Eight different vitamins make up the B complex, and they all play crucial roles in the body, such as producing energy, keeping our nervous system healthy, and supporting cell development. If eight sounds like a lot to keep track of, it might help to know that most research focuses on five in particular: thiamine (B₁), riboflavin (B₂), niacin (B₃), folate (B₉), and cobalamin (B₁₂).

The body has a limited capacity to store these vitamins, so regular consumption is needed to maintain adequate repletion and health. Humans get most of our B vitamins from animal and dairy sources, beans, eggs, nuts, seeds, and certain vegetables like leafy greens and legumes, as well as fortified grains and breakfast cereals. 

“A well-rounded, healthful diet provides adequate B vitamins for most people,” says gastroenterologist Joel Mason, senior scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) and professor at the Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy and Tufts University School of Medicine. “However, for certain segments of our society, a healthful diet may not suffice. Take, for example, the elderly, whose absorption of B₁₂ declines with aging, supplementation with B₁₂ may definitely be necessary. The same is true for people on vegan diets, those who are pregnant, or those who have had gastric bypass surgery.” 

Also, certain commonly used drugs, including the diabetes medication metformin and proton pump inhibitors such as omeprazole, inhibit B₁₂ absorption and can contribute to the risk of not meeting B₁₂ requirements. 

Plant-based foods do not contain cobalamin (B₁₂), which is why people who eat vegan diets may be susceptible to a deficiency. In most of the scenarios just mentioned, a multivitamin that contains 2.4 micrograms (mcg) of vitamin B₁₂ will fill the need. 

Adequate amounts of these essential nutrients are important for sustaining health, but don’t overdo it, Mason cautions, because more is not necessarily better in all instances. Excess amounts of certain vitamins can have adverse effects. 

One example is vitamin B₆, which is toxic in high doses. In the 1980s, taking large amounts of B₆ to treat perimenstrual pain became popular among women. The recommended daily allowance is about 2 milligrams (mg) for adult men and 1.6 mg for adult women, but some women were taking up to 200 mg a day. In many cases, it resulted in irreversible peripheral neuropathy, or permanent damage to the nerves in their arms and legs.

Another example is niacin (B₃), which is sometimes prescribed in large doses to treat high cholesterol. However, large amounts of niacin could cause severe flushing episodes, rashes, and itching.

B Vitamins and Pregnancy

In the United States, corn, rice, and wheat flours have been fortified with thiamine (B₁), riboflavin (B₂), and niacin (B₃) since the 1940s, which helps prevent the spread of diseases such as pellagra and beriberi. Folic acid, the synthetically produced version of folate (B₉), was added to the mandatory fortification process in the U.S. in 1998 to ensure pregnant people consumed enough of the vitamin to help prevent neural tube defects in developing fetuses. The neural tube develops into the brain and spinal cord of a fetus early in pregnancy, often before someone knows they have conceived.

Because as many as 50% of pregnancies are unplanned, the U.S. government is among approximately 70 countries that mandate fortifying grains with folic acid to prevent neural tube defects. Efforts like these to increase folate levels have resulted in as much as a 70% decrease in birth defects in the central nervous system. 

Folate and Cancer

Research over the past 30 years has produced compelling data that shows people who consume too little folate also have an increased risk of certain cancers. 

“Our research at Tufts, mostly focused on colon cancer, has shown in both animals and humans that those who chronically consume too little folate have a sizably increased risk of colon cancer, and perhaps also pancreatic and post-menopausal breast cancer,” says Mason. 

Mason’s research in animals has also suggested, however, that consuming too much folic acid might paradoxically increase the risk of certain cancers developing. And this is where controversy brews.

Through their research, Tufts scientists started to notice that while adequate levels of folate are good, a super-supply of folic acid yields more cancer in animals.

“Intuitively, this makes sense,” says Mason. “Folate is a fertilizer for cell growth. If you have a few cells floating around that contain mutations that could turn them into cancer cells, and mutated cancer and precancer cells multiply rapidly naturally, sprinkling a fertilizer like folate on them could likely make them multiply much more quickly.”

However, he adds, several large epidemiological studies have shown that the paradoxical cancer-promoting effect is not a widespread issue and, if the phenomenon actually exists, those high levels of intake are probably only achieved among people taking multiple sources of folic acid supplements.

Thiamine and Gastric Bypass 

Thiamine (B₁) deficiency is a poorly recognized complication of gastric bypass surgery, Mason says. “In the U.S. it used to be that the people with thiamine deficiencies were almost exclusively those with alcohol use disorder,” Mason recalls.

Malabsorption of this essential vitamin stemming from changes in the digestive tract after gastric bypass surgery, in combination with repeated vomiting and nausea after the surgery can result in patients appearing in the emergency room with listlessness, weakness, confusion, eye problems, and other symptoms. 

If not identified and treated, thiamine deficiency can result in beriberi, which can lead to irreversible nerve and brain damage and be life-threatening.

“Surgeons, ER clinicians, and others need to be aware to test for a thiamine deficiency if a patient who has had gastric bypass surgery presents with these symptoms. If there is a suspicion a deficiency exists, the individual should receive an injection of thiamine without waiting for the test results to return,” says Mason. “And if you suspect you may be deficient in or overconsuming B vitamins, contact your physician.”