When you read the research about low-carb diets, "low-carb" is defined in many different ways. One difference is whether they are looking at the amount of carbohydrate in the diet or the percentage of calories from carbohydrate. The other difference is "how low?" The usual carbohydrate recommendations from the U.S. Dietary Guidelines and similar sources for the general public are that 50-65% of the calories in a person's diet should be from carbohydrate, so anything less than that is sometimes considered "low-carb". Studies of low-carb diets have ranged from 45% down to 5% , and authors of diet books are similar. That said, there are three general approaches:
1. Just reduce carbohydrate - Basically, the thinking is that any reduction in carbohydrate is helpful, because increasing numbers of us have exceeded the amount of carbohydrate our bodies can easily handle. These approaches are mainly focused on reducing or eliminating added sugars and other refined carbohydrates. The "No White Foods" approach fits in this category, which means simply eliminating sugar, white flour, white rice, and potatoes from the diet.
2. Find the best amount of carbohydrate for each person - There is no doubt that each of us has a different level of carbohydrate tolerance. We can think of this as a continuum from people who can eat sugar all day long with no ill effects all the way to diabetics with very poor glucose control. As we age, our carbohydrate tolerance often erodes and we have a greater blood glucose response to carbohydrate, even if our blood glucose stays in the "normal" range. We may develop insulin resistance or creep closer to prediabetes - these are signs that there is probably damage in the pancreas to the beta-cells which make insulin.
Approaches that seek to help people find the optimal carbohydrate level usually advise reducing carbohydrate to a fairly low level and and then gradually adding carbohydrate until there are adverse effects such as a) weight loss ceases, b) weight gain occurs, c) carb cravings return, d) blood glucose control diminishes, e) blood triglycerides rise. Then the individual would reduce carbohydrate intake a bit from that point. Examples of this type of approach are the Atkins and South Beach diets.
3. Seek a ketogenic diet - A ketogenic diet is one which causes the body to primarily use fat for energy rather than glucose. (See What is Ketosis?) This puts the body into a state sometimes called "keto-adaptation" or "fat adaptation". This state has some metabolic advantages such as improved stamina, e.g. since the body is used to using fat for energy, there is less tendency to "run out of fuel", either during exercise or between meals. One type of ketogenic diet is used to treat epilepsy, and ketogenic diets are under investigation to treat other disorders. Although there is some individual variation, a diet moderate in protein and below 60 grams of carbohydrate per day is ketogenic in most people. Some can eat as much as 100 grams per day and remain in ketosis.