The Dark Side of Vitamins: When Too Much of a Good Thing Becomes Harmful

If you’ve ever checked out the supplement shelves at a drugstore, you might have noticed rows upon rows of vitamins. You might even have wondered if you’d need one too. Wouldn’t a multivitamin supplement make you healthier? Wouldn’t it give you radiant skin and boost your gut health? Better still, why not take two of them— the higher the dosage, the better, right?

If this thought has ever occurred to you, hit pause and read on before you buy yourself a multivitamin supplement—or two.

What are vitamins and what do they do?

Vitamins are compounds your body needs in small amounts to grow and work properly. There are 13 vitamins (Vitamins A, C, D, E, K, and 8 B-complex vitamins) and each of these vitamins has different roles to play in your body. For example, vitamin A helps to maintain healthy tissues and skin while improving your eyesight [1]. Vitamin C, on the other hand, is important for immune function and iron absorption [2].

Fat-soluble vs water-soluble vitamins: What is the difference?

Some of these vitamins dissolve in fat (Vitamins A, D, E, and K) while others dissolve in water. Fat-soluble vitamins are stored in fatty tissues and in the liver and can thus stay in the body for longer periods [3].

Water soluble vitamins (Vitamin C and B-complex vitamins) dissolve in water and excess is passed out with urine. They can’t be stored in the body for later use [4].

Do you need a multivitamin supplement?

As we already said, all of these vitamins are essential for a healthy body. Different vitamins are needed in different amounts and the required amounts vary depending on various factors such as your age, gender, and health status. How much of each vitamin you need per day is listed in the RDA (Recommended Dietary Allowance) tables [5].

Although your body needs vitamins for proper function, it can’t make most of them except for vitamins D and K, which are produced in small amounts [6]. This means you need to get your vitamin needs through your diet. A healthy diet composed of fruits, vegetables, whole grains, meat, and fish will provide almost all of these vitamins in the required amounts. Certain food types such as dairy products and breakfast cereals also come fortified with vitamins (meaning vitamins are added during production.)

However, given your busy lifestyle and less-than-healthy dietary practices, you might not get enough vitamins through your diet. Not getting enough vitamins can lead to vitamin deficiencies that can cause various health defects such as brittle nails, hair loss, or even depression [7], [8]. In such instances, your best bet would be vitamin supplements to fulfill your vitamin requirements.

Can you overdose on vitamins and why is it a bad thing?

In short, yes, you can. As we explained earlier, different vitamins have different RDA values as they are needed in different amounts. You can easily exceed this limit if you’re taking in these vitamins through multiple sources.

While at the first glance it may seem like higher vitamin doses give faster results, it’s not true. In fact, ingesting higher than required doses of vitamins can lead to hypervitaminosis—which is commonly known as vitamin toxicity. This is especially true for the fat-soluble vitamins which are stored in the body and get accumulated with each intake [9].

In some extreme cases, this might even lead to hospitalization or emergency care. For this reason, there are established daily upper limits for most of the vitamins, beyond which they can be dangerous.

Here we explain how different vitamins can cause adverse effects and in which doses they can be toxic.

Vitamin A Toxicity

Vitamin A toxicity can either be acute (ingesting high vitamin A doses over a short period) or chronic (taking in a large amount over a longer period) [9]. Oral vitamin A comes in two forms: Preformed vitamin A (which is found in animal food sources, fortified foods,  and supplements) and provitamin A (which is found in plant sources) [9]. Vitamin A toxicity is mainly due to pre-formed vitamin A. Plant-based foods usually don’t result in excess vitamin A.

RDA for vitamin A is 900 mcg (3,000 IU) per day for adult men and 700 mcg (2,333 IU) a day for adult women [10]. The upper limit for vitamin A is set at 3,000 mcg (about 10,000 IU) per day.

According to Mayo Clinic, a single dose higher than 200 000 mcg can cause symptoms such as nausea, vomiting, feeling dizzy and imbalanced, and vision problems [11].

Ingesting doses higher than 10 000 mcg per day over a long time can cause osteoporosis [12] and other bone problems [9]; liver injuries [13]; and other conditions such as diarrhea and nausea [11]. The World Health Organization (WHO) reports that chronic vitamin A toxicity can produce headaches, fatigue, mood changes, and anemia [14].

