I would ask each and every reader to try as best you can to check their nutritional biases at the door here as best you can and let's just talk semantics.
What does essential mean?
Something that is essential is something that we cannot do (or in the nutritional context live) without. By this definition, it is clear that yes, for most humans, carbohydrates are non essential. However, even this is too simplistic, because without looking very far, we can identify a group of humans for whom carbohydrates are "deadly" essential. As in imminent death kind of essential. Relatively rare as is their number, I'm talking about people with glycogen storage/breakdown disorders.
Now, let's look at the other macronutrients. Well, we cannot live without protein. Therefore it is essential. No argument. However, proteins are comprised of 21 distinct building block amino acids of which only 8 are actually identified as *essential*. A strict interpretation of this fact might be that ... eh ... 13 amino acids, or the majority are non-essential. We don't need them. They can be made by the body from other stuff.
Lastly let's look at fats. There are only two technically essential fatty acids. Yep! A measely two out of tens perhaps hundreds of fatty acids of varying chain lengths and degrees of saturation -- that are essential. That number can be increased slightly to include but three more "conditionally essential" FA's although theoretically these can be derived from other compounds. But let's stretch it to say that those other compounds are usually not present in sufficient quantities, so we have a need for five fatty acids. That's still a small portion of all fats. Notably absent from the essential list? Long chain saturated fatty acids, e.g. palmitic acid.
See where I'm going here?
If we're going to define essential as necessary to sustain life, humans can apparently survive on an emulsion of 8 amino acids and 5 fatty acids -- and I suppose we'd have to include vitamins and minerals etc.
Perhaps Stephan's onto something with that bland liquid diet from a straw study in his Food Reward series over at his blog. If we can create a liquid concoction of all these essentials we could solve malnutrition and obesity in one swoop!
Non Essential = Potentially Toxic in Excess:
The flipside of the "essential" argument coin seems to me to be that because we don't need carbs we are somehow predisposed to toxic effects of consuming them in excess. Let's look at a benefit/detriment balance then of the three macros.
Here things shake out rather worse for the "essential" macronutrients than for the beleaguered carbohydrate. How so? Well, the worst thing a carb ever did was cause some transient hyperglycemia. Granted down the line a chronic state causes issues, but sugar and starch are not potent *poisons*. Chronic hyperglycemia is due to unrestrained gluconeogenesis in the end. But even if totally from dietary sources, the toxic effects are long term and far more detrimental things are going on due to the underlying disrupted metabolism.
In this regard, I'm also willing to give a pass to omega 6 fatty acids. Their disease-agent qualities -- whatever they may be -- are still thought to be more long acting in nature, although some do seem to attach potential drug-like qualities to these.
Now the essential AA's are another story! We have phenylketonurics for whom even normal amounts of this essential amino acid poses a toxic threat. Another essential AA, leucine, is associated with pellagra.
My point? An argument based on excesses for the macros is irrelevant to the discussion on whether or not they are essential.
Essential v. Optimal:
There is no doubt that glucose is essential to sustain human life. But because we can produce glucose from non-carbohydrate substrates (amino acids), the rationale is that we don't have to consume glucose. True.
But do we *need* long-chain saturated fats then? It would seem not, because our bodies are more than capable of synthesizing saturated fatty acid from carbohydrate by de novo lipogenesis. By the logic above, we don't need to consume any saturated fats because we're more than capable of making our own.
So I'm going to go out on a Clintonesque limb here and say that it sort of depends on the meaning of the word "essential". Or more broadly the context. Essential for what, exactly? Just life? Because it is true: We die without protein. We die without fat. We live without carbs. So if I'm stuck on a desert island and a genie grants me only two macronutrients I'm picking protein and fat. But that's sort of a silly scenario to contemplate, no?
Rather than what is essential, the more relevant argument/discussion should be "what is essential for optimal life". We're often told that the O3's in flax are inferior to those in fish oil, because although we can convert them to the forms most beneficially utilized by humans, we're not particularly good at doing this. Similarly we should ask whether just because the body can make all the glucose it needs, whether that is optimal as a source vs. exogenous sources.
Another way is a bit more physiologically minded. Though carbs may not be essential, they do play a crucial role in metabolism. Despite the demonized glycation seen as universally negative, glycosylation is the normal process that activates/deactivates certain enzymes etc. Rather than the body having to scavenge and conserve and produce glucose for essential life-sustaining functions, I believe a case can be made that it is more optimal to supply the body with at least some of its glucose *needs* directly.
This boils down to the genie on the desert island offering me up two whole foods to pick from rather than the macros. I'm picking salmon and potatoes. Such a diet would serve my body better.
Immediate v. Long Term Needs
The last argument I'll present with regards to the essential nature of dietary carbs is that of the immediate need vs. the long term need. In a normal human, there is never an immediate need or short-term need (a few to several days) for any of the three macros. In the longer haul, the most essential of the three macros appears to be protein.
And yet for persons with metabolic disorders, it is only the carbs that pose the possibility of immediate danger of the ultimate lack of essential nutrients -- aka death -- if hypoglycemia develops and severely enough.
To Sum Up ...
So putting it all together, where does this leave us? I am of the opinion that the "you don't need carbohydrates" mantra of the L.Ron Hubbard of the Latter Day Church of MAD*, R. David Feinman, is the most meaningless argument in favor of carbohydrate restriction for weight loss or otherwise. Optimal human nutrition for longevity and performance inarguably includes considerable dietary carbohydrates over the centuries.
*church named by Anthony Colpo.