Safe starches, blood glucose and insulin

A reader asked me recently about a subject which is confusing many people in the paleosphere. 

"Jaminet in his debate with Rosedale suggests higher carb diets tend to lower blood sugar whereas low carb diets, RAISE it. Can you help me untangle what is going on here? It makes it sound like the more carbs you eat the better your blood sugar levels which does not seem right to me. Clearly, an important health goal is achieving low blood glucose so I would want to know what is the best way to eat to control them. 

I always assumed that the less sugar you eat, the less blood sugar you'll have. Is there a threshold? If Jaminet is correct, then shouldn't we see a higher fasting glucose associated with ketogenic diets that are totally carb restricted vs higher carb diets? I know excess protein might be converted into glucose but if you followed a ketogenic diet with low protein would you still see the rise in blood glucose? 

What is the mechanism through which blood glucose is being lowered in high carbers? Do they secrete more insulin to deal with it hence lower blood glucose? Would their insulin levels therefore be higher even if blood glucose was relatively low? Which is worse for health- low glucose/high insulin or moderate glucose/low insulin? 

What is the mechanism through which some cancers are being suppressed with ketogenic diets if not through lowered blood glucose? Is there something else going on?"

These are very important questions as they raise some concern in people which utilize a low carbohydrate diet for controlling their blood glucose (BG). Before trying to elaborate an answer, there are some facts that must be kept in mind:

• Hyperglycemia is not a disease, it is a symptom. 
• BG levels can be affected by non-dietary factors.
• The two principal energy substrates for humans (glucose and free fatty acids (FFA)) compete with each other for their utilization.
• Calories matter.
• There are differences between physiological insulin resistance (PIR) and pathological insulin resistance (PaIR). The term "insulin resistance" is very vague, it doesn't define explicitly which tissue(s) is IR.

The first issue to adress is whether ketogenic diets raise BG levels. The only evidence I have seen for this happening is in anecdotes from people in the internet. But if we want to have an objective look at the subect, we must see what happens in studies done with ketogenic diets (I will use low carbohydrate and ketogenic diets equally). 

Ketogenic diets and blood glucose levels

Most studies done on ketogenic diets are short-term and involve weight loss. All of them show a reduction in BG and insulin levels. A study by Grieb et al. (1) found that people eating an optimal diet (Kwasniewski) had on average a BG level of 87.9mg/dL, which is in the normal range; and very low values of HOMA-IR. Sharman et al. (2) have shown that the metabolic benefits of carbohydrate restriction are independent of weight loss. Given the evidence, it is only possible to speculate about the mechanisms by which BG levels rise in some people eating a ketogenic diet.

The goal of a ketogenic diet is to simulate fasting, but without the negative effects of prolonged nutrient restriction. Before going on, it is pertinent to remember the Randle Cycle (3):

In short, both FFA and glucose compete with each other for their uptake and oxidation. This can be translated as when BG is high, FFA utilization is low; and when BG is low, FFA utilization is high. Ketogenic diets are characterized by low BG levels, in part because of a drastic reduction in exongeous glucose. In turn, plasma FFA rise from dietary and endogenous sources (the contribution of each one depends on energy balance). During this scenario, plasma ketone bodies also rise. So we have the following metabolic milieu:

Cellular effects of a fasting-type metabolism

For exerting it metabolic effects, insulin needs to first bind its receptor (the insulin receptor, IR). Upon binding, insulin triggers an intracellular signaling cascade, which influences both the function of intracellular proteins and gene expression. The signaling pathway triggered by the binding of insulin include the recruitment of the IRS (insulin receptor substrate) to the cytosolic part of the insulin receptor dimer. The signaling cascade stimulated by insulin is essential for its function. If proteins involved in this signaling pathway are inhibited, there will be no cellular response to the binding of insulin and the activation of IR.

One level of inhibition of glucose utilization by FFA involves the inhibition of GLUT4 translocation to the plasma membrane. The translocation of GLUT4 is stimulated by insulin, by activation of IRS, PI3K and other proteins (4, 5). In vitro studies have shown that palmitate, the main fatty acid stored in mammalian adipose tissue, inhibits GLUT4 translocation and activity (6, 7). This results in reduced glucose uptake in skeletal muscle.

