This is a brief overview enlightening a surface understanding on Ketogenesis (creation of Ketone bodies). This is intended as an extended elaboration, to the parent manuscript’s very brief description of Ketogenesis in the chapter SKD / Keto.

Disclaimer

1. This article does not convey nor it intends to substitute over the reader's terminal and/or medical supervisions.
2. As an ordinary member of public with however limited & surface credentials in nutritional science (click here for details) this article amongst others relative ~ remain strictly surface-centric. Not academically-exhaustive. This article is only envisioned to facilitate entry-level dialogue to advance interests amidst many nuances of nutritional science.
3. This recap on ketogenesis is a work in progress. And as as such remain subject to rewriting and/or updates at This Author's (AW) own timely discretion and opportunity.

Metabolism, as a major study in Biochemistry often demands rigorous & lengthy discussion. Hence it is likely impossible to address all branching topics into just one (1) condensed writing in lay / accessible language.

This Author (AW) nonetheless apologises for the late and/or untimely arrival on this much needed basic biochemistry; considering the many elusive demands this concept initiative evolved throughout the years in both its written book(s) and ongoing Youtube® channel entirely on the sole effort and resources off one (1) individual) without any third party assistance or oversight.

Live-It-Forward,

AW.

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What is Ketogenesis?

The most agreeable definition is simply an alternative metabolic pathway at utilising Ketone bodies instead of glucose amidst low energy state, caloric deprivation and/or positive influx of fatty acid release (endogenously liberated from existing adipose, or exogenously from dietary source).

The most condensed one-paragraph one may ever summarise the entire mechanism behind ketogenesis, is the inhibition of oxaloacetate pathway within the Krebs Cycle, where the build up the first byproduct of fat oxidation ~ Acetyl COA ~ begin its intermediary conversion(s) to HMG-CoA, before then finally ~ the three (3) ketone bodies towards plasma circulation as an alternative energy source.

The three primary ketone bodies most referred throughout many literatures today are B-ohb (the shortened reference for B~hydroxybutarate), acetoacetoate, and acetone.

What the process or stage/s of Ketogenesis?

Based on general & current literatures, Ketogenesis takes place exclusively in the Liver from which then other organs soon reacts and respond / as follows:

1. The release (either as “free” or from existing tissue) or influx of dietary fatty acids proceeds as TAGs or triglyceride. This is provoked by either one or two commonly thought scenarios:
   1. Via lowered carbohydrate intake;
   2. Fasting state and/or;
   3. Overall calorie deficit.
2. These fatty acids then prompts the build up of Acetyl COA molecules and activates the enzyme Acetoacetyl-CoA Thiolase to begin converting these build up of Acetyl CoA into Acetoacetyl-CoA.
3. Conversion of the Acetoacetyl-CoA into HMG-CoA begins, by the enzyme HMG-CoA Synthase. These HMG-CoA intermediary conversions is thought to be the early stages of cholesterol and then consequently ¬ steroid hormone synthesis.
4. Finally, HMG-CoA then undergo final conversions into the the ketone bodies as (3) progression and/or types.
   1. First Ketone body is acetoacetate ~ which can be registered / indicated via its losses in the urine and detectable in the Diastix / Ketostix.
   2. Then,  beta hydroxybutarate, or famously known as B-OHB, circulates in the blood now soluble for the brain and amongst other tissues for immediate energy source.
   3. Then lastly, acetone, detectable by breath test. Noteworthy reminder / trivia is that the “smell” of these is usually proclaimed to that as “paint thinners” and/or “nail polish removers”.   
5. Meanwhile as #1-#5 ensues, insulin status likely becomes low, and thus obligates glucagon (glycogen breakdown) from the pancreas in exchange to keep blood sugar on a level to remain available for immediate utilisation.
6. If within the context of exercising state~ byproducts of energy metabolism namely lactic acids, and glycerol (byproduct triglyceride breakdowns) both expedite glycogen depletion and continues prompting the above glucagon activity by withdrawing stored glycogen into readily usable glucose (Ninjanerd.org).
7. As steps #1 to #6 repeats, ketone productions will likely ensue. Until at least the following inhibitory events occur:
   1. Reintroduction of bolus / influx of carbohydrates.
   2. Acute insulin spikes thereby inhibiting the early steps (lipolysis/betaoxidations/free fatty acid release).

