First, it is important to understand that ascorbate is asborbed in the small intestine modulated by glucose (1):
Na+ dependent glucose transport via SGLT1 receptors in enterocytes modulates the asborption of ascorbate, while transport of dehydroascorbic acid (DHA) is by facilitated difussion. As glucose content in the lumen and in the enterocyte increases, ascorbate transport is reduced. In layman's terms: ascorbate absorption is reduced by high glucose, so if you eat more glucose you will need to eat more ascorbate. This means, glucose increases your minimal requirement for vitamin C in your diet.
But once ascorbate and DHA enter the bloodstream, they must be transported inside cells.
"DHAA can be recycled back into vitamin C, but only inside cells. In order to enter cells, DHAA needs to be transported by glucose transporters. GLUT1, GLUT3, and GLUT4 are the three human DHAA transporters; GLUT1 does most of the work."Structurally, DHAA and ascorbic acid are similar to glucose:
|Ascorbic acid, reduced form (left) and DHA, oxidized form (right)|
"Glucose transporters are activated by insulin. Thus, DHAA import into cells is increased by insulin, leading to more effective recycling of vitamin C "The reference takes us to a study done on osteoblastic cells (3). When these cells were incubated with insulin, the uptake of DHA and the intracellular concentration of ascorbate from DHA increased. But if you read look into the results, you will find:
What? But isnt insulin, which is stimulated by glucose, supposed to increase DHA transport? Yes, but here is where interpretation problems arise. When we study cells in vitro we isolate the variable we want to study. This helps us understand the exact mechanism of action of certain molecule or pathway (because there is no other molecule/pathway influencing it) but this also creates a problem: our cells are not isolated. In fact, they are under many different convergent signals which might have sinergistic or antagonistic effects on one cellular process. If we focus only on the proximal cause (insulin increases DHA uptake) we miss the big picture for understanding metabolic processes (why insulin increases DHA uptake?). From my point of view, this is an evolutionary acquired mechanism. Vitamin C transport/recycling modulated by insulin/glucose can be illustrated using a different scenario: muscle protein balance. Insulin's role in different tissues is mainly inhibitory. In skeletal muscle, insulin inhibits muscle protein breakdown. But has a passive role on muscle protein synthesis, which is stimulated by plasma amino acid availability. On the other hand, the most powerful stimulus for insulin secretion, hyperglycemia, increases proteolysis (4). Hence, insulin counteracts the effect of hyperglycemia (until a threshold is reached) on muscle protein balance. This effect is also illustrated by glucose induced inflammation. Hyperglycemia is inflammatory (5, 6, 7) and contrary to popular beliefs, insulin is anti-inflammatory (8, 9). My guess is that glucose is the main stimulus for insulin secretion because it is inflammatory. Just like in skeletal muscle, insulin is released to prevent damage from glucose."Transport of [14C]DHAA was inhibited by D-glucose and 2-deoxyglucose, both of which are substrates of GLUT"
Getting back to the topic, these examples help to understand why insulin might increase DHA transport and recycling. When there is a higher level of glucose competing with DHA, insulin's role is to increase the number of GLUT receptors for achieveing a normal intracelular DHA level for proper conversion to ascrobic acid (AA).