Inactivation of PDH (pyruvate dehydrogenase) is one of the most important mechanisms for inhibition of glucose oxidation  by FFA. PDH activity is controlled by phosphorylation, by PDK (pyruvate dehydrogenase kinase) and PDP (pyruvate dehydrogenase phosphatase). Phosphorylation by PDK inactivates PDH, while dephosphorylation by PDP activates it. Fatty acid oxidation increases the mitochondrial ratios of [acetyl-CoA]/[CoA] and [NADH]/[NAD+], which inhibit PDH. Low carbohydrate diets have shown to reduce muscle PDH and increase PDK (8, 9), effects which are reversed by a carbohydrate refeeding (10). Fatty acids also increase the concentration of cytosolic citrate, which inhibits 6-phosphofructo-1-kinase, providing another mechanism of inhibition of glucose oxidation (3). Fatty acids can reduce phosphorylation of IRS-1 (11), GSK-3b and PKB/Akt (12), thereby acting also downstream of IR and IRS.

The metabolic response to fasting can show what are the effects on insulin signaling of very high levels of plasma FFA and ketone bodies, but within a physiological range. In a very interesting study, Soeters et al (13) found that insulin-mediated peripheral glucose uptake after 62h of fasting was significantly lower compared to 14h of fasting. They also found that after 62h of fasting, Akt phosphorylation at Ser473 and AS160 phosphorylation at Thr642 were reduced. This implies that insulin signaling was attenuated (reduced phosphorylation of Akt) as well as glucose uptake (phosphorylation of AS160 is involved in the translocation of GLUT4). The authors concluded:

"(...) it is possible that pAKT-ser⁴⁷³ is involved in the physiological adaptation to fasting, inducing a reduction in peripheral glucose uptake and protecting the body from hypoglycemia."

Intramyocellular triglyceride accumulation is thought to mediate fatty acid insulin resistance. This is one way by which some authors think that a high-fat diet leads to insulin resistance. Compared to fasting (67h) a very low carbohydrate diet (eucaloric) produces the same amount of IMTG accumulation, both produce glucose intolerance and reductions in insulin sensitivity (14). Thus, the factor for triggering this metabolic response seems to be the absence (or drastic reduction) in glucose availability (my bolds):

"Thus, we suggest that dietary-induced IMTG accumulation and insulin resistance in healthy humans may be largely influenced by circulating FFAs, whose availability (in turn) is regulated by dietary CHO intake. (...) our study provides support for the hypothesis that the physiological trigger for this coupling in the healthy individual may be a short-term challenge to dietary CHO availability. That we have observed these diabetogenic alterations in a physically fit population, which is purported to be insulin sensitive yet exhibits high IMTG concentrations (the ‘athlete paradox’) (Goodpaster et al. 2001), supports our contention that they represent an adaptive rather than pathological response. This substantiates our previous assertion that alterations in glucose tolerance and insulin sensitivity associated with dynamic changes to the plasma and/or lean tissue lipid profile are part of a normal co-ordinated adaptation to short-term changes in food availability (Stannard & Johnson, 2004) and perhaps, more specifically, to fluctuations of dietary CHO availability. (...) This short-term alteration is teleologically sound because it limits competition between skeletal muscle and glucose obligate tissues for circulating glucose substrate when its availability becomes limited. Irrespective of a causal relationship, the coupling between IMTG accumulation and reduced insulin sensitivity may also represent a co-ordinated adaptive (non-pathological) response to CHO stress  (Johnson et al. 2003). A concomitant resistance in muscle to the effects of insulin on glucose uptake during CHO stress maintains normoglycaemia and thus the preservation of plasma glucose for use by the CNS and glucose-obligate tissues (Reaven, 1998). Dissociation of insulin action by way of muscle insulin resistance rather than attenuation of insulin secretion means that residual circulating insulin levels can be maintained (Klein et al. 1993), thereby preventing rampant proteolysis (Fryburg et al. 1990), lipolysis (Kather et al. 1985) and perhaps hepatic glucose release, whilst unnecessary uptake of blood glucose by muscle is prevented."