Little trivias

• It is interesting to note that the Liver is the primary organ that produces ketones. But cannot accepts ketone bodies in return as its energy source (ATP)  (Dhillon KK & Gupta S 2022) due to the lack of enzyme (Beta Keto Acyl-CoA Transferase).
• Ketone bodies appears to rely on the natural HMG-COA Reductase response to the influx / large presence of fatty acids either released or dietary. HMG-Coa Reductase is also taught to increase overall cholesterol. A popular drug, as readers may already are far too aware of at aggressively inhibiting these complex HMG-COA Reductase intermediates is none other than Statin group of drugs (or technically known as “HMG-COA Reductase Inhibitors”) (Sato, T 1998).

“ The plasma levels of acetoacetic acid also significantly decreased from 37.7 +/- 22.6 to 28.4 +/- 13.4 mumol/l, and those of 3-hydroxybutyric acid and total ketone bodies also tended to decrease after pravastatin treatment. These results suggest that pravastatin decreases ketone formation in hepatic mitochondria besides cholesterol synthesis in hepatic microsome.” ~ Sato, T. 1998

What key molecules and enzymes involved in Ketone bodies formation? 

In speculative order of events~

1. Malonyl-COA
   Considered to be the precursor before the rest of obligate intermediary products and/or enzymes may take place to commence / continue ketogenesis (McDonald, L. 1998). When liver glycogen is full this is indicative of high level of Malonyl CoA, which as a result inhibits Acetyl-COA production and thereby cannot commence the CPT-1 enzyme for fatty acid metabolism (Ninjanerd.com).
2. Acetyl-COA
   An obligate shared necessity in the Citric Acid Cycle (aka. Krebs cycle) which these represents a shared process between glucogenesis (through oxaloacetatic pathway) and/or ketogenesis (depending on other substrates’ bias of influx / inputs). Acetyl COA is produced once select ketogenic amino acid have once been metabolised / oxidised.
3. Acetoacetyl COA
   An intermediary product of Acetyl-CoA oxidation from select ketogenic amino acids (particularly Leucine and Lysine) which then precrusors the HMG-CoA reductase to initiate cholesterol synthesis.
4. HMG-COA Reductase
   In concert with Hmg-COA Lyase, the Coa-reductase obligates the synthesis of Cholesterol.
5. HMG-CoA
   As pre-final form substrate before the actual creation of ketone bodies.
6. CPT-1 Enzyme
   Another enzyme thought important in-between all steps is that of  CPT 1 enzyme or Carnitine palmitate transferase #1. This transports the free fatty acid for oxidation within the mitochondria. Interestingly a deficiency in carnitine may compromise the transport (or "shuttling") the fatty acids for oxidation in the mitochondria, and thus inability for adequate energy production.

   "Carnitine is thought to be important on the ketogenic diet because the high fat intake means more fatty acids need to be transported into the mitochondria for oxidation, requiring more carnitine and therefore increasing risk of depletion of body carnitine stores." ~ Matthewsfriends.org

It is worth reminding that all of the above molecules and/or pre-cursors to metabolism listed below is not necessarily exclusive to Ketogenesis metabolic pathway alone; as many are also closely obligated for other cycles and/or metabolic pathways far too complex for us to highlight here in this feature Recap (to name a few ~ Gluconeogenesis, Cori Cycle, Citric Acid / Krebs cycle among others).

What organs and/or tissues are able to utilise Ketones?

In general, many literatures seemingly agree that the skeletal muscle, heart and “most” other organs readily utilise Ketones where and when available when glucose supply is low. However it is noteworthy whilst the brain cannot use free fatty acids directly as is ~ such fatty acids must be transported through easily penetrable means which is where and when ~ the three ketone bodies, B-OhB in particular becomes abundant which solubility is a prerequisite for transport.

What are the required scenarios for Ketogenesis?

