Three amino acids walk into a bar….the adventures of supporting glutathione synthesis in fatty liver disease using serine & nicotinamide riboside
-Betsy Redmond, PhD
Betsy is colleague and friend going back to our days at Metametrix Laboratory. I am thrilled she’s contributed our professional blog this month. Betsy is a creative, in-depth thinker, always paying attention to current research. Altered glutathione and NAD+ metabolism are primary features of fatty liver disease. The unique, upstream approach taken by Mardinoglu and team to increase endogenous glutathione synthesis and reduce fatty liver (while supporting methylation balance) based on their work using personalized genome-scale metabolic models, is worth us considering in clinical practice. Additionally, if you are analyzing plasma amino acids on your patients, the glutamate/(serine + glycine) index developed by Gaggini et al could be a sensitive indicator identifying risk in FLD. ~DrKF
Three amino acids walk into the bar, some sort of altercation ensues, and they leave as glutathione…
Beloved by functional medicine practitioners far and wide, glutathione (GSH) is a major endogenous antioxidant. GSH neutralizes reactive oxygen species (ROS) by donating a hydrogen, after which it becomes glutathione disulfide (GSSG) (only about 10% is in the GSSG form). With coenzyme NADPH, GSSG is readily reduced to form two GSH molecules again. This happy dance happens over and over again in the body, as need demands.
Depletion of GSH can lead to, among other things, mitochondrial dysfunction and cell death. Conditions associated with GSH depletion are far and wide: neurodegenerative, pulmonary, immune, cardiovascular, age-related, liver disease and more. The three amino acids that comprise GSH are glycine, cysteine and glutamate. Specifically, glutamate and cysteine complex to make gamma-glutamylcysteine,; glycine is added to the C-terminal, and glutathione is born.
The amino acids needed to make glutathione can be taken up from circulating plasma as they are available.
The rate limiting amino acid in de novo synthesis of GSH in metabolic disease appears to be glycine.
Glycine also participates in protein synthesis, in detoxification reactions, and has anti-inflammatory, cytoprotective and immunomodulatory properties; it’s also an inhibitory neurotransmitter. Glycine was best described by Pérez-Torres as “the smallest non-essential, neutral and metabolically inert amino acid, with a carbon atom bound to two hydrogen atoms, and an amino and a carboxyl group.” It sounds like an impressive amino acid. Research has found plasma glycine, serine and glutamate concentrations to be consistently altered in metabolic diseases, as are plasma branch chain amino acids (BCAA), isoleucine, leucine, and valine. Plasma levels of glycine and serine are lower, while glutamate and BCAAs are elevated.
Glycine and serine (serine is converted to glycine for GSH synthesis) are spent making GSH, but glutamate is increased once GSH exits the cell and is transaminated by GGT, releasing glutamate into circulation.
Non‐alcoholic fatty liver disease (NAFLD) affects about 25% of the population. Its mildest form is hepatic steatosis which is defined as the accumulation of fat in the liver with no evidence of hepatocellular injury. Progression of liver disease goes from hepatic steatosis, NAFLD, to non‐alcoholic steatohepatitis (NASH). Up to 30% of people with NAFLD develop NASH, a serious illness with inflammation and scarring. If untreated, NASH progresses to liver cirrhosis, scaring of the liver, and hepatocellular carcinoma. Some 20% of people with NASH develops cirrhosis. Although these stages are presented as an orderly progression, clinically that is not always the case. Since the liver is a major metabolic organ, liver disease is associated with metabolic diseases, such as obesity, insulin resistance, type 2 diabetes (T2DM), and cardiovascular disease. NAFLD is common in those with T2DM even with normal liver enzymes, so it’s often hiding in plain sight, putting them at a higher risk for progression. Researchers have noted that an impaired redox balance–primarily a deficiency of NAD+ and GSH–is a main feature of NAFLD.
Currently, the standard recommendation for treatment is: weight loss with diet, exercise. Sometimes vitamin E supplementation is recommended. Biomarkers that could identify metabolic disturbances and NAFLD early, as well as supportive therapies, could be beneficial, and to that end I reviewed two current articles:
Mardinoglu, M., et. al. (2017) noted that patients with hepatic steatosis have depleted NAD+ and glutathione as a primary feature of the condition. Direct supplementation with glutathione may not be effective, as it is cannot enter small intestine cells intact. In personal communications, the researcher noted that, “To our knowledge, GSH is an active substance and metabolized mostly in the small intestine. In order to be taken up by the liver it has to be degraded to amino acids.” [I also wonder- since glutamate is elevated in metabolic disease, and a residue in the GSH tripeptide, might it be that providing glycine or serine to support synthesis could be preferred over glutathione? ~KF] Glycine has been found consistently low in those with hepatic steatosis. Glycine can be synthesized from serine. Adequate serine levels can increase glycine levels, and subsequently glutathione. (Incidentally, those on plant-based diets generally have a higher intake of non-essential amino acids including glycine and serine and may be why they have lower incidence of NAFLD.)
