• Open Access

Serum D-Lactate Concentrations in Cats with Gastrointestinal Disease

Authors


  • Prior abstract publication: This work was presented as an oral abstract at the 2009 American College of Veterinary Internal Medicine Forum & Canadian Veterinary Medical Association Convention, Montreal, Quebec, Canada (Serum D-lactate concentrations in cats with gastrointestinal disease).

Corresponding author: R.A. Packer, Department of Basic Medical Sciences, Purdue University, Lynn Hall, 625 Harrison Street, West Lafayette, IN 47907-2026; e-mail: rpacker@purdue.edu.

Abstract

Background

Increased D-lactate concentrations cause neurological signs in humans with gastrointestinal disease.

Hypothesis/Objectives

To determine if serum D-lactate concentrations are increased in cats with gastrointestinal disease compared to healthy controls, and if concentrations correlate with specific neurological or gastrointestinal abnormalities.

Animals

Systematically selected serum samples submitted to the Gastrointestinal Laboratory at Texas A&M University from 100 cats with clinical signs of gastrointestinal disease and abnormal gastrointestinal function tests, and 30 healthy cats.

Methods

Case-control study in which serum D- and L-lactate concentrations and retrospective data on clinical signs were compared between 30 healthy cats and 100 cats with gastrointestinal disease. Association of D-lactate concentration with tests of GI dysfunction and neurological signs was evaluated by multivariate linear and logistic regression analyses, respectively.

Results

All 100 cats had a history of abnormal gastrointestinal signs and abnormal gastrointestinal function test results. Thirty-one cats had definitive or subjective neurological abnormalities. D-lactate concentrations of cats with gastrointestinal disease (median 0.36, range 0.04–8.33 mmol/L) were significantly higher than those in healthy controls (median 0.22, range 0.04–0.87 mmol/L; = .022). L-lactate concentrations were not significantly different between the 2 groups of cats with gastrointestinal disease and healthy controls. D-lactate concentrations were not significantly associated with fPLI, fTLI, cobalamin, folate, or neurological abnormalities (> .05).

Conclusions and Clinical Importance

D-lactate concentrations can be increased in cats with gastrointestinal disease. These findings warrant additional investigations into the role of intestinal microbiota derangements in cats with gastrointestinal disease, and the association of D-lactate and neurological abnormalities.

Abbreviations
HPLC

high-pressure liquid chromatography

NQ

not quantifiable

PLI/fPLI

pancreatic lipase immunoreactivity/feline pancreatic lipase immunoreactivity

SD

standard deviation

TLI/fTLI

trypsin-like immunoreactivity/feline trypsin-like immunoreactivity

D-lactic acidosis was first described in ruminants with grain overload in 1965,[1] and has since been well documented and characterized in ruminants.[2-15] Over a decade after the discovery in ruminants, D-lactate encephalopathy was first described in human medicine in a case of jejunoileostomy and intestinal resection.[16] The vast majority of the veterinary literature on D-lactic acidosis pertains to diarrheic calves and ruminant models of grain (adult ruminants) or milk (neonatal ruminants) overload.[6, 8, 9, 12, 14, 15, 17-19] The most common underlying conditions in human D-lactate encephalopathy are related to gastrointestinal tract abnormalities, including short-bowel syndrome from intestinal resections or jejunoileal (gastric) bypass surgery, malabsorptive syndromes, and exocrine pancreatic abnormalities.[20-22] The strong association with gastrointestinal tract alterations can be explained by mammalian physiology. The D-enantiomer of lactic acid is not normally present in appreciable quantities in serum from mammals.[23] D-lactate in these species must come either from exogenous sources, such as bacterial fermentation of carbohydrates in the gastrointestinal tract, or alternate metabolic pathways of synthesis, such as can occur in certain metabolic diseases and intoxications.[23-27]

Recently D-lactate encephalopathy has been identified in 2 cats with gastrointestinal disease, one with exocrine pancreatic insufficiency and probable intestinal bacterial overgrowth (now also termed small intestinal dysbiosis),[28] and the other with pancreatitis (O'Brien DP, unpublished data). Other reports of D-lactic acidosis in small animals remain sparse, and underlying conditions previously reported with D-lactic acidosis include diabetic ketoacidosis[25] and propylene glycol ingestion in cats.[24] The pathophysiology, clinical signs, and metabolic pathways associated with D-lactic acidosis in small animals have not been well described. The lack of reported cases of D-lactate acidosis in small animals might in part reflect the difficulty in detecting D-lactate in serum in these species. Routine clinical monitors only measure L-lactate, rather than total (D + L) lactate or D-lactate alone.[29, 30] As a result, D-lactic acidosis is not readily detected, and can be identified as a result of unexplained metabolic acidosis with an increased anion gap.