There are other vitamin C transport systems used by cells, namely SVCT1 and SVCT2, two sodium dependent transporters for AA, which are regulated among others, by AA plasma concentrations. This transport is not regulated by insulin. So we have two ways by which vitamin C is transported into cells (10):
|AA is transported inside cells by Na+ dependent transporters coupled to a Na+/K+ATPase, while DHA is transported through GLUT proteins. DHA is rapidly reduced to AA, which can excit the cell by different uncharacterized mechanisms (beyond the scope of this post).|
"Confirming the role of insulin in promoting vitamin C recycling, Type I diabetics (who lack insulin) have lower blood levels of vitamin C, higher blood levels of DHAA, increased urinary loss of vitamin C metabolites, and greater need for dietary vitamin C. [9, 10]"The first mistake is comparing no insulin (such as in type I diabetics) with low insulin. But why do type I diabetics have lower levels of AA and higher levels of DHA? Is it because a defective recycling via lower DHA cell uptake? Maybe. But I dont think it is the only cause. Hyperglycemia, as mentioned earlier, produces oxidative stress, increasing the formation of reactive oxygen species which react with AA, oxidizing it to form ultimately DHA. Because there is a defective transport of DHA into cells (because of competitive inhibition by glucose and abscence of insulin), the balance is shifted towards DHA. The main source of mitochondrial ROS seems to be complex III during the oxidation of complex I substrates (NADH dehydrogenase) (11). Without going into details (maybe on a different post), complete (aerobic) glucose oxidation produces a ratio of NADH to FADH of 5:1. This means that mitochondrial energy metabolism relies more on complex I than complex II, increasing the production of ROS. This scenario is also observed in type II diabetics (12, 13), who in fact have the opposite (chronic hyperinsulinemia), but glucoregulatory alterations. Despite having high insulin levels, they have low plasma vitamin C. Increased ROS production both by systemic inflammation, hyperglycemia, carbohydrate-based mitochondrial energy production and defective cellular transport are the likely causes of vitamin C alterations observed in these patients. In uncontrolled TIDM, abscence of insulin makes things worse. This is by no means applicable to ketogenic diets.
Then, Jaminet states:"Dehydroascorbate, the fully oxidized form of vitamin C, is reduced spontaneously by glutathione, as well as enzymatically in reactions using glutathione or NADPH. "
"Glutathione is recycled by the enzyme glutathione peroxidase, a selenium-containing enzyme whose abundance is sensitive to selenium status. One difficulty with zero-carb diets is that they seem to deplete selenium levels."For this assupmtion, he references a study on a sudden cardiac death on a ketogenic diet for seizure control (14). From the cited study:
"Selenium is an essential nutrient in the human diet. Sources of selenium include cereals, meats, and fish . However, depending on the intake of those foods and the source of cereals (e.g., soil rich or poor in selenium), selenium levels in humans are variable. Patients on the ketogenic diet have little cereal intake, and only moderate fish and meat intake, and thus are predisposed to low selenium levels."Even I said I wasn't going to refer to seizure-control ketogenic diets, its worth mentioning that according to the last recommendations of the International Ketogenic Diet Study Group (15), the "normal" diet used in this cases is 90% fat and 10% of carbohydrates and protein combined. They state that "Calories are typically restricted to 80%–90% of the daily recommendations for age". Not very similar to popular paleo-keto diets. To achieve this incredibly low level of protein, they have to restrict important sources of Se, specially those eaten in abundance by low carbers like meat and fish, being the former one of the foods with best Se bioavailability (16). The recommended Se intake seems to be around 40ug/day (17) and muscle meats, on average, have 0.3-0.4mg Se/kg (organ meats such as liver and kidney concentrate more Se, from 4 to 16 fold the amount on muscle). Remember that 1000ug = 1mg, so 0.4mg = 400ug. If you eat 100g of meat a day (excluding other animal sources) you are eating 40ug of Se, the recommended intake (for reference, 100g equals to 3.5oz/0.22lb).
"So here we have a second mechanism contributing to the development of scurvy on a zero-carb diet. The diet produces a selenium deficiency, which produces a glutathione deficiency, which prevents DHAA from being recycled into vitamin C, which leads to DHAA degradation and permanent loss of vitamin C."As we have seen, using a clinical case based on a ketogenic diet for seizure control leads to a flawed conclusion.
Finally, as said by one of the PHD readers in the comment section, ketogenic diets have shown to increase GSH levels (18).
The hypothesis proposed by Jaminet is not supported by facts, only by assumptions based on misinterpretation of the existing data. Proper ketogenic diets dont produce vitamin C, glutathione or selenium deficiency. Loss of glucoregulation does.