There are several fundamental problems with trying to evaluate long term dietary outcomes. Peoples diets change over the years, the definitions of low carb are especially flawed (where as most low carb diets rely on the presence of dietary ketones for weight loss, they do not necessarily advocate that for long term maintenance, rather suggesting the various sources of to tolerance (defined as not gaining unwanted weight). Don't know if anyone has seen the actual journal publication from Cell Metabolism thats been hitting the press lately, I have not. It in some form is claiming that a "high animal protein" diet is as bad for the 50+ cohort as smoking, at least as far as cancer is concerned. I be willing to bet that there is some distortion in the press reports, but hold my opinion unitl I can see the actual article.
So anyway, in the case of low carb diets,there are a lot of sub-programs out thee, low car w/ "adequate protein; low carb high protien; low carb high fat; and so one. There is not just one that we can point to and evaluate, so the research simple cannot be done, because of the somewhat vague and variable conditions. Its also not possible to have a well controlled long term study on human subjects, because its simple not possible to lock people up and control what they actually eat. People have been known to lie or make mistakes in recording what and how much they eat. Animal studies don't necessarily translate well to human metabolism and disease states.
This condition is the same for nearly all macronutrient balances. So asking for long term studies on the efficacy of any diet is not going to yield much satisfaction, its too much a moving target and way too variable for the way most people eat.
After all tis wind, I guess I am just saying that each of us will do what we think is best, and this forum should be about supporting peoples choices and encouraging them along the way, not slamming their choice. Asking for "proof" is fine and dandy, but it really doesn't exist and I can say that any article found, even in peer reviewed literature, will have limitations and detractors, that being the probing and questioning nature of science. Then there is the question of whats an appropriate source? Peer review? Popular published books/articles? Web Pages/ Blogs?
I can say that my own personal experience was my best blood fat profile, cholesterol, weight, body composition, blood pressure, and overall health indicators occurred when I was eating less than 100 grams a day of carbohydrate, 3-4 4 ounce servings of protein based food, and fatty foods to make up calorie balance to stop weight loss.
But alas, I lose the desire to eat that way as the pasta and bread monster comes to visit, and now I am sitting at 260, up from 170, and cannot get started in losing.. but thats another story all together.
So we all will have our opinions, and will eat the way we feel is best, for our own results and needs. Be kind and gentle, its difficult