So, from the studies above, we can conclude that:

• FFA supress glucose uptake and oxidation, resulting in muscular insulin resistance, without reducing insulin secretion.
• These effects seem to be dependent on dietary carbohydrate restriction.
• FFA-induced muscular insulin resistance is a physiological response to low availability of glucose. 
• Under normal conditions, this serves to maintain adequate BG levels. When FFA are in excess, there might be a rise in BG levels, because oxidation and release of FFA are not coupled. This leads to insulin resistance in other tissues like the liver (15), consequently failing to control hepatic glucose output.

Blood glucose levels and high carbohydrate diets

Without any biochemical explanation, logic dictates that if we eat a high carbohydrate diet, glucose oxidation pathways are stimulated. This is the opposite of what we observe with carbohydrate restriction, that is, stimulation of insulin signaling. Glucose and insulin both regulate GLUT4 and GLUT1 in muscle cells (16) to increase glucose uptake. The Randle cycle dictates that glucose stimulates its own oxidation and reduces fatty acid utilization. As shown above, glucose reduces PDK and increases PDH. Insulin inhibits lipolysis, further facilitating glucose oxidation. In healthy subjects, a high carbohydrate-low fat diet can improve insulin sensitivity (17, 18). It makes sense, carbohydrates supress fat oxidation and increse glucose oxidation. Overall, there should not be a rise in BG levels on a 24h basis, if anything, we can expect a reduction, because we are utilizing glucose as our main substrate. 

The common ground: calorie restriction

Until now, we have seen that carbohydrates stimulate glucose utilization (insulin sensitivity) and that FFA supress it. How can then a ketogenic diet produce such good results in people with diabetes? Diabetes and MetSyn are characterized by lipotoxicity and glucotoxicity (19). This means that there is an abnormal level of plasma FFA and glucose, produced by PaIR. Insulin cant supress hepatic glucose output, muscle cells do not respond to insulin, and adipocytes liberate FFA in an uncontrolled fashion. In very simple terms, there is an excess of both energy substrates, each one inhibiting the utilization of the other. This scenario can be improved both by restricting fat (thereby increasing glucose utilization) or restricting carbohydrates (increasing fat utilization). In either case, calories must be restricted (directly or indirectly). So, people who show signs of glucose intolerance and switch to a ketogenic diet can improve their BG and insulin levels (by reducing glucotoxicity), but if energy is in excess, BG can start to rise. On the contrary, reducing dietary fat alleviates lipotoxicity, increasing insulin sensitivity. This is why any diet which is calorie restricted, independent of macronutrient composition, produces weight loss and improves glucoregulation. Calorie restriction, by producing a calorie deficit, alleviates both gluco- and lipotoxicity. 

One of the important aspects for dealing with this subject is the fact that glucose intolerance can have many underlying causes. In this manner, a person with autoimmune diabetes may not tolerate carbohydrates as well as a person with only mild PaIR. The fact that PaIR may progress into beta-cell dysfunction (20, 21) can alterate the response to a high carbohydrate diet, and depending on the severity, extreme measures must be taken to achieve normal BG and insulin levels (such as severe calorie restriction). The distribution of body fat can also have consequences on glucoregulation (22). Last but not least, epigenetic changes produced in utero can affect glucose tolerance since the moment we are born (23). 

What is more important, in my opinion, is to address whether hyperinsulinemia causes or potentiates IR, or if hyperinsulinemia results from IR, by a compensatory mechanism. If the first hypothesis holds true, then a ketogenic diet would have an advantage over a low fat-high carbohydrate diet. Desensitization of target cells triggered by the same hormone (homologous desensitization) is a very common characteristic of hormone signaling. In short, high levels of a given hormone reduce the response of the cell to the hormone effects and a reduction in the level of this hormone resets sensitivity. Excess hormone signaling is harmful, so the cell's attempt to restore normality is mediated by reducing its response. This is exactly what happens with insulin: 

• Chronic hyperinsulinemia (in vivo and in vitro) causes a reduction in the number of receptors per cell and glucose transport (24, 25).
• Pre-incubation of 3T3-L1 adipocytes with high levels of insulin and glucose increase PTEN activity, which is correlated with decreased PtdIns(3,4,5)P3 (26). This metabolite is very important for intracellular signaling transduction of insulin.
• Hyperinsulinemia has shown to induce insulin resistance in humans (27).
• Overall, hyperinsulinemia is proposed to be a result and a driver of insulin resistance (28).

