Waiting to Exhale

Losing weight is a big issue. The CDC estimates that ~42% of adult Americans, roughly 100 million individuals, are obese (defined as having a BMI Index >30). Obesity is responsible for many diseases and accounts for nearly $173 billion in medical expenditures (in 2019 dollars). 

There is no shortage of weight loss services and products. In fact, the weight management market was valued at $143 billion in 2022. Ozempic anyone? But ever wonder how we actually lose weight? Where does the fat go? Stumped? Well, you're not alone...there seems to widespread mystery and confusion about the metabolic process of weight loss even among health professionals, including doctors and dieticians.  

Writing in the SciTechDaily, Ruben Meerman and Andrew Brown of the University of New South Wales say the correct answer is that "fat is converted to carbon dioxide and water. You exhale the carbon dioxide and the water mixes into your circulation until it’s lost (as vapor,) urine or sweat." The key relationship, which we all intuitively recognize, is that:

                                               ENERGY IN = ENERGY OUT

According to government figures, Australians consume 3.5kg of food and beverages every day. Of that, 415g is solid macronutrients and the remaining 3kg is water. Messrs. Meerman and Brown note that what's not usually reported is that a typical person also consumes over 600g of oxygen every day, which is crucial to maintaining the metabolic balance, as shown in the diagram below. Including oxygen, 4.1kg of stuff goes in, so at least 4.1kg of stuff must come out, otherwise we gain weight.     

In steady state, the 415g and 640g of oxygen is metabolized into 740g of carbon dioxide, 35g urea, and 280g (one cup) of "new" water. (Every O₂ molecule we inhale gets split into two separate oxygen atoms which are welded together with hydrogen atoms obtained from food making two brand new water molecules) (h/t rubenmeerman.com). The 3kg of water we drink plays a crucial role in all of this by acting as a solvent, a reactant, and a transporter of nutrients, but remains unaltered in the metabolic process.

What this means is that of the 4.1kg of food, water, and oxygen a person typically consumes each day about half of it exhaled out as carbon dioxide and water vapor. The remainder is excreted out as urine, sweat, and feces. The first part is crucial for weight loss. If we are not producing enough carbon dioxide, we're likely adding pounds.

So, if fat turns into carbon dioxide, can simply breathing more cause weight loss? Umm, no. Meerman and Brown note that "huffing and puffing more than you need will cause hyperventilation and will only make you dizzy, or possibly faint." Unfortunately, the only way you can increase the amount of carbon dioxide your body is produces is by...moving your muscles. Yes, exercising, doing chores, walking to the store half a mile away rather than driving or, worse, Instacart-ing. At least you'll appreciate with every breath that you take, you're exhaling away fat.