Some of my thoughts on the matter.
In the gut lactate is usually
rapidly metabolised to butyrate. An imbalance of lactate producing versus utilising bacteria may promote lactate accumulation; as may an
excessively low pH. This classically occurs in
short bowel syndrome where Lactobacilli and Leuconostoc are increased, and others such as Clostridium and Bacteriodetes decreased. Common bacteria in
probiotics which can produce D-lactate include lactobacillus acidophilus, plantarum and bulgarius.
In CFS research the Sheedy culture-based study found increased levels of D-lactate producing Streptococcus and Enteroccoccus. The most recent gut microbiota study using updated methodology (16s rRNA gene sequencing) found Strep levels were usually below 5% of total bacteria, but up to 15-60% in a few patients. These results may suggest an increased potential to generate D-lactate in
some with CFS.
I think increased gut lactate production in some with CFS may relate to SIBO. This would allow bacteria increased access to dietary carbohydrate which can be metabolised to L/D-lactate. There was
one study ages ago which found SIBO in 77% of CFS patients (31), and its eradication led to symptom improvement. Gut bacteria involved in SIBO were identified in
one study although the overall lactate metabolism is not clear. The main risk factors for SIBO are low stomach acid (HCl) and slowed intestinal motility or constipation. Anecdotally many people with CFS feel they have low HCL.
Gastric emptying is also slow in CFS which might also imply upper intestinal motility is too in some people.
There is still no study directly measuring D-lactate levels in ME/CFS. Other studies finding increased lactate levels in muscles upon exertion are consistent with impaired oxidative energy metabolism and a shift toward anaerobic metabolism. This may similarly explain the increased brain lactate in CFS. Hence in these studies the lactate is not likely of gut origin. However some people with CFS may have increased D-lactate levels; although probably not severely high.
I think removing SIBO is probably key (improve HCl and digestion?). You could avoid most lactic acid bacteria (LAB) probiotics and
FOS which may promote more lactate accumulation. However
some pre and probiotics may be able to improve D-lactate acidosis; perhaps mainly by boosting Bifidobacteria. Carbs could be removed initially then you could boost butyrate-producing bacteria (e.g. Roseburia, Feacalibacterium and Blautia coccoides) by consuming lots of vegetables, fruit, fibre and resistant starch.
It is worth remembering that whilst current probiotics are typically based around lactate-producing Lactobacilli bacteria, this genus should be a very minor component of the normal gut microbiota. Like with many things in science what we first learn to appreciate attracts most research attention and becomes a bias exploited by mass marketing.