Jessica, who saw my TEDx talk about how fat is metabolised to CO₂ and H₂O, wanted to know if the CO₂ in carbonated water can make you fat.
The short answer is, no, but because this is such a frequently asked question, I penned her the long answer, too. Like Jessica, you probably knew most of these facts but here’s how to connect the dots.
Plants make fat by rearranging the atoms in carbon dioxide and water in a process called photosynthesis. Olive trees convert CO₂ and H₂O into olive oil, for example. Sunflowers make sunflower oil and palm trees make palm oil, but all those oils are essentially identical apart from some relatively minor differences in the ratios of saturated to unsaturated fatty acids. Here’s how plants do it, and why you and I can’t:
STEP 1: is to “suck” water (H₂O) out of the soil and up to the leaves, and then pluck the hydrogen atoms off those water molecules i.e. to literally remove the two “H” atoms from H₂O. Plants use the energy in sunlight and an enzyme called chlorophyll to do this. Because sunlight is required for this biochemistry to proceed, these are called the “light reactions” to distinguish them from the reactions in STEP 2 of photosynthesis, which are called the “dark reactions”. The oxygen atoms removed from the water molecules (i.e. the “O” in H₂O) are the waste product of the “light reactions” and are released into the atmosphere as O₂ via stomata in the leaves.
STEP 2: is to “suck” carbon dioxide out of the air and stick the hydrogen atoms from STEP 1 to these molecules. It takes six CO₂ molecules plus six H₂O molecules to make one glucose molecule, which has the chemical formula C₆H₁₂O₆. While STEP 1 and STEP 2 both involve numerous steps, the entire process can be summarised in this simple equation:
6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂
STEP 3: is to convert the glucose molecules to fatty acids. For example, three glucose molecules can be rearranged to make one stearic acid molecule. Stearic acid is one of the most common saturated fatty acids found in nature and has the chemical formula C₁₈H₃₆O₂. Olive oil is 2-3% stearic acid and around 66% oleic acid, with the chemical formula C₁₈H₃₄O₂. Oleic acid is a mono-unsaturated fatty acid, and is really just stearic acid minus two hydrogen atoms (removing two hydrogen atoms from any fatty acid is called a “desaturation”.)
The energy in olive oil is released when you convert it back to CO₂ and H₂O. That energy was once sunlight, which was used to rip the hydrogen from water molecules and then “stored” by sticking those hydrogen atoms to carbon atoms, which is the closest thing to real, actual, magic that happens on our planet every time the sun is shining. The point is, once you have converted fat to CO₂ and H₂O, only a plant can start the process of putting it back together again.