Obesity seems to be characterized by an increased amount of insulin being secreted, compared to lean subjects (29). So, while calorie restriction per se is responsible for improved glucoregulation, there might be a short-term benefit in consuming a high-fat ketogenic diet in T2DM and MetSyn patients. As the bodyfat mass and associated hormones regulate, the differences between hypocaloric diets with different macronutrient profiles might be eliminated. This seems reasonable for diet-induced insulin resistance, but not for autoimmune or severe diet-induced glucose intolerance. There seems to be a threshold in which many people cant fully recover their insulin sensitivity with dietary measures. This is where a more integrative and previously uncharacterized approach kicks in (this is the subject of my future post). 

Glucose and cancer

Glucose restriction for cancer treatment seems reasonable given the evidence on the dependence of most types of cancer on glucose for cell growth and proliferation. Unfortunately, the picture is not that simple (30). Although restricting glucose is a good idea, specially for glucose-dependent tumors, the evidence shows that cancer cells also feed on glutamine. More surprinsingly, some types of cancer can grow on fatty acids (31). Restricting glucose reduces insulin levels, which promotes cancer growth. Nevertheless, ideal levels of blood glucose and insulin for treating cancer can only be achieved via calorie restriction. In fact, many supporters of ketogenic diets for cancer often cite the study of Zuccoli et al (32) on the management of glioblastoma. But very few mention what is stated in the study:

"Due to the hyperuricemia the patient was gradually shifted to a calorie restricted non-ketogenic diet, which also delivered a total of about 600 kcal/day. This diet maintained low blood glucose levels and slightly elevated (++) urine ketone levels due to the low calorie content of the diet."

Despite switching to a non-ketogenic (by definition) diet, the patient still showed progress. In my opinion, besides glucose and protein restriction, calorie restriction (and probably fasting) is the dominant factor for achieving success during cancer treatment. 

Summary and key points

• Both energy substrates (glucose and fatty acids) support their own oxidation and inhibit the metabolism of the other.
• A diet high in fat and low in carbohydrates will reduce glucose metabolism and increase fat metabolism. Conversely, a high carbohydrate-low fat diet increases glucose utilization and decreases fatty acid metabolism.
• Increased glucose utilization implies upregulation of glucose membrane transporters and enzymes involved in glycolysis. Additionally, it reduces the activity of enzymes involved in fat metabolism. 
• Increased fatty acid metabolism inhibits key glycolytic enzymes, as well as GLUT membrane translocation. It also interrupts glucose/insulin signaling and stimulates lipolytic enzymes. 
• Chronic hyperinsulinemia, caused by peripheral insulin resistance and energy excess, aggraviates glucose intolerance. Both high glucose and high FFA levels promote this state, by different mechanisms. 
• Under energy balance, a high fat ketogenic diet might produce muscular insulin resistance, reducing glucose tolerance. This should not be compensated by an increase in blood glucose levels. However, if energy intake exceeds calorie expenditure and/or the body utilizes predominantely FFA for energy for extendend periods of time, there can be a rise in blood glucose to non-pathological levels. This is specially relevant if there is little exercise being done (exercise promotes muscular insulin sensitivity) and/or there is an abnormal condition.
• The etiology of glucose intolerance is very important for the proper treatment. Although calorie restriction is the primary solution for obesity/diet-induced insulin resistance, people with autoimmune (both congenital/perinatal or diet-induced)  and beta cell dysfunction should adopt a very low carb approach. 