Homocysteine has been identified as a pathogenic feature of NAFLD in humans and alcoholic fatty liver disease in a rodent model. Using serine verses glycine (or glutathione) also supports one carbon metabolism in its metabolic journey to glycine. Glycine can be synthesized via the interconversion of serine through serine hydroxymethyltransferases with concomitant conversion of tetrahydrofolate (THF) into 5,10‐methylene‐THF. During the conversion of serine to glycine, an additional carbon unit is provided for one‐carbon metabolism.
After glycine, cysteine may become the limiting amino acid in glutathione production. L-cysteine can be provided efficiently with N-Acetylcysteine (NAC). Additionally, NAD+ is also needed to support fatty acid oxidation. Supplementation with serine and an NAD+ precursor, like tryptophan, niacin, or nicotinamide riboside (NR), along with NAC are expected to increase NAD+, glutathione, and fat oxidation in the liver, leading to lower levels of oxidative stress, resulting in lower hepatic steatosis and its progression. Adding carnitine may help with the full process of fatty acid metabolism.
To test the hypothesis Mardinoglu, M., et. al (2017) put mice on a Western diet and supplemented for two weeks (a long time for a mouse!) with serine, NAC, and nicotinamide riboside (a NAD+ precursor). Following supplementation, plasma glycine and serine were increased, along with decreased triglycerides and hepatic steatosis. Following the animal study, the researchers conducted a proof-of-concept human study (N=87) with hepatic steatosis using only serine. The participants received L-serine (200 mg/kg/day grams; ~20g/day) for 14 days. Plasma levels of liver enzymes (ALT, AST, ALP), and triglycerides all decreased, and MRI showed decrease hepatic steatosis. Simple support helped.
Given his results, while I might try just high dose serine first, adding NAC, nicotinamide riboside, glycine and carnitine also makes sense, as Mardinoglu’s research suggests.
The Glutamate/(serine + glycine) Index (you can calculate in practice this with a plasma amino acid panel)
Another key study by Gaggini, M. et. al. (2018) may help to identify who is most at risk. It included subjects with NAFLD, who were or were not obese, and evaluated if, and how, their plasma amino acids were altered, and the relationship with insulin resistance. Compared to healthy controls, those with NAFLD who were not obese had statically elevated levels of glutamate, isoleucine and valine, while those with NAFLD who were obese had significantly elevated levels of glutamate, all BCAA, and significantly lower levels of glycine and serine. Similar findings were noted with levels of inflammation. Glycine had a significant inverse correlation with hepatic insulin resistance, and glutamate had a positive correlation. The authors realized that glutamate, glycine and serine were consistently impacted by metabolic disease and developed the GSG-Index, Glutamate/ (Serine+Glycine). In their study the GSG-Index was found to be elevated in those with NAFLD, more elevated in NAFLD plus obesity, and highly correlated to liver enzymes. Control subjects had a mean GSG Index of .38. The association of the GSG-Index with increased liver enzymes was assumed to be due to increased glutathione transamination by GGT, releasing glutamate. The GSG-Index was also able to discriminate mild to severe liver fibrosis. The GSG-Index did not correlate with obesity without NAFLD. Older research has consistently found those with NAFLD have degradation of glycine and serine, with release of glutamate.
While these are smaller single studies, taken together these articles identify possible new tools in an overall assessment, and highlight the benefits of using functional nutrition and lifestyle medicine. This is the area where functional medicine really shines, paying attention to the details and supporting in the right spot.
Betsy Redmond, PhD, MMSc, RDN
Betsy Redmond, PhD, MMSc, RDN has worked at a functional diagnostic laboratory in content development and research for more than 10 years, and also runs her own private practice nutrition & consulting business. She is well versed and experienced in nutritional, microbiome and environmental toxin testing, was one of the researchers of the first clinically available fecal microbiome tests, and a contributing author to Laboratory Evaluations for Integrative and Functional Medicine.