The name “B-Vitamins” refers to a group of essential micronutrients that share some characteristics, however, chemically they are totally different substances.
By now we know 8 different vitamins that are members of this category: Vitamin B1 (Thiamine), B2 (Riboflavin), B3 (Niacin), B5 (Pantothenic Acid), B6 (Pyridoxine), B7 (Biotin), B9 (Folic Acid) as well as Vitamin B12 (Cobalamin). One thing they have in common is their water solubility, and many people believe that excessive amounts simply get excreted through urine. Also, for some B-Vitamins, no upper tolerable upper intake level can be defined yet, which makes it look like it is impossible to overdose. However, this might be a dangerous misconception, and here is why…
Thiamin — Vitamin B1
In the case of Vitamin B1, the general conception holds true for the most part. No serious adverse effects were reported even at doses up to 50 mg per day or more which is around 42 times higher than the RDA for an healthy adult. Some reports using intravenous Thiamine showed allergic reactions, but the amount needed is higher than a normal human body can absorb. However, large oral doses can cause digestive issues.¹
Riboflavin — Vitamin B2
Some of you may have experienced that taking a multivitamin or Vitamin-B complex product can change the color of your urine. This phenomenon is mostly caused by high amounts of Vitamin B2, which indicate that excessive amounts get flushed out of the body without causing any serious harm. While there is no upper tolerable intake level, high amounts of Vitamin B2 in your system can cause more adverse effects than just brightly colored urine, such as itching, numbness, burning or prickling sensations and sensitivity to light.²
Niacin — Vitamin B3
Vitamin B3 plays an essential role in human energy metabolism and is often used to maintain healthy cholesterol levels. However, higher intakes of this vitamin can cause a flushing sensation, which means that the skin of your face and hands can turn red and itchy, but this is not considered a serious side effect. Chronically high intakes, however, are correlated with multi-organ failure and may decrease insulin sensitivity¹,². The tolerable upper intake level is set at 30 mg per day for adults¹. So, it is important to be aware of how much Vitamin B3 you take through your supplements.
Pantothenic Acid — Vitamin B5
Vitamin B5 seems to be quite safe in terms of serious adverse effects. However, large doses can cause gastrointestinal side effects like nausea and heartburn. No tolerable upper intake level exists for Vitamin B5.¹,³
Pyridoxine — Vitamin B6
Vitamin B6 is one of the more common vitamins among dietary supplements. This vitamin is needed for more than 100 enzymes in the body and is highly involved in protein metabolism and immune function. It also plays an important role in brain development in pregnancy and infancy⁴. However, too much of a good thing can be bad. Excessive intake of these nutritional supplements can cause severe and progressive sensory neuropathy characterized by loss of control of bodily movements. Other effects of high vitamin B6 supplementation include painful skin lesions, photosensitivity, nausea and heartburn¹,⁵.
…for some B-Vitamins, no upper tolerable upper intake level can be defined yet, which makes it look like it is impossible to overdose. However, this might be a dangerous misconception…
Recent studies have indicated that the form of intake plays a critical role. Usually, Vitamin B6 is taken as pyridoxine. However, it has to be converted into its active form pyridoxal-5-phosphate by the liver in order to serve its purpose. If you take too much pyridoxine, the liver may can’t keep up with the conversion and excessive pyridoxal could block the receptors for the active form of pyridoxal-5-phosphate⁶. The result would actually be symptoms of Vitamin B6 deficiency. The tolerable upper intake level for pyridoxine is set at 100 mg. However, because of the concerns above, the EU cut this value down to 25 mg per day for adults.
Biotin — Vitamin B7
Vitamin B7 is often used in beauty products because deficiency has shown to cause hair loss⁷. High amounts of Biotin can cause Vitamin B5 deficiency as both vitamins share the same receptor for uptake⁸. However, to this date no tolerable upper intake level has been defined¹.
Folic Acid — Vitamin B9
Vitamin B9 is crucial for the synthesis of genetic material and protein metabolism. It also plays an important role in the elimination of specific toxins in the body⁴. High levels of folic acid can mask a vitamin B12 deficiency and additional amounts can exacerbate anemia caused by vitamin B12 deficiency. However, if the vitamin B12 absorption is sufficient, folic acid intakes up to 250% RDA seem to be save¹.
Cobalamin — Vitamin B12
Vitamin B12 is often used by vegans and vegetarians because the main sources of this vitamin are animal products. No tolerable upper intake level has been defined for Vitamin B12, which means that there is not enough evidence to support serious adverse effects caused by higher intakes¹. However, high blood levels of Vitamin B12 could be a sign of serious, potentially life-threatening disease. Some diseases, like leukemia, can cause impaired breakdown and excretion of Vitamin B12. Therefore, increased levels are used as diagnostic criteria for those diseases⁹.
If you thought that you could just take high amounts of B-Vitamins without risking adverse effects, you might have been wrong. As this seems to be the case for some, it’s a total misconception for others. Also, if you already have enough of a specific vitamin through your diet, why wasting money on supplements? Measuring your nutrient levels and only taking supplements you really need can save your wallet and your health.
- Institute of Medicine (US) Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, 1998. Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline. National Academies Press (US).
- Ellsworth, Marc A., et al. “Acute liver failure secondary to niacin toxicity.” Case reports in pediatrics 2014 (2014).
- Hendler, Sheldon Saul, and David M. Rorvik. PDR for nutritional supplements. Thomson Reuters, 2008.
- Coates, Paul M., et al. Encyclopedia of dietary supplements. No. Ed. 2. Informa Healthcare, 2010.
- Gdynia, Hans-Jürgen, et al. “Severe sensorimotor neuropathy after intake of highest dosages of vitamin B6.” Neuromuscular Disorders 18.2 (2008): 156–158.
- Vrolijk, Misha F., et al. “The vitamin B6 paradox: Supplementation with high concentrations of pyridoxine leads to decreased vitamin B6 function.” Toxicology in Vitro 44 (2017): 206–212.
- Zempleni, Janos, Yousef I. Hassan, and Subhashinee SK Wijeratne. “Biotin and biotinidase deficiency.” Expert review of endocrinology & metabolism 3.6 (2008): 715–724.
- Said, Hamid M., et al. “Biotin uptake by human colonic epithelial NCM460 cells: a carrier-mediated process shared with pantothenic acid.” American Journal of Physiology-Cell Physiology 275.5 (1998): C1365-C1371.
- Ermens, A. A. M., L. T. Vlasveld, and Jan Lindemans. “Significance of elevated cobalamin (vitamin B12) levels in blood.” Clinical biochemistry 36.8 (2003): 585–590.