I have switched to flavored carbonated water to reduce my calorie intake, but I have a question: What happens to the carbon in carbonated water? In photosynthesis, CO2 is converted into sugar and free oxygen, right? Since I don't expel a bunch of gas, I wonder what happens to the CO2 once it's inside of me. I suspect the carbon and oxygen are coming apart inside. Does the human body have some mechanism to turn this CO2 into sugar and oxygen, just like in plants? And would that in turn create an even larger me? Is "zero calories" a misnomer? Can the gas be turned into products that are the building blocks of our bodies without ever needing to have calories used or released?
I'm just wondering because my switch from beer to sparkling water has had no net effect on my physique.
Your question is making Science pretty damn sad. Really sad. For your curiosity, you are commended. For your attempt to think through a question? Only praise! Science just wishes your high school (hell, middle school) science class did a better job instructing you.
The carbon in carbonated water is carbon dioxide (CO2). When you drink carbonated water, it remains CO2 and you breathe it out as CO2—not just as burps, but also as breath. At no point in this process is the carbon split from the oxygen and turned into sugar—only plants can do that. And you're not a plant! So, good news: Switching from beer to carbonated drinks (without sweeteners) is a reasonable way to try to lose weight. The CO2 in such a carbonated drink is truly calorie free.
Plants split up CO2 into hydrogen and carbon chains (sugars, basically) plus oxygen by capturing sunlight and stuffing high-energy electrons onto the carbon in the carbon dioxide, ripping apart water for the hydrogen, and releasing oxygen as a waste product. Animals, including humans, can do none of this photosynthesis.
Photosynthesis was discovered in halting steps, starting all the way back in the 1600s. The chain of thought started by scientists noticing that plants grew and gained weight without really changing the weight of the soil they're grown in. In other words, the plant had to grow not by consuming the soil, but by remaking water and the air around it. Later experiments revolved around how an animal (like a mouse) would perish shortly after being placed in a closed glass container, yet if a plant was placed in the glass container, the mouse would live.
Humans can't photosynthesize because we lack chloroplasts—the parts of the plant that perform this splendid trick. Just like the mouse in the jar could not break down the carbon dioxide surrounding it for oxygen and energy, you can't convert your seltzer water into food and air.
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