In fact this was one of the biggest surprises I had when exploring ketosis. For years I have been following a cyclical lower carb diet. For years I wouldn’t consume a carb until later in the afternoon (ala Carb Backloading style). After eating 5 days without any carbs I tested my ketone levels… they were 0.1 mmol. This reading was done first thing in the morning (10 hours fasted) after 5 days without a carb in my diet.
Again, there are very interesting animal studies plus some single case reports and small uncontrolled trials of humans with neurodegenerative disease and cancer given ketogenic diets and/or exogenous ketones (Murray 2016, Poff 2015, Roberts 2017, Newport 2015, Cunnane 2016). In some cases where the patient does not have the cognitive resources to comply with a well-formulated ketogenic diet, or where target blood levels of BOHB that work in animals are hard to achieve in humans by diet alone, supplemental ketones may have an important role to play in the prevention, management, or reversal of these disease categories.

d-βHB was measured immediately on whole blood using a handheld monitor and enzyme-based reagent strips (Precision Xtra, Abbott Diabetes Care, UK). Samples were stored on ice, centrifuged and duplicate plasma aliquots stored at −80°C. All urine passed during the visit was collected, the total volume recorded, and 1 ml aliquots taken, frozen and retained for analysis.
There are enticing anecdotes of supplemental ketones being used to boost human physical performance in competitive events, notably among elite cyclists. Given that BOHB can deliver more energy per unit of oxygen consumed than either glucose or fatty acids (Sato 1995, Cox 2016, Murray 2016), this makes sense. But what we do not know is if there is any required period of adaptation to the use of exogenous ketones, and thus how to employ them in training. It is clear that exogenous ketones decrease adipose tissue lipolysis and availability of fatty acids, the exact opposite to what happens on a well formulated ketogenic diet. This distinction between exogenous ketones and ketogenic diets on adipose tissue physiology and human energy balance underscores an important reason why these two ketone-boosting strategies should not be conflated.
Intermittent fasting is using the same reasoning – instead of using the fats we are eating to gain energy, we are using our stored fat. That being said, you might think it’s great – you can just fast and lose more weight. You have to take into account that later on, you will need to eat extra fat in order to hit your daily macros (the most important thing). If you’re overeating on fats here, you will store the excess.
As KE drinks achieved a significantly higher d-βHB concentrations than KS, we investigated factors that may be important in the use of ketone drinks to achieve nutritional ketosis. Initially we determined the repeatability of blood ketosis following KE drinks and found little variation in kinetic parameters between individuals. Variability between participants was less than within the population, and accurate individual prediction of the d-βHB Cmax following a body-weight adjusted KE drink was achieved. Variability within individuals was likely due to normal daily changes in GI function, including gastric emptying, portal blood flow or intestinal transit time, which may alter KE hydrolysis and absorption.

But there have also been studies done showing that the Inuit Eskimo’s do not actually reach a state of ketosis. This is due to numerous factors. One being that the diet the eskimo’s eat ‘would not be expected to cause ketosis, because the calculated anti-ketogenic effect of the large protein ingestion was somewhat more than enough to offset the ketogenic effect of fat plus protein.” 
Exogenously delivered ketone supplements significantly altered rat weight gain for the duration of the study (Fig. 6). However, rats did not lose weight and maintained a healthy range for their age. Rats have been shown to effectively balance their caloric intake to prevent weight loss/gain [97–99]. Due to the caloric density of the exogenous ketone supplements (Table 1) it is possible for the rats to eat less of the standard rodent chow and therefore less carbohydrates while maintaining their caloric intake. Food intake was not measured for this study. However, if there was a significant carbohydrate restriction there would be a signifcant change in basal blood ketone and blood glucose levels. As the hallmark to the KD, carbohydrate restriction increases blood ketone levels and reduces blood glucose levels. Neither an increase in basal blood ketone levels nor a decrease in basal blood glucose levels was observed in this study (Fig. 7). Additionally, if there were an overall blood glucose decrease due to a change in food intake, this would not explain the rapid reduction (within 30 min) in blood glucose correlated with an elevation of blood ketone levels after an intragastric bolus of ketone supplement (Figs. 2, ​,33 and ​and44).

Effects of ketone supplementation on blood glucose. a, b Blood glucose levels at times 0, 0.5, 1, 4, 8, and 12 h (for 10 dose) post intragastric gavage for ketone supplements tested. a Ketone supplements BMS + MCT and MCT significantly reduced blood glucose levels compared to controls for the duration of the 4-week study. BMS significantly lowered blood glucose only at 8 h/week 1 and 12 h/week 3 (b) KE, maintained at 5 g/kg, significantly reduced blood glucose compared to controls from week 1–4. BD did not significantly affect blood glucose levels at any time point during the 4-week study. Two-Way ANOVA with Tukey’s post hoc test, results considered significant if p < 0.05. Error bars represent mean (SD)

After a minimal amount of internet "research," I decided to try my first exogenous ketones. I have used the ketogenic diet off and on for at 15 years and my body is pretty efficient at fat adapting. (Usually by the end of 2 strict days, I am in ketosis, but not without symptoms and intense cravings.) I can consistently fast from carbs for 20 - 24 hours and do this consistently. However, around hour 20, my mind begins to negotiate that intermittent fasting is advantageous too and that I can afford to have some carbs once a day. Hence the yo-yo effect.

