I will concur with Eric and TD that to some extent many people who have enormously positive short term benefits on Very Low Carb, seem to hit a point where their bodies cant keep it up over long periods of time.
That being stated, I do not believe that there is some universal biological imperative which makes Very Low Carb diets unsustainable in the long term. What I see is that most people who fail are not going far enough to ensure an optimal balance. There are a number of factors involved which if understood more fully could help people maintain the benefits of Low Carb/ High Fat more optimally.
You cant simply use cuts of muscle meat and fat trimmings with some occasional organ meats and think you can build a complete optimal long term Low Carb diet upon such an incomplete foundation. As one of the few long experienced Very Low Carbers , the only way I have able to make this work for the long term is to eat the entire animal. Much in the same way the Reindeer herders are able to live almost entirely off of meat based diets, by eating the entire animal.
I drink the Blood, eat the brains, tongue, eyes, marrow, glands, gonads, intestines, stomach...practically everything expect the bones and hide...many Low Carb people are unable or unwilling to take it to that level of commitment, and fail to maintain optimal wellness as a result. Though I have moved away from AV style high meat, I will eat aged meats regularly as well as unwashed intestine smoothies, also my greens are grown with animal compost, so I would say that over the years my gut flora have optimized to this diet in ways that more antiseptic dieters may not have been able to?
There are indeed many factors that if understood could explain why some people adapt better than others.. genetics, epi-genetics, microbiome composition, for example.
Many people harbor bad habits which may not get discussed that sabotages the success of Low Carb Paleo. Two much sweet reliance on sweet fruits, starchy foods, nuts seeds, or stubborn use of Dairy, supplements, stressful lifestyle... and so on....
I am an advocate for quality, and most grassed meat and fat on the market does not have the qualities needed to sustain a ling term extremely Low Carb Diet. Bad water, wormer/drugs/vaccines, poor breeding, overgrazing, sub quality hay will all take its toll on the health those up the food chain. Typically cows feed hay and limited forage have a much less optimal protein and fat composition. Animals slaughtered too young do not have optimal micro nutrient content( CLA, DHA, Carotene, etc). I also consume a lot of Mutton which is suppose to have a Higher DHA content than beef. These factors of good quality, and good variety are also of the utmost importance
Ive preached over the years how (for me personally) Coconut Butter has played a key role in my diet. There is a synergy coconut fat has when consumed with copious amounts of animal fats and proteins. I believe it adds balance, and primes my metabolism toward optimal fat assimilation. The coconut carbs are extremely low glycemic, and the fiber may act in the way that Resistance starch in supporting a healthy gut flora balance. Many others on this forum have not seemed as impressed as I with this combination, so I have been left wondered if it would be possible for someone else to share this affinity? hmmmm.....Perhaps this HoneyBadger will be my first disciple?