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While we believe there are valid reasons for limiting protein intake, experimental evidence does not support this one. In our opinion, it makes sense physiologically for GNG to be a demand-driven rather than supply-driven process, because of the need to keep blood glucose within tight bounds.
Gluconeogenesis (GNG) is a carefully regulated process for increasing blood sugar. It is stimulated by different hormones, including glucagon — the primary hormone responsible for preventing low blood sugar. GNG produces glucose slowly and evenly . It was once thought that the main determination of the rate of GNG was how much glucogenic substrate, that is, raw materials for it, was available, but further experiments have shown that this is not the case . Instead, it now appears that GNG is relatively constant over a large variety of conditions .
As an example of this stability, a study by Bisschop et al. in 2000 showed that subjects following a keto diet for 11 days had only a small (14%) increase in glucose production from GNG after overnight fasting, as shown in this graph. This works out to a difference of less than a gram of glucose per hour.
Note that 11 days might be too little time for all of the subjects to , and it is possible that the rate of GNG would change in subsequent weeks.
In another experiment (this time in subjects on a glycolytic, or carb-based, rather than a ketogenic diet), ingesting 50g of protein resulted in the same amount of glucose production as drinking water . In other words, the amount of glucose that was made after ingesting that protein wasn't any more than would have been produced without it. While it's possible that this protein doesn't count as "excess", it was likely to be nearly half of their daily required protein intake, and eaten in one sitting, and so it is enough to cast serious doubt on the idea.
There are other experiments in which increasing the available material for GNG to high levels didn't increase GNG , . In these experiments GNG substrates were infused directly into the blood rather than eaten.
The problem with applying the results of these experiments to the question of excess protein consumption is that infusion might bypass some mechanism that increases GNG when the protein is actually eaten. For instance, it is known that protein consumption stimulates a great deal of glucagon (along with insulin) , and it might be suggested that this glucagon would thereby increase GNG. A counterargument to that possibility is that although glucagon stimulates GNG in many conditions, its action appears to always be overridden by insulin . This means that the insulin that is produced when eating protein will counteract the glucagon and GNG will not be affected (except in the case of insulin-dependent diabetes, where insulin is neither created nor responded to in the normal fashion).
Both the argument from infused substrates and the counter-arguments outlined here are plausible mechanism arguments — taking physiological processes known to occur in one context and arguing that they will occur in another context. Plausible mechanism arguments should be used with caution.
(We owe a debt of gratitude to a special friend from Windy City for helping us access full texts, as our previous access has expired. Thank you!)
Evidence type: observation
Please note
We have cited some people here as making what we believe to be an unsupported assertion. This does not imply any disrespect for the authors! To the contrary, we believe that writers such as these contribute to scientific knowledge even when they make mistakes. By writing specific and falsifiable statements and by posting them publicly where others can cite them, they give others a chance to learn from both their accurate statements and their mistakes.
We, too are fallible, and we expect that errors of our own will come to light sooner or later; such is the nature of science. There is no shame in this, and we intend none. Please see for our philosophy about this.
Scientific progress is made in large part through discovering errors and correcting them. We sincerely hope that those we have quoted will be glad to either learn from this post, or conversely, to point out to us where we have erred. In either case, an issue that was obscure will have been clarified for everyone.
Also, keep in mind that a significant percentage of protein consumed that is in excess of what you actually need for your daily maintenance and repair will convert to sugar and get used exactly the same way.
As I’ve said before, I’m trying to minimize my use of glucose, whether exogenous or endogenously produced. If I’m eating so much protein that the excess is being converted to glucose, I’m not really minimizing it, am I?
34:37 JM: Well, and I would think that if you're sensitive to carbohydrate then you would be sensitive to eating too much protein as well, because you want to stave off the effects of gluconeogenesis from happening, which would provide too much glucose in your body, tantamount to eating a lot of carbs. EW: That's a good point, that some of the protein that we eat can be turned into the glucose through gluconeogenesis, and that may be a reason why someone is not able to get to ketosis -- that too much protein is being converted to glucose.
(Update 2012-08-21)
When you eat more protein than your body needs to replace and repair body parts, excess protein is largely converted into glucose and burned as fuel.