In the end, the level of carbohydrates proposed by the Jaminet's is in the safe side. The alarmism promoted by some people is not supported. While severely restricting carbohydrates is, in my opinion, the best approach for MetSyn and obesity, once fat mass has reduced, one can tolerate more carbohydrate without problems. If the choice is restricting carbohydrates for life, you should expect a very abnormal response to any carbohydrate (being "safe" or "unsafe"). Nevertheless, lets not forget that the Perfect Health Diet is not a high carbohydrate diet, but a high-fat, low carbohydrate diet. Despite my obvious differences with Paul (33), his dietary advise is very reasonable and his diet is the first I recommend. This template, plus calorie restriction and/or fasting, is the best dietary measure one can implement. Everyone should adjust their individual carbohydrate needs, but in the end, the key is controlling and preventing inflammation. And carbohydrates per se are not inflammatory. 


*Certainly, there are people who are in the extremes of the Gaussian distribution. For these persons, extra measures should be taken. I will write about my approach in the following post. 

Bioenergetics

Metabolic efficiency can be defined as the capacity for producing useful energy with the least amount of metabolic by-products and heat. In a machine, for example, efficiency can be measured as the ratio of the work done to the energy supplied to it. You can do more with less. There are other definitions of metabolic efficiency around the web such as the capacity for fat utilization for a given activity. 

For aerobic metabolism, the main pathway for ATP production in eukaryotes, there are two substrates which compete with each other: glucose and fatty acids. Amino acids can also be used but are not (or should not) be an important source of fuel. For knowing which fuel is more efficient (giving the higher amount of energy per mole, joule per mole) we must review the pathways for glucose and fatty acid catabolism before they converge into the Krebs Cycle.

Glucose oxidation, ie. glycolysis, is the pathway for oxidizing glucose to pyruvate. The preparatory phase of glycolysis requires the "investment" of two molecules of ATP. This is "payed off" in the following steps, producing 4 ATPs. So the net gain of the conversion glucose --> pyruvate is 2 ATPs. This process also produces 2 NADH+.  Pyruvate must be converted to acetyl CoA, producing one more NADH+ and CO2. So the total oxidation of one molecule of glucose to acetyl CoA yields:

1 glucose molecule to 2 pyruvates: 2 NADH+
2 pyruvates to 2 Acetyl CoA:  2NADH+, 2CO2

Total = 4 NADH+ plus 2CO2

On the other hand, fatty acid oxidation is relatively simpler. Once triglycerides are hydrolyzed, free fatty acids are taken up by cells. In the cytosol, they are esterified to conezyme A to form a fatty acyl CoA. The fatty acyl group is transferred to carnitine and moved across the inner mitochondrial membrane by an acylcarnitine transporter protein. Once in the matrix side, the fatty acyl group is released from carnitine and reattached to another CoA molecule. The mitochondrial fatty acyl CoA is oxidized in a series of reactions to produce Acetyl CoA. The conversion of a fatty acid to acetyl CoA yields (taking palmitate as an example):

1 palmitate to 1 palmitoyl CoA: 2 ATP
1 palmitoyl CoA to 8 Acetyl CoA: 7 FADH2 + 7 NADH+

Overall, in these preliminary reactions before entering the Krebs Cycle we have that 1 molecule of glucose can yield 2 Acetyl CoA while 1 molecule of palmitate can produce 8 Acetyl CoA. Oxidation of glucose until this step produces 2 molecules of CO2 and 4 NADH+, while oxidation of palmitate produces an equal number of FADH2 and NADH+, 7.

Once acetyl CoA reacts with oxaloacetate, forming citrate, it enters the Krebs Cycle. This pathway produces 3 NADH+, 1FADH2, 1 GTP and 2CO2. So, overall we have:

1 molecule of glucose produces: 

2 Acetyl CoA
6 CO2
10 NADH+
2 FADH2

Ratio NADH+:FADH2 = 5:1

ATPs produced from complete oxidation: 30-32 (assuming 2.5 ATP from NADH+ and 1.5 ATP from FADH2)

1 molecule of palmitate produces:

8 Acetyl CoA
16 CO2
31 NADH+
15 FADH2
Ratio NADH+:FADH2 = 2:1 (depending on carbon length)

ATPs produced from complete oxidation: 108 (assuming 2.5 ATP from NADH+ and 1.5 ATP from FADH2)

The final electron acceptors are the coenzymes NADH+ and FADH2 which transfer electrons to the mitochondrial respiratory chain. Understanding this process is of utmost importance for understanding the differences between glucose and fatty acid metabolism. For a basic tutorial in oxidative phosphorylation please read this. 

