I almost posted this as an "answer" to this old question that recently popped up again, but decided to make it a question.
I recently watched this companion video for my intro-to-bio textbook. Lots of questions came to mind; like: Is this true -- all excess protein is converted to glucose or fat? How is the conversion determined? Is glucose preferred? Does this explain why protein shakes cause blood sugar/insulin spikes? Is a "fat-adapted" ketogenic body less likely to convert protein into glucose? Are high-protein Low-Carbers regularly converting protein to glucose for their energy needs? (Are they thusly not in ketosis?)
Someone (Jon) stated in his answer to the other question that protein may be oxidized in the citric-acid cycle, instead of being converted to glucose or fat. Is this true? What does he mean by "oxidized"? used to make ATP? Or "burned" some other way?
Are there benefits to eating more protein than 'required'? Do you eat considerably more protein than is recommended?
asked byg_ (3631)
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on October 14, 2011
at 05:41 PM
I don't think the oversimplification in that presentation is "criminal", but it is a case of oversimplifying leading to small errors. The overarching point seemed to be that excess protein doesn't get stored as such to build muscle. Whatever the fate, it must be stripped of the nitrogen thus "taxing" your kidneys. I would contend that the potential for this actually taxing the kidneys is vastly overstated in the nutrition field.
In the other post, the graphic I posted came from a text/website ... I just can't recall which ... but clearly a more advanced one. In intro bio, understanding the fate of protein as was stated is sufficient and not overly misleading. In more advanced biochem or physiology, a text is likely to contain something closer to my graphic. Different amino acids must all be stripped from the "amino" but they can feed directly into various pathways without needing to be converted to glucose first. Indeed I think conversion to fat for storage is the least likely path.
on October 14, 2011
at 05:11 PM
I'm a bio major and no, not all excess protein gets converted to glucose or fat. The kidney excretes them out of your body through urination. But yes protein can get converted to glucose if you're eating low carb. Gluconeogenesis is made from NON-CARBOHYDRATE sources. That's why we don't necessarily need to eat carbs (or at least the recommended 130 g/day) for energy. You should only eat carbs for your red blood cells. Oxidation is the process of burning or breaking down something. So yes if there is excess protein, your body will use some of that for energy. This is generally not what you want your body to be doing which is why there is so much emphasis on fat here. If you can divert your body from burning protein (since you're not eating carbs), then the body will burn fat (ketosis). I'm only supposed to be eating about 55 g of protein but I eat over 140 g EASILY every day. My macros look something like: fat: 50-60%, protein: 20-30%, carbs: 10-20%. Eating lots of protein is beneficial for building muscle. So do a lot of lifting and your body will utilize protein in a good way (build muscle) rather than oxidizing it for energy.