I highly recommend the book "The Art and Science of Low Carbohydrate Living" by Phinney and Volek.
Even they say that low carb is not for everybody. They estimate that 3/4 of the population does well on low carb and 1/4 on higher carb levels. There are lots of factors that make the difference but two major ones are your degree of insulin sensitivity/resistance and your degree of leptin sensitivity/resistance.
This is why young studs like Choco are perfectly fine with lots of watermelon and sweet potatoes. His metabolism functions like Grok. Those of us who have done some damage to our metabolisms, or are overweight , or are older may need to go lower carb.
I can see how one could have gotten the impression a while back from the forum pundits that Primal was all low carb all the time but that has really changed for the better.
The thing is, even with fruit and tubers added, a Primal diet is still lowER than the SAD fare.
I do not claim any expertise in MS specifically. That said, as Terry Wahls pointed out, it's all about the mitochondria. Oxidative stress is hard on them. As shown in that link Nechammer posted, there is a more efficient burn going on in terms of how much you put in/how much energy you get out plus there is a lot less oxidative stress going on with fat urning as compared to sugar burning.