Electrons carried by NADH+ are transferred to Complex I, also called NADH dehydrogenase. Complex II, succinate dehydrogenase, is the same enzyme encountered in the Krebs Cycle, which oxidizes succinate and transfers electrons to FADH2. Studies have shown that Complex I and complex III are the main producers of ROS in the electron transport chain (1, 2, 3, 4)*. So, the ratio of NADH+:FADH2 is important for the potential ROS production in the mitochondria. Because glucose generates 5 times more NADH+ than FADH2, complex I activity is increased. Fatty acid oxidation produces only twice the amount of NADH+ than FADH2, shifting to a more balanced utilization between complex I and complex II. There are other ways to pass electrons into the respiratory chain. In the first step of the beta oxidation pathway, catalyzed by acyl-CoA dehydrogenase, electrons from the substrate are transferred to the FAD of the dehydrogenase, then to electron-transferring flavoprotein (ETF), which in turn passes its electrons to ETF:ubiquinone oxidoreductase. This enzyme reduces ubiquinone directly, without the need for other complexes. Glycerol-3-phosphate derived either from glycerol or from glycolysis is oxidized by glycerol-3-phosphate dehydrogenase and reduces ubiquinone directly. 

The overall message is that glucose's complete oxidation involves more complex I activity, ultimately promoting more ROS production by the mitochondria. A more in depth review of the implications of glucose and fatty acid metabolism, and mitochondrial complex utilization in aging and disease can be found in this paper by Mobbs et al. (yes, the same guy from the nephropathy reversal study):

"Therefore shifting away from glucose utilization toward lipid and amino acid utilization would be expected to substantially reduce the production of reactive oxygen species, without necessarily reducing ATP production. As described below, other beneficial effects also occur as a result of this altered pattern of glucose fuel use, including a shift toward producing antioxidizing NADPH and increased protein and lipid turnover, which reduces the accumulation of oxidized protein and lipids."

Ok, so we have seen why depending on fat as an energy source seems a better idea than depending on glucose. But what about ketones?

Kurt Harris thinks ketosis is metabolically stressful. Considering that the ketogenesis pathway only demands one molecule of NADH+ and one H2O, I dont see the "stressful" part.

Let's see how ketones alter mitochondrial energy metabolism**. Remember that the respiratory electron transport chain is the process which generates ATP by creating an electrochemical proton gradient. Electrons carried through redox couples are ultimately combined to H+ and O2 to form H2O. The total energy available from the movement of electrons up the respiratory chain is determined by the difference between the variable redox potential of the mitochondrial NAD couple and the O2 couple (this is because the first step in the chain is NADH dehydrogenase and the last step is production of H2O). We know experimentally the reduction potential of both couples (5): 

[NAD]/[NADH+] couple: -0.28V
[O2]/[H2O] couple: +1.2V
So we can calculate the total energy yield given by:

∆G' = -nF∆E

Where: n is the number of electrons, F is 96.485 kJ/mol/V (Faraday's constant) and ∆E the difference between potentials. Replacing in the equation with the values we have:

∆G' = -(2)(96.485)(0.92)

∆G' = -177.5324 or -178kJ/2mol

The energy of the proton gradient created by the electron transport chain is given by: 

∆G' [H+] cyto/[H+] mito = RTln[H+] cyto/[H+] mito + FEmito/cyto

Where  Emito/cyto is the electric potential of the mitochondrial relative to the cytosolic phase.

3 ATPs are generated by the transport of one electron pair through the respiratory chain. Since the maximum energy available from the redox reactions of the chain is -178kJ, the energy available for the synthesis of each ATP cannot be more than -59.2kJ/mol (178/3). Experiments done on different cells have shown that this energy is in the range of -54 to -58kJ/mol (very efficient). 