The clinical presentation of D-lactic acidosis in humans is characterized by episodes of neurological impairment, specifically encephalopathy, in which altered mental status, slurred speech, ataxia and gait disturbances, disorientation, behavior changes, nystagmus, and vision abnormalities may occur.[20] Clinical signs reported in ruminants, and the limited information in cats, are similar to the presentation in human D-lactic acidosis (DP O'Brien, unpublished data).[14, 15, 28]

The purpose of the present study was to measure serum D-lactate concentrations in cats with gastrointestinal disease, and to evaluate whether or not D-lactate concentrations were correlated with signs of encephalopathy or with other gastrointestinal function test results. Our hypothesis was that cats with gastrointestinal disease have increased serum D-lactate concentrations compared to healthy controls, and that D-lactate concentrations are correlated with encephalopathy or other neurological deficits. The results of this study should promote awareness of D-lactic acidosis in small animal patients with gastrointestinal disease, advance the current understanding of D-lactic acidosis in cats, and provide a reference range for serum D-lactate concentrations from a population of healthy cats.

Materials and Methods

Sample Population

Surplus serum samples from 100 cats with abnormal gastrointestinal function test results (fPLI, fTLI, cobalamin, or folate) that were submitted to the Gastrointestinal Laboratory at Texas A&M University were systematically selected for the study from the most recent consecutive incoming samples that fit the inclusion criteria. Because established reference ranges for D-lactate are not available, serum samples from 30 healthy cats were used as controls. Questionnaires were sent to veterinarians of the cats included in the sample population to survey for gastrointestinal and neurological signs, and results of other laboratory tests. Cats were excluded from the study if they had concurrent diabetes mellitus or if the questionnaire was not completed. Additional serum samples were selected to replace any excluded cases. The 30 healthy control cats were reported to be free of clinical signs of disease, and had normal complete blood count, serum biochemical profile, and serum cobalamin, folate, fTLI, and fPLI concentrations.

Lactate Measurements

Serum D- and L-lactate concentrations were measured by stereospecific high-pressure liquid chromatography using a 3-mm octadecylsilane column coated with N,N-dioctyl-L-alanine as the chiral selector for the separation of lactic acid enantiomers. Detailed methodology is described in Omole et al.[31] For the quantification of D-lactate concentrations below the reliable range of primary assay, secondary standards were used in which the lower limit of quantification of the assay was set to 0.05 mmol/L. Concentrations below the lower limit of quantification are reported as not quantifiable (NQ). For statistical analysis, NQ values were arbitrarily considered 0.04 mmol/L, to avoid artificially increasing variance.

Clinical Assessment

Clinical history and physical examination information was collected retrospectively through questionnaires submitted to the primary care veterinarian of each case. Questionnaires included a structured request for the following information: type (vomiting, diarrhea, hematochezia, tenesmus, melena, anorexia, weight loss) and history (duration, frequency, diagnostic testing, and results) of the gastrointestinal signs; pertinent serum biochemical, blood gas results, or both; treatments administered; tentative diagnosis (if available); and neurological signs. The description of neurological signs included an assessment of mental status, according to the following definitions that were provided: (1) bright, alert and responsive (actively exploring and interacting with the clinician, owner and environment), (2) lethargic (will pay attention to their environment and respond appropriately but does not actively seek interaction and would rather withdraw), (3) obtunded (does not pay attention or interact with their environment unless stimulated; once responds, response is appropriate but sluggish), (4) stuporous (does not attend to their environment; only responds to strong stimuli such as pain and response is not appropriate), and (5) coma (no response even to painful stimuli). Additional neurological queries included whether or not abnormal behaviors (none, disorientation, pacing, aggression, insomnia, excess sleeping, head pressing, or others), seizures, weakness, ataxia, loss of balance, or any other neurological abnormalities or clinical signs were reported or observed.

