Hydroxy- and Oxoacid Digestion

KEY Fermenters: Oxidation or fermentation by gut bacteria producing acid; Use: Utilisation or assimilation by gut bacteria; Common: Combined use and O/F for widespread, moderate and minor gut colonisers (total bacteria: 500);

hydroxyacids	Common	Species examples	All consumers	Prevalence in food	Human digestion, metabolism, interactions	Structure
α-Ketobutyrate	6	Anaerotruncus colihominis; Gordonibacter urolithinfaciens; Intestinibacter bartlettii; Lactiplantibacillus plantarum; Megasphaera elsdenii; Stomatobaculum longum;	26	Detected in lima beans, lettuces, some cabbages, and grass pea.	Produced from crystathione, an intermediate in the biosynthesis of cysteine; metabolite of threonine, methionine and homocysteine degradation. The ketobutyrate is transported to the mitochondria where it is converted to propionyl-CoA before entering the citric acid cycle.
α-Ketovalerate	4	Anaerotruncus colihominis; Gordonibacter urolithinfaciens; Lactiplantibacillus plantarum; Megasphaera elsdenii;	34	Detected in ducks, chickens, pigs and various herbs (e.g., sweet trefoil).	Has been detected in human blood, but is not involved in amino acid biosynthesis, although norleucine is derived from this ketovalerate in Serratia marcescens.
Glycerate	0		24	From peanuts, grapes, tomatoes, some bananas, cows milk, and apples.	It has been reported that dietary fat enhances small intestine metabolism of fructose, which releases glycerate into the blood stream. High and constant levels induce glucose intolerance and damage pancreatic islet cells, leading to T2DM (Wu2022).
DL-Glycerate	6	Citrobacter farmeri; Citrobacter freundii; Citrobacter koseri; Cronobacter sakazakii; Enterobacter cancerogenus; Serratia marcescens;	57	From peanuts, grapes, tomatoes, some bananas, cows milk, and apples.	It has been reported that dietary fat enhances small intestine metabolism of fructose, which releases glycerate into the blood stream. High and constant levels induce glucose intolerance and damage pancreatic islet cells, leading to T2DM (Wu2022).
Glycolate	2	Paraclostridium bifermentans; Serratia marcescens;	21	Occurs in all green plants and is a product of photorespiration.	Rapidly absorbed by hepatocytes and is usually interconverted with glycine. Glycolate has been linked to oxalate biosynthesis (Danpure2001).
Levulinate	0		14	From fruits, green vegetables, chickens, root vegetables, and cereals (rice, wheat bread). Also a food additive (as esters or calcium salt).	Calcium levulinate is often used as an injectible calcium supplement. Levulinate has been detected in urine, along with its reduction metabolite, 4-hydroxyvalerate. There is a suspicion that the levulinate is metabolised to the alcohol in the liver (Harris2011).
Quinate	6	Enterobacter hormaechei subsp. hormaechei; Klebsiella oxytoca; Klebsiella pneumoniae; Klebsiella quasipneumoniae subsp. quasipneumoniae; Lactiplantibacillus plantarum; Raoultella ornithinolytica;	48	Broadly abundant in plants, with higher levels in coffee beans, tobacco leaves, carrot leaves, apples, peaches, pears, plums and vegetables. Often presented as chlorogenic acid (quinate esterified with caffeic acid).	Many people ingest ~0.5g of quinate derivatives/day, but only a limited amount is absorbed in the small intestine. Most of it is hydrolysed by gut bacteria in the colon (Clifford2020). Free quinate is poorly absorbed. Serum concentrations increase in patients with chronic kidney disease (Toyohara2010).
Pyruvate	46	Acetivibrio ethanolgignens; Anaerotignum lactatifermentans; Anaerotruncus colihominis; Bacteroides caccae; Bacteroides stercoris; Bilophila wadsworthia; Blautia hydrogenotrophica;	389	Apples are high in free pyruvate (~0.5 g/apple); fermented food items, such as cheese, beer and wine also contain higher-than-normal levels.	Free pyruvate is readily absorbed by humans, and the ingested sodium salt at 0.1 g/Kg increases blood pH (alkalosis; Olek2014). Can be utilised in glycolysis or the TCA cycle. Too much ingested pyruvate can cause diarrhoea and increased LDL cholesterol (Onakpoya2014).
Methyl-Pyruvate	4	Anaerotruncus colihominis; Gordonibacter pamelaeae; Intestinibacter bartlettii; Rothia mucilaginosa;	61
α-Hydroxybutyrate	1	Rothia aeria;	26		2-Hydroxybutyrate is released as a byproduct when cystathionine is cleaved to cysteine and is produced when threonine and methionine are catabolised. High serum levels is a potential biomarker for increased glucose intolerance.
3-Hydroxybutyrate	7	Citrobacter freundii; Enterobacter asburiae; Enterobacter cloacae; Intestinimonas butyriciproducens; Klebsiella quasipneumoniae subsp. quasipneumoniae; Megasphaera elsdenii; Raoultella ornithinolytica;	154	Eggs and milk. Sometimes taken as a supplement.	Levels of 3-hydroxybutyrate are raised during ketosis and the metabolism of fats in the liver. Used as an energy source by the brain when glucose levels are low.
g-Hydroxybutyrate	0		15	A minor product of alcoholic fermentation, especially in fruity white wine production.	A naturally occurring neurotransmitter (weak GABA receptor agonist). Used to treat narcolepsy, and is a nervous system depressant.
Lactate	16	Anaerobutyricum hallii; Bacteroides pyogenes; Clostridium sporogenes; Clostridium sporosphaeroides; Coprococcus catus; Desulfitobacterium hafniense; Desulfovibrio desulfuricans;	167	Typically from LAB fermented foods, such as kefir, some cheeses and sour milk/cream. Also in sourdough and some beers.	Produced during glycolysis. Blood resting concentrations are typically ~1-2 mM, but can reach 25 mM after intensive exercise. Humans produce L-lactate from pyruvate. Serum lactate can be reconverted back to pyruvate and used as an energy source.Serum concentrations decline in patients with chronic kidney disease (Toyohara2010).
L-Lactate	3	Anaerostipes caccae; Coprococcus catus; Rothia aeria;	29	See 'Lactate'	See 'Lactate'
D-Lactate	0		3	Fermented foods, including yoghurt, sauerkraut and some cheeses, can contain appreciable amounts of D-lactic acid.	Poorly absorbed carbohydrates can result in gut fermentation and increased concentrations and absorption of D-lactic acid. Excess acid can lead to acidosis. Oxidised to pyruvate in the mitochondria with D-2-hydroxy-acid dehydrogenase.
DL-Lactate	18	Anaerostipes caccae; Anaerotignum lactatifermentans; Citrobacter farmeri; Citrobacter freundii; Citrobacter koseri; Coprococcus catus; Cronobacter sakazakii;	173	Produced during fermentation, often by a mixture of D- and L-lactate forming lactobacilli.	See above
Methyl-D-Lactate	1	Intestinimonas butyriciproducens;	8