Necessity is the mother of invention. Put differently, if they can get by with the bare minimum smarts, most people will.
All the low-carb people in the clip were fatter before they went low-carb. All the vegan people in the same clip were never fat to begin with.
I agree with Rose, and I can cite the precise mechanism that keeps us fat in spite of all our efforts. It's a consequence of the years of hyperinsulinemia. It's called insulin-induced lipohypertrophy. Fat tissue grows bigger not only because it contains more fat, but also because it's made up of more fat cells. Those extra fat cells won't go away just because we cut carbs. The simplest explanation is that for equal insulin (or equal hormonal milieu counting all hormones), two fat cells will have twice as much fat as one fat cell. There is no reason for two fat cells to contain only half as much as they should. It's their job to contain a set amount of fat based on the hormonal signals they receive. Furthermore, those extra fat cells need to be fed just as much as every other cell so the supporting tissue like blood vessels will also grow bigger for that. That won't go away either just by cutting carbs.
But the clip title is "Low Carb vs. Plant-Based". So, to keep with the subject, here's a list of 16 experimental studies that compares low-carb to other diets:
http://www.dietdoctor.com/weight-lo...ing-the-science
The clip is not science though it is compelling, isn't it. It's compelling because it does not draw any conclusion,
we do. We're allowed to make up our own mind based on a simple comparison of fat people and lean people, and an associated dietary approach with each group. But then, the list of 16 experimental studies also allows us to make up our own mind, however it does not allow us to imagine a falsehood, namely that "If these guys follow their own diet... Their pictures speak a thousand words." The falsehood is that if they do follow their own diet, then that's why they are fat. That list doesn't allow this falsehood because when they put fat people on the same diet those fat gurus presumably follow, they lost more weight than on any other diet.
In other words, the clip here is an observation, and the list of 16 experimental studies is a test of the hypothesis that the observation generated, namely "low-carb makes us fat".
I also put in my $0.02 but from my experience, the vegan crowd doesn't like science.