Statistical Analysis

Data distribution was tested for normality by the Shapiro-Wilk test.1 Serum D-lactate and L-lactate concentrations and clinical signs were compared between 30 healthy cats and 100 cats with gastrointestinal disease using a Wilcoxon rank sum test.1 Of the 100 cats with gastrointestinal disease, D-lactate concentrations of cats with neurological signs were compared posthoc to D-lactate concentrations of cats with no neurological signs using a Wilcoxon rank sum test.1 This analysis was performed using only those cats with definitive neurological signs of ataxia, balance problems or nystagmus, with a duplicate analysis of those cats with other (subjective) signs that were probably neurological in origin. Association of D-lactate concentration with gastrointestinal dysfunction (cobalamin [ng/L], folate [μg/L], fTLI [μg/L], and fPLI [μg/L] concentrations) and neurological signs were evaluated after log transformation with multivariate linear and logistic regression analysis, respectively.1 Where cobalamin concentrations were out of the range of the assay (> 1,000 ng/L, or < 150 ng/L), values of 1,001 ng/L and 149 ng/L, respectively, were used in the statistical analysis to minimize the introduction of variance into the model. Statistical significance was considered < .05.

Results

A total of 100 cats with gastrointestinal disease and 30 healthy control cats were used in the analysis. Two cats with incomplete questionnaires and 3 cats with diabetes mellitus were excluded from the study and replaced with systematically selected serum samples as previously described. All 100 cats in the final study population had a clinical history of gastrointestinal signs and abnormal gastrointestinal function test results. All serum samples were collected within a 6-week period.

D-lactate concentrations of cats with gastrointestinal disease (median 0.36, range 0.04–8.33 mmol/L) were significantly higher than in healthy control cats (median 0.22, range 0.04–0.87 mmol/L; = .022; Fig 1). In contrast, L-lactate concentrations were not significantly different between the 2 groups. D-lactate concentrations were not significantly associated with serum concentrations of fPLI, fTLI, cobalamin, or folate or with the presence of neurological abnormalities; however, the cat with the highest D-lactate concentration (8.33 mmol/L, Table 1) was reported as lethargic, listless and weak.

Figure 1.

Concentration of D-lactate in serum of healthy cats and cats with GI disease. Data are presented as a box and whisker plots. The upper and lower edges of the box represent the 75th and 25th percentiles, respectively; the line within the box is the median value. Whiskers represent the range. Outliers are indicated by a closed diamond. Cats with GI disease had significantly higher concentrations of D-lactate (P = .022).

Table 1. Summary of serum D-lactate concentrations in cats with gastrointestinal disease
D-lactate Concentration (mmol/L)Number of Cats (n = 100)
  1. a

    D-lactate, 8.33 mmol/L; NQ, not quantifiable.

≥ 5.001a
4.00–4.990
3.00–3.990
2.00–2.991
1.00–1.999
0.05–1.0084
NQ (< 0.05)5

Four of the 100 cats with gastrointestinal disease had definitive neurological deficits (ataxia, balance problems or nystagmus). A total of 31 cats exhibited 1 or more subjective clinical signs that may be attributable to D-lactic acidosis and/or encephalopathy, including mentation and/or behavior changes, or other signs (Table 2). A total of 25 cats had lethargic (n = 23) or obtunded (n = 2) mentation. Serum D-lactate concentrations of cats with definitive (median 0.65, range 0.43–0.97 mmol/L) or cats with either definitive or subjective (median 0.48, range 0.04–8.33 mmol/L) neurological deficits were not significantly different from cats without neurological signs (median 0.29, range 0.04–2.54 mmol/L).

