No offense intended, but you have some misunderstandings regarding the behavior of the bioenergetic pathways (read: how your body makes energy/ATP)
Your body doesn't "switch" from using carbs to using fats. Rather, it is using fat (oxidative pathway), carbs (glycolytic pathway) and creatine phosphate (phosphagen pathway) simultaneously at all times. When we are at rest, most of our energy is produced by the oxidative pathway (read: fat).
If this was not the case, we would not need to breathe and could live on the moon pretty easily without any space suits.
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Realistically speaking...the only nutrient one NEEDS in their diet is protein. ...
Chris, a few important corrections on your statements above (I bolded the incorrect ones for clarity).
It sounds like you are equating carbs just with the glycolytic pathway and equating the oxidative pathway just with fat. Both of those are incorrect. Sugars and carbs in general do give you some quick ATP anaerobically in the first step of glycolysis but the large majority of their energy is extracted later from the extra pyruvate produced in the process, starting with its oxidative decarboxylation, passing through the usual citric acid and ending with a whole lot of oxidative phosphorylation.
So while it's certainly true that fat metabolism also uses citric acid and oxidative phosphorylation after going through lipolysis and beta oxidation, it is certainly not the only or even the major substrate for it, both carbs and fats get to the same acetyl-coA molecule as an intermediate for aerobic energy pathways.
Similarly, fat metabolism is not as you seem to claim the reason why we need to breath, and in fact you can grow cells on just a substrate of pyruvate/sugar and a few aminoacids alone and they will still live indefinitely only provided they are allowed access to oxygen.
So again I think the important thing not to confuse is the actual end pathway (e.g., oxidative pathway) with the initial available substrates (sugar/carbs or fat). Carbs just happen to have a few more 'emergency' options available in a pinch like anaerobic lactate energy pathways.
And in terms of switching between fuels, your body does indeed use various combinations of these pathways and generally uses multiple energy sources to different degrees but it does have preferences and ranking orders and will not bother with much beta oxidation or fat metabolism while it still has large amounts of the same substrate for oxidative metabolism from carbs/glycogen. People have been trying many strategies to try to change that but had very limited success.
Also I should point out that technically you don't 'need' proteins more than you need other nutrients. Speaking about survival alone you can go for very extended periods of time without any protein just as you can do for carbs or fat (it's not good for you of course but you can live through it fine). As long as you have air and water, you just need things like vitamins,minerals and the 8 essential aminoacids (plus other molecules that are very good for you but not quite life-or-death good, like essential fats) to survive well and an energy source, either sugar/carbs or even just fat stores from your body. Everything else can be made from anything else, you can make all other aminoacids and therefore any protein from just sugar and nitrogen recovered from other pathways. That's one of the reasons that allows you to survive for so long on just water (and vitamins) if you are overweight, something that was quite an evolutionary advantage in times of famines (or for animals hybernating several months every year).