Evidence type: experimental
(Emphasis ours)
In the process of protein metabolism, the complex protein molecule is split in the intestinal tract to amino-acids. These are absorbed into the blood stream and transported to the liver where oxidative deamination occurs. Here the glycogenic amino-acids are split to form urea and glucose. That this process is a slow one is shown in the charts by the slowly rising blood urea nitrogen. Glucose is, therefore, liberated into the blood stream in this process at a slow and even rate over a prolonged period of time. Under these conditions the diabetic is able to utilize a greater total amount of glucose without glycosuria in the eight hour period. Therefore, the inability of a diabetic to dispose of large quantities of glucose is partially compensated if the glucose is presented for utilization slowly and evenly. There appears, then, to be some advantage to the diabetic of this slow liberation of glucose from protein foods.
Evidence type: review of experiments
(Emphasis ours)
Gluconeogenesis plays an integral role in the maintenance of glucose homeostasis in humans, contributing about one-third of glucose produced in the postabsorptive state and all glucose produced when hepatic glycogen is depleted by starvation (6, 23-25). Because the results of in vivo experiments in humans and animals (12-15, 20) and in vitro perfused rat liver studies (11, 27) have demonstrated a close correlation between the rate of glucose production and the flux of gluconeogenic substrates, it is believed that gluconeogenic precursor supply plays a major role in the regulation of glucose production (12,13,20). Several studies in vivo support this concept. For example, we and others have demonstrated that the hyperglycemic response to severe burn injury and sepsis is a direct result of an increased rate of glucose production, which is associated with a concomitant increase in the fluxes of alanine and lactate, major gluconeogenic substrates (15, 39). The proposed regulatory role ofprecursor supply received further support in the quest to rationally explain the paradox of a reduced glucose production rate (and hypoglycemia) in starvation, despite a hormonal-substrate milieu that would normally favor stimulation of gluconeogenesis (2, 7, 12, 13, 28), thus glucose production. After prolonged starvation (3-4 wk), human subjects had low levels of gluconeogenic precursors associated with hypoglycemia and a reduced glucose production rate (6, 7, 12, 25). Infusion of unlabeled alanine caused hyperglycemia and an increased incorporation of [ 14C]label from alanine into glucose in this circumstance (12,13). It was therefore proposed by Cahill, Felig, and Marliss and their associates (7, 12, 13, 20) that the reduced glucose production rate in starvation was due to the reduced availability of gluconeogenic substrates; hence, gluconeogenic precursor supply was rate-limiting for glucose production rate. In contrast, the findings of several kinetic studies performed in humanand dog do not support this proposal (1, 30, 34, 38). These studies in postabsorptive subjects employed either the isotope dilution or hepatic vein catheterization techniques and failed to show any significant change in glucose production rate in response to infusions of substantial quantities of alanine, lactate, and glycerol even when there was a fivefold increase in the hepatic uptake of the infused substrate (1, 30, 34, 38) These conflicting findings suggest that the relationship between gluconeogenic substrate supply and gluconeogenic enzyme activity in prolonged starvation may be different from that of the postabsorptive state. Alternatively, it is possible that the response to an increase in precursor supply is different from the response to a decrease. This latter possibility could occur if the endogenous supply of gluconeogenic precursors is just sufficient to maximally satisfy the capacity of the gluconeogenis enzyme system or of a particular key-limiting enzyme. [...] Ourdata so far indicate that under almost any physiological situation, an increase in gluconeogenic precursor supply alone will not drive glucose production to a higher level, suggesting that factors directly regulating the activity of the rate-limiting enzyme(s) of glucose production normally are the sole determinants of the rate of production; hence, there will be no increase in glucose production if the increase in gluconeogenic precursor supply occurred in the absence of stimulation of the gluconeogenic system. On the other hand, results of the DCA experiments suggest a coupling between precursor supply and gluconeogenic enzyme capacity. In this light, if there is a stimulation in gluconeogenic enzyme capacity (for example because of hyperglucagonemia of severe trauma), then there will have to be an increased rate of uptake of gluconeogenic precursors to meet the requirements of such a stimulated system. Thus the rate of uptake of gluconeogenic substrates and the rate of glucoseproduction will be closely related, but the increased uptake of gluconeogenic precursors will be a consequence of a stimulated gluconeogenic enzyme system rather than the cause of an increased rate of gluconeogenesis.
If anyone having access to this paper would like to share it with us, we would be grateful, because it is the most relevant experiment we could find on the topic, and further details may be important.
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