Table 2. Frequency of neurological signs reported in cats with gastrointestinal disease. A total of 31 of the 100 cats had an identifiable neurological sign
Clinical SignNumber of Cats (n = 100)
Altered mentation25
Weakness10
Ataxia or balance problems3
Somnolence3
Disorientation2
Aggression2
Pacing2
Nystagmus1
Vision difficulty1
Insomnia1

Diagnostic imaging or histopathological diagnoses were not available for any of the 4 cats with neurological signs. Histopathological diagnoses of gastrointestinal disease were only available in 3 cats, 1 each with (1) severe chronic interstitial pancreatitis and chronic lymphocytic enteritis, (2) lymphocytic plasmacytic infiltration of colonic mucosa, and (3) mild to moderate lymphocytic, plasmacytic gastritis, and duodenitis with helicobacter gastritis and moderate villous blunting of the duodenum. None of these 3 cats had increased D-lactate concentrations or neurological signs.

Discussion

Although the present study did not identify a relationship between D-lactate concentrations and individual gastrointestinal function tests, cats with gastrointestinal disease had significantly higher serum concentrations of D-lactate than cats without gastrointestinal disease. For the reference population, the serum D-lactate concentrations reported in 30 healthy cats in the current study comprise the largest group of healthy cats evaluated to date. These results are consistent with previous concentrations reported in a limited number of healthy cats (n = 3 and n = 6, respectively)[24, 28] as well as other species, including calves (n = 21),[4] goat kids (n = 35),[10] rats (n = 12),[32] and humans (n = 72).[21]

Mammals can synthesize/metabolize the L-lactic acid enantiomer from/to pyruvate via L-lactate dehydrogenase. In contrast, although recently D-lactate dehydrogenase has been identified in mitochondria of mammals,[33] the amounts do not appear to contribute significantly to D-lactate metabolism. Thus, D-lactate in cases of D-lactic acidosis must come either from metabolism through the methylglyoxal pathway, such as occurs in diabetes mellitus or propylene glycol toxicity, or from exogenous sources.[23-27] An increased carbohydrate load to the colon, leading to excessive bacterial fermentation, is one of the primary exogenous sources of D-lactate.[26, 34] Digestible carbohydrates are mainly absorbed in the small intestine. Carbohydrates that reach the colon, however, are subject to anaerobic fermentation by intestinal bacteria. Depending on the species, intestinal bacteria possess 1 or both of a D- or L-lactate-specific dehydrogenase and/or lactate racemase. Accordingly, D- and L-lactate are metabolic products of anaerobic fermentation of carbohydrates by the colonic microbiota.[26, 34]

The tendency for D-lactate to accumulate in serum is thought to be due, in part, to the lack of significant amounts of D-lactate dehydrogenase in mammalian cells, as well as competitive inhibition by acidic pH and/or L-lactate (produced by certain colonic bacteria in racemic mixture along with D-lactate), which interfere with D-lactate oxidation to pyruvate by D-2-hydroxy-acid dehydrogenase.[23, 27, 34, 35]

Similar to the pathophysiology of D-lactic acidosis in ruminants and humans, there is evidence for an association between increased D-lactic acid production and high carbohydrate load to the gastrointestinal tract in cats, as illustrated by the occurrence of D-lactic acid encephalopathy in a cat with exocrine pancreatic insufficiency and presumed intestinal bacterial overgrowth.[28] This association is further supported by an experimental study in which cats were fed a variety of 8 different high carbohydrate diets.[36] The results of this study indicate that cats have a poor ability to digest carbohydrates. D-lactate concentrations were 3–8 times higher in the feces of cats fed the high carbohydrate diets (range of means for the carbohydrate diets, 1.6–4.8 mmol/kg) than the low-carbohydrate control diet (mean [SD], 0.6 [1.1]). Serum D- and L-lactate concentrations, acid-base status, and clinical observations of the cats in their study population were not reported.

The clinical manifestation of D-lactic acidosis in humans includes recurrent episodes of encephalopathy and high anion gap metabolic acidosis.[20] Neurological signs observed commonly include an altered mental status (ranging from drowsiness to coma), dysarthria, and ataxia or other gait disturbances, whereas disorientation, weakness, aggression, nystagmus, hallucinations, and other neurological signs have been reported less frequently.[20, 21] In the present study, no association between serum D-lactate concentrations and neurological signs was observed. Possible explanations for the lack of association include the following: the number of cats with definitive neurological abnormalities was low, there was significant overlap of D-lactate concentrations between cats with and without neurological signs, and the threshold D-lactate concentration, above which neurological impairment occurs, may be higher than the typical concentration observed in our study population. For these reasons, a posthoc comparison of D-lactate concentrations among groups with and without neurological deficits is of little clinical importance in this study population. This comparison would more appropriately be evaluated in a prospective study designed specifically to compare D-lactate concentrations in cats with and without neurological signs, in which sample sizes within the neurological and control groups were more evenly distributed.

