, an average 70kg person, “replacing one hour of rest with exercise that raises the metabolic rate to seven times that of resting… removes an additional 39g of carbon from the body, raising the total by about 20 percent to 240g.” As promising as this sounds, it takes very little of that processed food to offset all that hard work. “Physical activity as a weight loss strategy,” the team concludes, “is, therefore, easily foiled by relatively small quantities of excess food.”


Breathe in, breathe out

Knowledge of the physical processes isn’t new, the scientists say, “but for unknown reasons it seems nobody has thought of performing these calculations before.” It may seem, on some level, as an abstract chemical process that doesn’t alter the “eat less, exercise more” mantra but it speaks volumes about the persistence of common misconceptions and received ideas, even among experts in the medical sciences.

Professor Andrew Brown, head of biomolecular sciences at UNSW, says that Dr. Meerman has “exposed a completely unexpected black hole in the understanding of weight loss amongst the general public and health professionals alike.” Dr. Meerman himself admits that the study grew out of a desire to plug the holes in his own ignorance: “I lost 15 kilograms in 2013 and simply wanted to know where those kilograms were going. After a self-directed, crash course in biochemistry, I stumbled onto this amazing result.”


A breath of fresh air

There is, he says a surprising amount of misunderstanding and misinformation when it comes to weight loss. A survey by the British Medical Journal (BMJ) found that a large number of health professionals including dietitians, personal trainers and even doctors have believed that fat is converted to energy (usually heat) during exercise.

This, says Dr. Meerman, “violates the law of conservation of mass.” According to BMJ figures more than 60 percent of physicians and dietitians involved in the survey believed that fat was turned into energy or heat with the number of personal trainers believing the same not far behind.

Einstein was one of the first to outline what happens to mass when it’s turned into energy (hint: it’s not the kind of weight loss you’d be interested in). Other responses from experts had fat turning into feces, sweat and muscle. The equally fallacious and widely discussed belief that fat turns to muscle and vice versa is just as contradictory to the rules of biology as the “heat” solution is to physics.

The team says that one of the questions most asked of them is whether breathing more can help with weight loss. Their answer, of course, is a resounding no: “breathing more than required by a person’s metabolic rate leads to hyperventilation, which can result in dizziness, palpitations and loss of consciousness.”

So, though the moral of the story may be the same on one hand — eat right, eat less, exercise and get some sleep — on the other, it is a rather eye-opening truism: always question and always look for answers, especially when you’re convinced you already have them.