According the the CDA(Canadian Diabetes Association) in 2013, "the current recommended minimum intake for CHO is not less than 130 g/day, to provide glucose to the brain" (32).
Source Chapter 11
The number 32 refers to : Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for
Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino
Acids. Washington, DC: National Academies Press;
But the original recommendation of that institute is way more subtle.
Summary
If your ancestors are Inuits or Masai, a LC diet is ok and you need less than 30g per day for your brain of glucose.
If not, please wait until we have enough data on the long-term consequences. In the meanwhile, we recommend
provisionally 130g per day of glucose at least for the brain need. But we know that a carbs-free diet is sufficient without the need of exogenous carbs if you eat sufficient fat and protein.
The long version
My comments are in green.
Curious about that, I have read the recommendations of that institute and compiled for you only the relevant parts.
No modifications, only quotes but Very interesting:
If you want to read the full text, quite long, many pages in fact
"Clinical effects of inadequate intake: The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed. However, the amount of dietary carbohydrate that provides for optimal health in humans is unknown".
"There are traditional populations that ingested a high fat, high protein diet containing only a minimal amount of carbohydrate for extended periods of time (Masai), and in some cases for a lifetime after infancy (Alaska and Greenland Natives, Inuits, and Pampas indigenous people).(Du Bois, 1928; Heinbecker,1928)".
"There was no apparent effect on health or longevity. Caucasians eating an essentially carbohydrate-free diet, ressembling that of Greenland natives, for a year tolerated the diet quite well( Du Bois, 1928) However, a detailed modern comparison with populations ingesting the majority of food energy as carbohydrate has never been done".
"It has been shown that rats and chickens grow and mature successfully on a carbohydrate-free diet (Brito et al.,1992; Renner and Elcombe, 1964), but only if adequate protein and glycerol from triacylglycerols are provided in the diet as substrates from gluconeogenesis".
"The ability of humans to starve for weeks after endogenous glycogen supplies are essentially exhausted is also indicative of the ability of humans to survive without exogenous supply of glucose or monosaccharides convertible to glucose in the liver (fructose and galactose). However, adaptation to a fat and protein fuel requires considerable metabolic adjustments".
"The only cells that have an absolute requirement for glucose as an oxidizable fuel are those in the central nervous system( i.e., brain) and those cells that depend upon anaerobic glycolysis (...), such as red blood cells, white blood cells and medulla of the kidney".
"The central nervous system can adapt to a dietary fat-derived fuel, at least in part (Cahill,1970; Sokoloff,1973). (...)
Therefore the marginal amount of carbohydrate required in the diet in an energy-balanced diet is conditional and dependent upon the remaining composition of the die"t.
"Nevertheless, there maybe be subtle and unrecognized, untowards effects of a very low carboydrate diet that may only apparent when populations not genetically or traditionally adapted to this diet adopt it". No source were provided so it is just an opinion that some population could be not genetically adapted.
"This remains to be determined but is a reasonable expectation".
"Of particular concern in a Western, urbanized society is the long-term consequences of a diet sufficiently low in carbohydrate such that it creates a chronically increased production of β-hydroxybutyrate and acetoacetic acides (i.e., keto acids)".
"The concern is that such a diet, deficient in water-soluble vitamins and some minerals, may result in bone mineral loss, may cause hypercholesterolemia, may increase the risk of urolithiasis (Vining, 1999), and may affect the development and function of the central nervous system. (..)It also may not provided for adequate stores of glycogen. The latter is required for hypoglycemic emergencies and for maximal short-term power production by muscles (Hultman et al.,1999). Nobody promotes to eat only animal-based food lacking in vitamins in a VLC diet like the Inuits.
The endogenous glucose production rate, and thus the utilization rate depends on the duration of starvation.(....)
The minimal amount of carbohydrate required, either for endogenous or exogenous source, is determined by the brain's requirement for glucose.
The brain is the only true carbohydrate-dependent organ in that it oxidizes glucose completely to carbon dioxide and water. Normally, the brain uses glucose almost exclusively for its energy needs.(...)
The requirement for glucose has been reported to be approximately 110 to 140 g/d in adults (Cahill et al.,1968).
Nevertheless, even the brain can adapt to a carbohydrate-free, energy-sufficient diet, or to starvation, by utilizing ketoacids for part of is fuel requirements.
When glucose production or availability decreases below that required for the complete energy requirements for the brain,
there is a rise in ketoacid production in the liver in order to provide the brain with an alternative fuel. This has been referred as "ketosis". (...).
It is associated with approximately a 20 to 50 percent decrease in circulating glucose and insulin concentration (Carlson et al.,1994;Oweb et al.,1998; Streja et al.,1977)
(...) In individuals fully adapted to starvation ketoacid oxidation can account for approximately 80 percent of the brain's energy requirements (Cahil et al.,1973).
Thus, only 22 to 28 g/d of glucose are required to fuel the brain.
(...)Presumably, this also would be the obligatory glucose requirement in people adapted to a carbohydrate-free diet(...)
..The required amount of glucose could be derived easily from ingested protein alone if the individual was ingestion a carbohydrate-free, but energy-adequate diet containing protein sufficient for nitrogen balance
page 280
Infants ages 0 through 12 months: "The infant brain is fully capable of using ketoacids as fuel"
page 285 "Glucose utilization by the Brain. Long-term data in Westernized populations, which could determine the MINIMAL amount of carbohydrate compatible wit the metabolic requirements and for optimization of health, are NOT AVAILABLE .
"Therefore, it is PROVISIONALLY suggested that an EAR for carbohydrate ingestion in the context of overall food energy sufficiency be based on an amount of digestible carbohydrate that would provided the brain (i.e., central nervous system) with an adequate supply of glucose should be sufficient WITHOUT the requirement for additional glucose production from ingested protein or triacylglycerols". But the temporary EAR became permanent since 2005.
This amount of glucose should be sufficient to supply the brain with fuel in the absence of a rise in circulating acetoacetate and β-Hydroxybutyrate concnetrations greater thant that observed in an individual after an overnight fast(...)
In summary, the EAR for total carbohydrate is set at 100g/d. This should be sufficient to fuel central nervous system cells without having to rely on a partial replacement of glucose by ketoacids.
((....)
Nevertheless, it should be recognized that the brain can still receive enough glucose (...)when a very low carbohydrate is consumed
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