Whilst debate-able the following outlines various scenarios, from order of most likelihood, to less that may play a possible contributing role :

1. Prolonged Fasting. This is arguably the most assured route. One trial (Yang MU & Itallie VT 1976) examining obese subjects response to either 800kcals on mixed diet vs ketogenic as well as vs complete prolonged fasting ~ strongly suggests that prolonged fasting two days onwards produce a markedly higher ketogenesis response.  
2. Free fatty acid release. The term “free” here is key importance since it suggests the release and liberation of fatty acids, TAGs or triglycerides namely from the liver ~ and thought to be one of the many precursors to Ketogenesis.
3. Fats intake emphasis comparatively over all other major macronutrients.
4. Carbohydrate restrictions typically ranging less than <100 grams. Though this may subject to individual nuances as many proclaim that less than 50 grams of net carbohydrate are taught to be more expedient to the process.
5. Overall caloric restrictions (assuming ISOcaloric macronutrient intake ~ proteins, fats, carbohydrates, fibres and alcohol). Or a uniform deficit of calorie intake from presumably all three macronutrients. Nonetheless, whether or not deficit severity expedites ketogenesis remains debate-able.     

What possible confounders are there that inhibits and/or preventing ketogenesis?

A number of factors despite much remaining subject to debate are curated as follows.

• Vitamin C requirements. At the time of writing this Recap there is unfortunately a scarce amount of research  determining whether Vitamin C (Ascorbic Acid) status have an impact on ketogenesis. One study on guinea pigs (Debons HB et al. 1959) suggests that the ketogenic response to fasting is reduced. Another study (Mueller PS & Cardon PV 1960) on guinea pigs fed with "scorbutic" (Vitamin C deficient)  diets showed a rise of serum free fatty acids. However that study did not test ketone bodies. We may only gleam, be it however speculative based on what we know so far at the following ketogenesis precursor  :
  • L-Carnitine, a precursor to acetylated carnitine and thus a plausible contributor to fatty acid oxidation. Much consensus appears to suggest this tobe be reliant on Vitamin C status (Savini I. et al. 2005) , (Linus Pauling Institute). However, so far under This Author's (AW) awareness ~ there is one (1) in-vivo study (Furusawa, H et al. 2008) on genetic knockout mice (to induce experimental Vitamin C synthesis inhibition) and concluded that Vitamin C was "not required".
• Vitamin B3 / Niacin / Niacinamide / Nicotinamide Riboside. Intriguingly there are mixed receptions on Vitamin B3's outcomes on insulin sensitivity in exchange for HDL lipid profile improvements. However specific literatures ascertaining whether or not it impacts ketogenesis remains, thus far with the writing of this Brief Recap ~ uncertain. At the very least, a study amongst overweight volunteers (X Chen, et al. 1999) on Nicotinic acid administration + 24 hours of fasting compared against those without supplementation were found to exhibit initial reduction of serum free fatty acids and ketone production, but both markers dramatically rises only after 21 hours+ of fasting on the NA supplemented group. There are also anecdotal sources (1 , 2) proclaiming there is a “temporary” inhibitory effect on FFA (endogenous liberation).
• Presence of citric acid. Citric acid is present in almost all citrus fruits and many plant foods at providing source of citrate. Citric acid was firstly noted by handful of individuals, though not all circumstances ~ whose low ketone readings appear to be suspected by citric acid intakes. This remained highly subjected for debate for nearing a decade which sadly - remains to this day only a mystery without definitive causality (McDonald, L. 1998). However speculatively from mechanistic perspective ~ Citrate, being a close obligate relationship with Acetyl-CoA does in some respect maintain in some way or shape form at allowing some possibilities for the oxaloacetate pathway to continue at producing glucose. Unfortunately whether or not citric acid intake, be it synthetic or from wholefood sources play an independent role at inhibiting ketogenesis remains lacking in research; except only one source (Mcdonald, L. 1998) suggests this phenomenon to be variable from individual to individual.  
• Excess protein intake. Particularly gluconeogenic group of amino acids are widely suspected to prevent ketogenesis.
• Excess intake of Xylitol. A hypocaloric sweetener off a sugar-derived alcohol ~ there appears to be multiple studies (Larysa H. 2019) & (Yamagata S. et al. 1969) & (Goto Y. et al. 1969) all of which both human and animal models suggesting Xylitol's "antiketogenic" by way of inhibiting free fatty acids. In light of these, it may seem reasonable to thus avoid xylitol intakes.

