If you are on a diet or preparing to start one you may have wondered where does fat go when you lose weight? You probably wrongly assume that it is transformed into some kind of energy or it just burns itself away.

In fact, it is no mystery among scientists and doctors that most of the fat you lose when you are dieting is exhaled out of your body. Yes that is right, you breathe out most of the weight you lose!

Despite knowing this for some time, a study was recently published which actually measured, for the first time, just how much weight is lost in your breath while dieting. The study was published in the British Medical Journal by researchers at the University of New South Wales in Sydney (Australia), according to which the lipids are eliminated from the body just like with carbon dioxide.

We spoke with Dr. Monica Giuffre, nutritionist of Bariatric Surgery at Humanitas hospital, to find out more.


Runner breathing heavily while taking a break from his morning exercise with his earphones in


What is the correlation between your diet and breathing?

“First we have to explain the difference between respiration and cellular respiration. The first is the act by which the human body introduces oxygen from the external environment (inhalation) and eliminates carbon dioxide (exhalation). The oxygen is absorbed into the blood and thanks to the red blood cells it travels to various tissues where cells perform cellular respiration. Cells use the oxygen to break down precisely the nutrients contained in the foods we eat and derive energy for vital activities”, answers Dr. Monica Giuffre.

During the research, the authors studied the mechanism by which the human body is freed from fat during a diet. Firstly they observed that excess carbohydrates and proteins are converted into chemical compounds called triglycerides, made up of carbon, hydrogen and oxygen. These substances are then stored in lipid droplets present in the fat cells.


Losing 10 kg of fat means you are losing 8.4 kg of carbon dioxide

Scientists have discovered that when you lose weight the carbon content in the fat cells is released and subsequently expelled through your breath. Specifically they have noted that during a diet losing 10 kg of fat entails the elimination of 8.4 kg of carbon dioxide. The remaining 1.6 kg become water, which is expelled from the body through urination, bowel movements, sweat, breath, tears and other body fluids.


What role do the lungs play in the success of a weight loss program?

The researchers then showed the crucial role played by the lungs in weight loss. However, an effort to breathe more than necessary will not help you lose weight. It may instead cause hyperventilation, followed by dizziness, palpitations and loss of consciousness.

“Lungs are mainly used in gas exchange of oxygen and carbon dioxide. The ratio between the eliminated molecules of carbon dioxide and those of inhaled oxygen is called a respiratory quotient (rq) that depends on the composition of the diet.”


Do I exhale the same amount of carbon dioxide in all kinds of weight loss programs, or are there differences based on the type of diet?

“The gas exchange varies because fats, sugars and proteins require different amounts of oxygen to be oxidized. For example, carbohydrates require one oxygen molecule to release a carbon dioxide molecule, while fats will require a greater amount of oxygen. The respiratory quotient is high if we eat mainly carbohydrates and it is low if we consume more foods high in fat”.

“During aerobic exercises the respiratory quotient is initially high because your muscles get energy mainly from sugars. This gradually drops as there is no sugar available to draw energy from for the oxidation of the fat accumulated in the adipose tissue. You will start to lose weight because the muscles begin to consume reserves. However, this process varies depending on the state of health of the individual, his/her gender and age.”


Are there any tips for those on a diet?

“For those who follow a low calorie but balanced diet it is recommended to perform adequate aerobic exercises and adjust them to optimize weight loss. When practicing sports, our muscles work harder and therefore need a greater amount of oxygen. If the carbohydrate intake is moderate, we can consume the reserve of fat that is deposited in the fat cells of our body around the belly, hips, buttocks, etc”, says Dr Giuffre.