If you spend enough time reading up on B3 vitamins, you’ll inevitably come across a bunch of terms that start with methyl: methyl group, methyl donor, methylation. But before you get caught up in the hype surrounding the potential dangers of methyl donor depletion, it’s worth learning more about the existing research around B3 vitamins and methylation.

Historically, concerns about B3 vitamins and methyl donors have stemmed clinical studies of high-dose niacin supplementation. So far there is no evidence from human or animal studies that nicotinamide riboside reduces methyl donors enough to contribute to any metabolic or other physiological problems.

Making Sense of Methylation: What Does It Mean for Your Cells and Health?

Many metabolic processes involve a chemical reaction known as methylation. These methylation reactions involve the transfer of a “methyl group”—a small collection of carbon and hydrogen atoms (CH3)—from one molecule to another. The molecule on the receiving end of this transaction is said to be “methylated” while the molecule giving up a methyl group is called a “methyl donor.”

In extreme circumstances, the body’s supply of methyl donors can become depleted, leading to metabolic and physiological dysfunction. Some people have genetic variations in the MTHFR gene that can disrupt how the body makes more methyl donors.1 Not getting enough folate, methionine, betaine, or choline in the diet can also lead to methyl donor deficiencies,2 though this is not typically a problem for people who eat a balanced diet.

More recently, scientists have raised concerns over consuming high levels of methyl donors through supplementation.3, 4 Getting too much or too little of the methyl donor folate during pregnancy can negatively affect fetal development.3, 5 Newer studies are also beginning to tease apart the complex relationship between methyl donor supplementation, the gut microbiome, and overall gut health.6, 7

While maintaining a sufficient supply of methyl donors is important, these findings show that excessive intake of methyl donors can be too much of a good thing. Our bodies are equipped to maintain balance, using multiple dietary sources of methyl donors and redundant pathways to maintain healthy methyl donor levels. Large changes to this system in either direction can therefore have unintended outcomes.

Looking to the Liver for Answers: Biomarkers of Methylation

Several biomarkers can help track what’s happening with methyl donors in the body. The amount of homocysteine in circulation in the blood is one of the most common. Homocysteine levels go up as methyl donors get used up. Blood levels of methyl donors themselves, including methionine and betaine, can also be measured.

Markers of liver health can also provide insight into the state of methyl donors in the body. Many chemical reactions – including methylation reactions – happen in the liver. Methyl donors are necessary for maintaining normal metabolism and keeping the liver functioning properly.2 Blood levels of the liver enzymes aspartate aminotransferase (AST) and alanine aminotransferase (ALT) provide a common measure for liver health: elevated levels of these enzymes are a sign of liver damage. Accumulation of liver fat can also be a sign that low methyl donor levels are disrupting metabolism.

Digging into the Data: B3 Vitamins and Methylation

The B3 vitamins—niacin (nicotinic acid), nicotinamide, and nicotinamide riboside—eventually become methylated to remove waste products from the body. Supplementation with B3 vitamins can increase levels of these methylated B3 byproducts in blood plasma and urine.8, 9 However, methylated waste products are normally found in the urine of healthy individuals, and especially low levels can be a sign of a vitamin B3 deficiency.10

Because B3 vitamins are methylated by the liver, there has been some speculation that very high doses of B3 vitamins could put a strain on the body’s methyl donors and even lead to methyl donor depletion. Historically, a handful of severe individual case studies have linked high-dose niacin supplementation with liver dysfunction or failure.11 More recent clinical trials have linked high-dose niacin supplementation to unfavorable changes in liver health and methyl donor levels. One study measured increased homocysteine levels in dyslipidemic men who consumed 2 grams of niacin per day for 8 weeks.12 Another measured increased ALT and AST levels in sickle cell anemia patients taking 1.5 grams of niacin per day for 12 weeks.13

A head-to-head study compared the effects of smaller 300 mg/day doses of niacin and nicotinamide in healthy adults.8 These two B3 vitamins had different effects on blood plasma levels of homocysteine. Nicotinamide significantly increased homocysteine levels while niacin had a much smaller, non-significant effect. Although both molecules decreased blood plasma levels of the methyl donor betaine, nicotinamide had a much stronger effect. These findings reveal that despite being lumped together as B3 vitamins, these molecules do not always have the same effects on our bodies.

So far two published clinical trials of nicotinamide riboside have measured outcomes associated with liver health. The first saw no change in blood AST levels or blood ALT levels after six weeks of supplementation with 1,000 mg/day nicotinamide riboside.14 The second trial administered 2,000 mg/day nicotinamide riboside to obese men for 12 weeks.9 They observed a trend toward lower ALT and a tendency for NR to reduce fat deposition in the liver. These findings suggest that nicotinamide riboside supplementation is not likely stressing the methyl donor system enough to cause physiological problems in the liver.

Untangling the Evidence: Not All B3s Are Created Equal

When examining the available evidence about B3 vitamins, methyl donors, and liver health, it’s also important to keep in mind that the three B3s are not equivalent. Not only do the B3 vitamins use different biosynthetic pathways to create NAD+, but they’ve also shown unique pharmacokinetic properties in mouse liver tissue.15 And only niacin is known to cause flushing at high doses.

Nicotinamide riboside has a much larger molecular weight than the other B3 vitamins, which means that their milligram doses are not directly comparable. Two grams of niacin or nicotinamide is not equivalent to two grams of nicotinamide riboside. These doses deliver very different numbers of molecules, and this matters because it’s the total number of molecules and not their overall weight that determines how many methyl donors get used. To get the same number of molecules found in two grams of niacin—and thus put a similar strain on your methyl donor system—you would need to ingest nearly five grams of nicotinamide riboside. The highest dose clinical trial of NR to date (2 grams per day for 12 weeks) actually showed trends toward improved liver health.9

While there is evidence that niacin and nicotinamide can affect the body’s methyl donor resources, no such evidence has yet been established for nicotinamide riboside. In fact, preclinical models of liver dysfunction16-18 and a recent clinical study9 all point toward nicotinamide riboside’s potential to support rather than harm liver health, which is an important physiological marker for the state of our methyl donor system. As research in this area advances, we expect to learn even more about nicotinamide riboside’s unique effects on our cells and our health.

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