As the electrons liberated by complex I are carried by coenzyme Q to complex III, the difference between the redox potential of the [NAD]/[NADH+] mitochondrial couple and the coenzyme Q couple determines the energy of the proton gradient generated by the mitochondria. This difference determines the energy of hydrolysis of ATP generated by the mitochondrial F1 ATPase. 

bOHB and AcAc are in near-equilibrium with the free mitochondrial [NAD]/[NADH+] ratio. Veech and colleages showed that the metabolism of ketone bodies in the heart produced a reduction of the mitochondrial NAD couple and oxidation of the coenzyme Q couple, increasing the redox span between the two couples, making more energy available for ATP synthesis because: 

∆G = -nF∆EQ/NADH

In this classic study (6), the authors found that***:

"The Eh7 of the mitochondrial NAD couple, Eh7 NAD+/NADH (Eq. 16), was -280 mV and decreased to about - 300 mV on addition of insulin, ketones, and the combination.

The ratio [fumarate2-]/[succinate2-] increased about 1.5-fold on addition of insulin, about 2.5-fold on addition of ketones, and about 4-fold on addition of the combination (Fig. 1). Taking this change to indicate the redox state of the [Q]/[QH2] couple, which is the cofactor for the succinate dehydrogenase (EC 1.3.5.1) reaction, the observed increase in [fumarate2-]/[succinate2-] indicates a 1.5- to 4-fold oxidation of mitochondrial [Q]/[QH2] on addition of insulin, ketones, or the combination. The Eh[Q]/[QH2], calculated from the succinate dehydrogenase reaction (Eqs. 20-22, Fig. 2), increased progressively from -4 mV during perfusion with glucose alone to + 15 mV on the addition of insulin and ketones.

When the oxidation of the mitochondrial Q couple was combined with the reduction of the mitochondrial NAD couple resulting from these additions to the glucose perfused hearts, the estimated energy available in the transfer of 2 e from the mitochondrial NAD to the Q couple (Eq. 23 and Eq. 24) catalyzed by the NADH dehydrogenase multienzyme complex, ∆G QH2/NAD+, increased from -53 kJ/2 mol e during perfusion with glucose alone to -60 kJ/ 2 mol e with addition of insulin and ketones. This increase was paralleled by an increase in the cytosolic free energy of ATP hydrolysis, ∆GATP (Eq. 7 and Eq. 8), determined independently using NMR spectroscopy, which increased from -56 kJ/mol in control hearts to about -59 kJ/mol on addition of insulin or insulin plus ketones. 

The potential between mitochondrial and cytosolic phases, E mito/cyto (Eq. 9, Fig. 2, Table 3), was -143 mV in hearts perfused with glucose alone and unchanged on the addition of insulin but decreased to -120 mV on addition of ketones or -130 mV on addition of the combination. "

Summarizing (from the study data)****: 

For ketones (without insulin):
Eh7 NAD+/NADH = -299mV or -0.299V
Eh7 Q/QH2 = +7.8mV or 0.078V
Cytosolic pH = 7.05
Mitochondrial pH = 7.52

∆G = -nF∆E

∆G = -(2)*(96.485)(0.015-(-0.3))

∆G = -59.2 kJ/2mol

For glucose:

Eh7 NAD+/NADH = -280mV or -0.28V

Eh7 Q/QH2 = -0.4mV or -0.004V

Cytosolic pH = 7.05

Mitochondrial pH = 7.09

∆G = -nF∆E
∆G = -(2)*(96.485)(-0.004-(-0.28))
∆G = -53 kJ/2mol

Recall that the energy of the proton gradient is calculated by:

∆G' [H+] cyto/[H+] mito = RTln[H+] cyto/[H+] mito + FEmito/cyto

Expressing ∆G in terms of pH instead of concentration, we get:

∆G' [H+] cyto/[H+] mito = 2.3RT (∆pH) + FEmito/cyto

where, R is the gas constant (8.315 x 10^-3 kJ/mol.K) and T the absolute temperature (in this study, 311.15K).  

