"Depending on whom you talk with, a low carb diet is many different things to many people. I think most misinterpret a Paleo or Primal lifestyle as somehow low carb. It can be, but most folks eat a diversity and quantity of whole plants that exceed that of the average American – often by a long shot. It can sometimes be a little low carb-like due the absence of high caloric foods made from grains. But I often find people who skip grains, sugar and the like as really paying attention to whole plants in their diet – which is, of course, a good thing. But a bona fide low, low carb eater is another animal all together. Whether you draw that line at 25, 50, or 75g a day of carbs, its low I’m afraid from the perspective of your gut bugs. Especially if those carbs contain a limited amount of resistant starch and other dietary fibers – food for gut bacteria.
That said, even though someone who eats as much as 200-500g of carbs a day can still be starving their guts bugs if those foods contain little to now indigestible substrates (fiber), a generic rule of thumb (albeit an ugly measure) is less overall carbohydrates – especially when you start dropping below 75-100g a day – translates into a dramatic drop in the amount of food reaching your colon where the vast majority of your intestinal microbial community resides."
Read more: Human Food Project