The best time to start a one day fast is in the evening (neither morning nor the night) – preferably, around 6 pm. It won’t make you lose your vital energy during the daytime workouts, nor does it let you sleep with undigested foodstuff in your stomach. Taking late meals and sleeping with undigested food doesn’t allow your body to rest. So the natural healing mechanism of your body fails during the sleep time as the entire resources are busy digesting your food.
In a subset of participants (n = 7) the effect of 3.2 mmol.kg−1 of βHB as KE and KS on blood pH and electrolytes after ketone drinks was investigated. Blood d-βHB kinetics were similar to those in the initial experiment (Figure ​(Figure3A).3A). After 60 min, blood pH declined from 7.41 to 7.31 following a KE drink (p < 0.001, Figure ​Figure3B).3B). Bicarbonate fell significantly from 23.6 ± 0.7 to 17.0 ± 0.8 mM following KE drinks (p < 0.001), but remained within the normal range (Figure 3C). Both ketone drinks significantly decreased blood potassium concentrations by 0.7 mM (both drinks p < 0.05, Figure 3D) and increased sodium and chloride concentrations (Sodium: both drinks p < 0.05, Chloride: KE = p < 0.05, KS = p < 0.005, Figures 3E,F).
Given that blood βHB after identical ketone drinks can be affected by factors such as food or exercise (Cox et al., 2016), the accuracy of tools for non-invasive monitoring of ketosis should be investigated. Breath acetone and urinary ketone measurements provide methods to approximate blood ketosis without repeated blood sampling (Martin and Wick, 1943; Taboulet et al., 2007). However, breath acetone did not change as rapidly as blood βHB following KE and KS drinks. Acetone is a fat-soluble molecule, so may have been sequestered into lipids before being slowly released, resulting in the differences observed here. Similarly, significant differences in blood d-βHB between study conditions were not reflected in the urinary d-βHB elimination. As the amount of d-βHB excreted in the urine (≈0.1–0.5 g) represented ~1.5% of the total consumed (≈23.7 g), it appears that the major fate of exogenous d-βHB was oxidation in peripheral tissues. These results suggest that neither breath acetone nor urinary ketone measurements accurately reflect the rapid changes in blood ketone concentrations after ketone drinks, and that blood measurement should be the preferred method to quantitatively describe ketosis. That said, it should be noted that although commercial handheld monitors are the most practical and widely available tool for measuring blood ketones, they can overestimate blood D-βHB compared to laboratory measures (Guimont et al., 2015) and these monitors do not measure L-βHB and so may not provide accurate total blood ketone concentrations, especially if a racemic ketone salt has been consumed.
Most supplements rely on BHB as the source of their exogenous ketone bodies. BHB is converted to acetoacetic acid with a small quantity converted to acetone through a acetoacetate decarboxylase waste pathway. Some of the acetoacetic acid will enter the energy pathway using beta-ketothialase, which converts acetoacetic acid to two Acetyl-CoA molecules (see diagram below2).
Her clients have had similar success. One woman, for instance, has gone from around 170 pounds to 140 pounds since April without making any initial dietary changes. She’s started to gravitate towards more keto foods over time, but still eats her favorite high-carb treats. As for exercise? Her routine consists of a couple of walks each week, Heverly says.
Ketone supplements contain exogenous ketones—synthetic ketones made in a lab. Most use a type of ketone called beta-hydroxybutyrate (BHB), which is the same as the ketones the body produces naturally. “We’re literally biohacking," says Amie Heverly, who began taking a ketone supplement called Prüvit last year and now works as a promoter selling Prüvit products. "You’re not adding a foreign substance to your body, because BHB is identical to what your body would naturally produce,” she explains.
An alternative to the ketogenic diet is consumption of drinks containing exogenous dietary ketones, such as ketone esters (KE) and ketone salts (KS). The metabolic effects of KS ingestion have been reported in rats (Ari et al., 2016; Kesl et al., 2016; Caminhotto et al., 2017), in three extremely ill pediatric patients (Plecko et al., 2002; Van Hove et al., 2003; Valayannopoulos et al., 2011) and in cyclists (O'Malley et al., 2017; Rodger et al., 2017). However, the concentrations of blood βHB reached were low (<1 mM) and a high amount of salt, consumed as sodium, potassium and/or calcium βHB, was required to achieve ketosis. Furthermore, dietary KS are often racemic mixtures of the two optical isoforms of βHB, d-βHB, and l-βHB, despite the metabolism of l-βHB being poorly understood (Webber and Edmond, 1977; Scofield et al., 1982; Lincoln et al., 1987; Desrochers et al., 1992). The pharmacokinetics and pharmacodynamics of KS ingestion in healthy humans at rest have not been reported.
Some people follow more of an Ultra Low Carb diet approach. This is generally around 50g or less of carbs per day. A ULC is more supportive of reaching a ketogenic state, but again total carbs are not the only variable when it comes to reaching ketosis (other factors such as types of carbs, protein consumption, portion size, ingredients, supplements used etc. all play a role and will be covered in more detail below). 
Exogenous ketones are created in a lab to accelerate both physical and mental performance. These ketone drinks were actually used in pro cycling races back in 2015, trading at prices that would make using your kidney as a bartering tool seem like a cut price deal. Fortunately, they’ve now come down in cost and are used often in between meals as a way of blackmailing your body into getting into ketosis way faster.
In addition, the body regulates ketone production via ketonuria (peeing out excess ketones) and ketone-induced insulin release, which shuts off hepatic ketogenesis (the liver making more ketones when you have enough).   The insulin from this process could be increasing glucose disposal which, when coupled with PDH activation, could drive glucose levels quite low.
In the second of these posts I discuss the Delta G implications of the body using ketones (specifically, beta-hydroxybutyrate, or BHB, and acetoacetate, or AcAc) for ATP generation, instead of glucose and free fatty acid (FFA). At the time I wrote that post I was particularly (read: personally) interested in the Delta G arbitrage. Stated simply, per unit of carbon, utilization of BHB offers more ATP for the same amount of oxygen consumption (as corollary, generation of the same amount of ATP requires less oxygen consumption, when compared to glucose or FFA).

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