For ketones, Emito/cyto = -0.12V, so:

∆G = 2.3 (8.315*10^-3)(311.15) (∆pH) + (96.485)(-0.12)
∆G = (5.9 kJ/mol) (∆pH) + (-11.52)
∆G = (5.9 kJ/mol) (7.05-7.52) + (-11.52)
∆G = -14.2 kJ/mol

For glucose, Emito/cyto = -0.14V, so:

∆G = 2.3 (8.315*10^-3)(311.15) (∆pH) + (96.485)(-0.14)

∆G = (5.9 kJ/mol) (∆pH) + (-13.5)

∆G = (5.9 kJ/mol) (7.05-7.09) + (-13.5)

∆G = -13.7 kJ/mol

The mean calculated ∆G of cytosolic ATP hydrolysis was:

∆G ATP = -57.6 kJ/mol for ketones
∆G ATP = -56.6 kJ/mol for glucose

Concluding from this data we see that ketones cause a decrease in the potential between the mitochondria and cytosol (ie. Emito/cyto) while increasing the ∆G of ATP hydrolisis, paralleled by the increase in ∆G QH2/NAD+ and ∆G[H+]. Increased efficiency. 


But why bOHB is such a "superfuel"? This can be illustrated in the following table (7):

The table above shows the heats of combustion for different nutrients metabolized by the Krebs cycle. As shown, there is a wide variation in the inherent energy of different C2 units which enter the cycle, derived from different sources. For instance, as we have seen, one molecule of pyruvate produces 15 ATP. The ∆G' of ATP hydrolisis cannot exceed 59-60kJ/mol and the range of variation in cells is very narrow, less than 10% (-53 to -60kJ/mol). So, by simple maths, for producing 15 ATP, -795 to -900 kJ/15 moles are needed (15*-53; 15*-60). There are only -778kJ available from pyruvate, so the ∆G' of the ATP produced is of low energy (-778/15 = -51kJ/mol). C2 from fatty acids (ie. palmitate) have too much inherent energy to be accomodated in the 15 ATPs, metabolically implausible (-1247/15 = 83). If we look at the table, bOHB has -1021kJ/C2, only 13% excess energy (-1021/15=68). Compared to pyruvate, bOHB is more reduced (ie. more H+ per C). 

When interpreting this data, one obvious question arise. If the inherent energy of palmitic acid is greater than bOHB, why does palmitic acid is not a "superfuel"? The answer lies in the nature of beta oxidation. If we remember the sites for entering the respiratory chain, we can see that some reducing equivalents produced by the oxidation of fatty acids enter the chain via the ETF:ubiquinone oxidoreductase, resulting in a loss of approximately 1 of the 6 possible ATP produced, because of its redox potential (8). The remaining reducing equivalents are metabolized in the Krebs cycle normally, but in contrast with metabolizing bOHB, the Q couple is reduced and not oxidized, decreasing ∆E available for ATP synthesis. Last but not least, free fatty acids produce enzymatic/transcriptional changes which account for the difference in efficiency between palmitic acid and bOHB. Elevation of plasma FFA induce the expression of uncoupling proteins (9, 10) and peroxisomal beta-oxidation (11). 

Summing up

The response Dr. Harris gave me a couple of months ago included some statements like:

"It [bOHB] is only efficient if you think the steps it took to get it don't count."

""Glycolysis" is anaerobic and only happens by itself when speed is necessary - like weight lifting."

"Ketosis is an adaptive state that beats the alternative of glycogen depletion or muscle wasting that it is designed to counter. This makes it "good". In the same way, muscle wasting to keep your brain supplied with glucose is "good". Good in the sense of beating the alternatives. That does not mean we should seek to live continuously in either state, though does it?"

I hope that after this post (and after comparing the "alternatives"), my position on the subject is clear.

* This is a subject of much debate. Studying ROS production by mitochondria under physiological conditions is not an easy task. Ultimately, every complex is capable of producing ROS. 
** This explanation is summarized from the referenced article by Veech. A nice article on the subject can be found here.
*** Eh stands for the redox potential of a half reaction. Eh7 = Eh at pH 7.
****Im omitting some units for the sake of clarity. 

Sato K, Kashiwaya Y, Keon CA, Tsuchiya N, King MT, Radda GK, Chance B, Clarke K, & Veech RL (1995). Insulin, ketone bodies, and mitochondrial energy transduction. The FASEB journal : official publication of the Federation of American Societies for Experimental Biology, 9 (8), 651-8 PMID: 7768357