What would happen if an inactive person went from the USDA recommendation to a 40% carb/30% fat/30% protein diet?
For most people (not just "inactive" ones, but anyone who isn't a bodybuilder or
extremely "active" otherwise), it would basically just be (nutritionally) pointless. For some others, it could exacerbate other health problems, if they already exist, but it would be unlikely to cause even long-term harm to most people.
Excess protein does put some degree of unnecessary stress on our digestive systems, though. Whatever isn't used "as protein" will eventually be broken down into much smaller, otherwise usable components, eventually just resulting in fat - like most everything else we eat or drink - unless it's used for something else before it gets there. All that "extra" digestion/metabolizing puts some amount of extra stress on the kidneys, first and foremost, since they have to filter out the useless and in a few cases, harmful by-products of those metabolic processes, but on other bodily parts involved in digestion as well, to one extent or another. But healthy humans have way more kidney-real-estate than we strictly need to begin with, and as organs go, they're very resilient (though not as amazingly so as our livers).
fructose skips a few steps on the way to fat.
The jury's definitely still out on that, but before you even get to what happens to fructose in our bodies, you have to consider how much fructose you're consuming in the first place. And since very few people consume significant amounts of pure glucose, I assume you're referring mostly to the "dreaded" HFCS, and are comparing it to "regular/ordinary sugar"? Well, the latter is sucrose, and sucrose is exactly 50% glucose and 50% fructose and while there are different formulations of HFCS, they're all close enough to 50-50 for any difference in fructose content to be irrelevant. (NB: "
High fructose corn syrup" gets its name by comparison to "regular" corn syrup, which is in fact mostly glucose.)
And while we're on the subject of HFCS, I'll give my standard rant... That there's never been any "hard" evidence that fructose in and of itself is "bad for you" in any way, shape, or form. Yeah, I know about the studies "correlating" the rise of obesity to increased consumption of HFCS, but my response to those can be summed up as: "spare me"... On the macro level, one could correlate also the rise in obesity to climate change, the fall of the Soviet Union, or for that matter (considering I'm posting this on a tech forum) Intel x86 CPUs breaking the 640K RAM barrier....
/soapbox