What / which amino acids are gluconeogenic and ketogenic?

To date and under this author’s (AW) own humble understanding ~ only two (2) amino acids are thought supportive towards Ketogenic metabolism ~ Leucine and Lysine (Newsholme P et al. 2011). Leucine, being a prime component of BCAA, branch chain amino acids certainly comes to mind amongst those who frequent ergogenic aids. This raises a valid question whether or not BCAA status represents a valid marker of Ketogenesis. Unfortunately research on this remains very scarce or at least thus far at the time of writing this recap ~ unattainable at This Author's (AW) own attempts.

However this does not necessarily suggests that all other amino acids are immediately anti-ketogenic. ISOLeucine, valine, tryptophan, Threonine, and Tyrosine are thought to share both glycogenic and ketogenic pathways (AK Lectures).   

What which types of fatty acids contribute more towards ketogenesis?

Despite its seemingly simple question to address, it is unfortunately one of the many broad and difficult to answer as it demands subsequent enquiries; from firstly:

1. Determining which types of fats are more readily oxidised (and thus thought to be more readily ketogenic) versus those that are not.
2. Determining the fate and/or implications of ketone utilisation, if any that may yet be further confounded by not only the types of fats above, but also the metabolic state of the organism ~ whether active or sedentary, energy surplus or deficit.

One human study (Delany JP et al, 2000) suggests that after comparing various types of fats - Linoleate, Oleic, Lauric,  palmitic, and stearic fatty acids before then  stratified by # of Carbon-length chains ~ the medium chains are as widely expected the fastest to oxidise than the long-chain unsaturated fatty acids of oleic and linoleates. However, oleate was found to be more oxidised than linoleate. Where as longer chain stearic and palmitic appears to be the slowest to oxidise.

"In summary, this study is the most complete investigation to date of the oxidation of individual fatty acids in humans. Laurate, a medium-chain fatty acid, was the most highly oxidized fatty acid, followed by the unsaturated fatty acids; the long-chain saturated fatty acids were the least oxidized. For the saturated fatty acids, oxidation was inversely related to carbon length. Of the unsaturated fatty acids, the n−3 fatty acid linolenate was the most highly oxidized and linoleate was the least oxidized. " ~ DeLany JP et al. 2000

Next, we need to then determine markers of ketone utilisations especially amidst exercise / energy expenditure conditions. Interestingly, ALA (alpha linoleic acid or plant based derivative of PUFA Omega-n3)  appear to be moderately ketogenic, though to a lesser extent than MCT / medium chain triglycerides.

"It turns out that ALA is moderately ketogenic (Likhodii et al., 2000;Dell et al., 2001)not as ketogenic as medium chain fatty acids but more ketogenic than other common dietary long chain fatty acids. ALA is ketogenic because it is the most easily beta-oxidized of the common dietary long chain fatty acids. " ~ Cunnane SC 2018

Another study (on rats) suggests that MUFA, compared to SFA feeding for 12 weeks does not affect ketone bodies, irrespective exercise and/or non exercising conditions.

"Reduction in circulating levels of ketone bodies was mainly induced by exercise training, mediated by adecrease in total hepatic ketone body synthesis activityas well as increased muscle ketone body use as measured by 3-ketoacyl CoA transferase activity." (Somboonwong J et al. 2015).

Nonetheless, we must strongly caution against the simplistic conclusion of reduced ketones in the serum equate overall utilisation. That is simply not the intended interpretation as the above study implored the importance at quantifying utilisation not just from the serum, but also how exercise and thereby muscular ketone usage markers (3-Ketoacyl CoA Transferase) are arguably - the more important mediator to such fuel use.

Interim WIP Conclusion

Understandably there awaits uncountable # of questions that are far too nuanced and cannot be condensed for pragmatic reading. Hence these may be subject for further revisioning at a later stage or possibly be split into separate articles at This Author's (AW) own timely discretion.

Your mileage to all scientific enquiring / experimentations remains entirely your own conviction to willingly indemnify yourself at your own responsibility. If this article nor this entire concept initiative is inadequate at addressing your enquiries and/or needs you'd be well advised to therefore seek outside and reputable advices at your willingly expense. Live-it-forward, AW.