The specific threshold for expression of neurological signs in cats with D-lactic acidosis has not been established and warrants additional investigation. The threshold concentration in humans for expression of encephalopathic clinical signs from D-lactic acidosis is generally considered greater than 3.0 mmol/L.[20-22] In an experimental ruminant study the mean (SD) D-lactate concentrations in acidemic diarrheic calves with abnormal mentation, weakness, and abnormal palpebral reflex were 10.8 (3.6), 11.0 (3.6), and 10.0 (4.4) mmol/L, respectively, which were all significantly different than D-lactate concentrations in acidemic diarrheic calves without these clinical signs.[8] Only 1 cat in our study population had a D-lactate concentration above 3.0 mmol/L. This cat had a D-lactate concentration of 8.33 mmol/L and was described as being lethargic, listless, and weak. The cat with the second highest D-lactate concentration (2.33 mmol/L) had no reported clinical signs of encephalopathy or neurological impairment. In a previous case report, a cat with exocrine pancreatic insufficiency and D-lactic encephalopathy had a D-lactate concentration of 17.7 mmol/L.[28]

The array of neurological signs observed in the cats in the present study were largely subjective, and one could argue whether or not the signs observed were attributed to encephalopathy rather than a general indication of systemic illness. Similarly, in early reviews of human D-lactate encephalopathy, authors indicate that additional historic cases of D-lactate encephalopathy might have been overlooked because of the vague nature of the clinical signs.[20, 21] With the exception of 4 cats with clear neurological deficits, clinical signs of the remaining 27 cats, although consistent with signs of encephalopathy, cannot conclusively be a result of neurological impairment. Prospective studies that include determination of blood pH, serum strong ion difference, D-lactate concentration, and controlled evaluation of neurological status (ideally at referral lefts with EEG capability) would make it possible to objectively characterize mild cases of encephalopathy.

There are several limitations inherent to this study. The study population was selected from banked serum samples of cats with confirmed abnormal gastrointestinal function tests, as performed through the Gastrointestinal Laboratory at Texas A&M University. It is possible that our study population is biased, such that cats with neurological impairment concurrent to gastrointestinal signs may have been less likely to have serum samples submitted; the presence of neurological signs or cats with a greater severity of illness may have diverted attention away from pursuing a gastrointestinal workup. Furthermore, although our analysis of D-lactate was performed in batched samples specifically for this study, clinical information was collected in a retrospective manner through questionnaires and medical records review, which precluded the use of a standardized means of assessing neurological dysfunction. This is of particular importance in the subjective assessment of encephalopathy (eg, neurological causes of mentation changes, versus lethargic mentation attributable to general systemic illness).

This study provides evidence that gastrointestinal disease is associated with increased serum D-lactate concentrations. An association between D-lactate concentrations and neurological signs was not identified in this study; however, because of limitations in study design, subjective signs of encephalopathy may have been overlooked. Also, neurological signs such as depression are likely multifactorial in origin and may preclude a clear relationship to D-lactate. Previous studies indicate that there is a threshold D-lactate concentration (approximately 3–4 mmol/L) required for the development of neurological signs in other species. The cat with a D-lactate concentration above this threshold showed clear signs of neurological impairment. Although at present the association between D-lactate concentrations and neurological signs is not fully elucidated in cats, D-lactic acid encephalopathy should remain a consideration in cats with gastrointestinal disease and concurrent neurological signs. Future studies should focus on monitoring clinical cases of feline gastrointestinal disease for serum D-lactate concentration, metabolic acidosis (typically normochloremic, high anion gap metabolic acidosis), and standardized neurological assessments, including potential signs of encephalopathy.

Acknowledgment

Funding: This work was funded by ACVIM Foundation grant 07-10D.

Footnote

  1. 1

    STATA/SE ver. 11.1, StataCorp, LP, College Station, TX