So is low carb anything below 300 grams, then? Well, few people would argue that eating 280 grams of carbs a day constitutes a low-carbohydrate diet. Most would consider less than 150 grams to be low carb, or moderately low carb at the upper limit. Of course, there's a big difference between 15 grams of carbohydrates vs. 149 grams per day. Also, are we talking about total carbs or digestible ("net") carbs, calculated by subtracting the fiber from the total carb content? Some people count total carbs (starch, sugar, and fiber), some count only digestible carbs, and some follow the recommendation of the American Diabetes Association to subtract half the carbs from the total if the item in question has more than 5 grams of fiber. I prefer counting only digestible carbs, and I eat about 60 grams per day, which equates to 80-90 grams total carbs. More important for me is the amount of carbohydrates per sitting, because anything over 25 grams usually results in higher blood sugar readings than I'm comfortable with.
Is there a carbohydrate level that is too low to be considered healthy? That depends on which studies you feel are reliable and, I would argue, your feelings about carbohydrate restriction. There also isn't a lot of research out there on low carbohydrate diets that provide enough calories; most were conducted on subjects consuming very-low-calorie diets, anywhere from 500-1000 calories per day. (There was also at least one that provided excessive calories, which opens up a whole other issue). Another problem is the short duration of some of the studies, since it typically takes time for the body to acclimate to lower carbohydrate intake. As long as there is sufficient protein and fat in the diet, gluconeogenesis will provide additional glucose to be used to fuel certain portions of the brain, the retinas, the red blood cells, and the kidneys, and ketone bodies produced from the metabolism of fat will supply energy for the cells and organs that don't require glucose (all of the others, including parts of the brain).
There is research suggesting that a minimum of 50 grams of carbohydrates per day is required to maintain proper conversion of the thyroid hormone T4 to the active form, T3, and to prevent excessive production of reverse T3. However, there is a fair amount of anecdotal evidence that some people can go well below 50 grams without developing this problem. Those who follow Atkins and other low-carb plans typically start on an induction phase of less than 20 grams daily for several weeks or months, gradually adding back carbs as they approach maintenance, and many appear to maintain normal thyroid function while eating very little carbohydrate.
Daily carb intake of 20 grams or less on a consistent long-term basis is too low, in my opinion. For one thing, that amount would allow only about 2-3 cups of vegetables for the entire day, and I believe we need at least twice that much plant food in order to get adequate amounts of fiber and highly beneficial phytonutrients. Consuming 30-35 grams is still considered a very-low-carbohydrate diet, and for some individuals this may be too low for optimal thyroid function. However, I personally know a few people who comfortably subsist on this amount or less. Dr. Richard Bernstein is an example of a person with Type 1 diabetes who has strictly maintained a 30-gram-carb-per-day intake for over 40 years with no apparent ill effects nor any diabetes-related complications like retinopathy, neuropathy, nephropathy, or cardiovascular disease. He also appears fit and still maintains a busy practice at the age of 76. Obviously, this very-low-carbohydrate diet is definitely working well for him, as I know it has for many others!
So back to the original question: How low is too low? As with almost everything else, how one responds to carbohydrates is highly individualized. Some people will do well on 30 or fewer grams of carbs indefinitely, while others may feel best eating 100 grams or more, and most will fall somewhere in between. Optimal carbohydrate intake is fluid and may vary from day to day, month to month, and year to year. We need to take into consideration how we feel energy-wise as well as how various carbohydrate levels affect our weight, blood sugars, blood lipids, and thyroid hormones.
I don't think it's right to assign a minimum carbohydrate requirement of 130 grams for brain function -- some of the most brilliant doctors and scientists out there consume far less than this -- and I believe 300 grams is too high for the vast majority of adults, although I do have friends who regularly consume 300-400 grams daily and manage to stay in amazing shape with great energy and normal labs. I respect the right of every person to make the choice about what is right for himself or herself, and I would never try to "push" low-carbing on anyone who wasn't interested. However, I feel strongly that carbohydrate restriction should be presented as a healthy option, given its many benefits and the success that so many have had with it. While I'm fortunately not the only dietitian who feels this way, I'm definitely in the minority. I remain hopeful that the powers that be will accept carbohydrate restriction as a legitimate way of eating someday very soon!
1. Panel on the Dietary Reference Intakes for Macronutrients, Institute of Medicine. Dietary reference intakes for energy, carbohydrates, fiber, fat, fatty acids, cholesterol, protein, and amino acids. Washington, DC: National Academies Press, 2002.
2. Burman KD, et al: Glucose modulation of alterations in serum iodothyronine concentrations induced by fasting. Metabolism ,1979 Apr;28 (4): 291–299
3.Volek JS, et al. Body composition and hormonal responses to a carbohydrate-restricted diet. Metabolism, 2002 Jul; 51 (7): 864-870.