If an expecting mother ingests a vitamin A dosage of more than 10 000 IU per day (3 000 mcg/d), there’s a higher chance of them having birth defects [15].

Vitamin D Toxicity

Vitamin D comes from some animal-based food sources and it’s also produced in your body when you’re exposed to sunlight [16]. Vitamin D toxicity, however, usually results from supplement overuse, and not from sun exposure or your diet [17].

The Endocrine Society recommends a dosage of 15 mcg/d (600 IU/d) for adults aged 19-70 and a dosage of 20 mcg/d (800 IU/d) for adults over 70 [16]. An amount more than the upper limit of 50 mcg (2000 IU) per day is considered excess for vitamin D [10].

Hypervitaminosis D can result in calcium buildup in your blood (known as hypercalcemia) and it can lead to nausea, vomiting, and lethargy [17]. While this condition is not life-threatening, vitamin D toxicity can also lead to serious problems such as kidney failure, heart issues, and confusion [18]. Vitamin D toxicity can also cause stomach pain, polyuria (abnormally large amounts of urine), and extreme thirst [19].

Vitamin E Toxicity

Vitamin E is another fat-soluble vitamin that, if taken in high doses can accumulate in the tissues. Vitamin E is found naturally in nuts, seeds, fruits, and vegetables among other foods. However, vitamin E toxicity is mainly due to the overuse of vitamin supplements.

RDA for vitamin A is 15 mg/d for men and women [10]. (This is equal to about 22 IU from natural sources or 33 IU from synthetic sources.) The upper limit for vitamin E is set at 1000 mg (which is equal to 2200 IU of synthetic vitamin) [10]. However, Johns Hopkins Medical School reports that amounts higher than 400 IU of vitamin E can increase the risk of death [20]. Excess vitamin E can also cause serious bleeding disorders [21]. Vitamin E toxicity can also result in serious conditions such as kidney failure, heart failure, stroke, and hypothyroidism [21]. Other complications of hypervitaminosis E include weakness, nausea, vomiting, and diarrhea [21].

Vitamin K Toxicity

Vitamin K comes in three forms—K1 which is found in leafy vegetables, K2 which is found in animal foods, and K3 which is artificially produced [22]. Of these, two natural vitamin K forms are broken down quickly and are expelled with urine or stool [23]. Hence, they don’t usually cause toxic effects, and so there is no upper limit set for vitamin K [24].

The only reported adverse effects of excess vitamin K are mild stomach issues and skin rashes [25].

Synthetic vitamin K3, however, can be toxic and can cause allergic reactions, liver toxicity, hemolytic anemia, and jaundice [22] [24]. Therefore, this form of vitamin K has been banned from over-the-counter products in the US [24].

Vitamin C Toxicity

Vitamin C, also known as ascorbic acid, is a water-soluble vitamin. You can find high amounts of vitamin C in citrus fruits, certain vegetables, and fortified drinks. Daily vitamin C requirement for men is 90 mg/d and 75 mg/d for women [10]. The upper limit for vitamin C for adults is 2000 mg per day [26].

Although vitamins that dissolve in water rarely show toxicity, megadoses of 3000 mg and above have been shown to cause diarrhea, increased risk of kidney stones, and excess iron absorption in those who have a condition called hemochromatosis (excess iron buildup) [2].

Vitamin C overdose can also result in conditions such as headache, diarrhea, nausea, vomiting, and heartburn [26].

Vitamin B1 (Thiamin) Toxicity

Vitamin B1, also known as thiamin, is another water-soluble vitamin that is essential to maintain health. It is naturally found in meat, fish, whole grains, and thiamin-fortified foods. RDA for vitamin B1 is 1.2 mg/d for adult men and 1.1 mg/d for women [10].

Cases of thiamin toxicity have not been reported and thus, there is no upper limit for thiamin intake [27]. Excess vitamin B1 is thought to be excreted with urine without causing any harmful effects [27].

Vitamin B2 (Riboflavin) Toxicity

Vitamin B2 is also a water-soluble vitamin that is naturally present in dairy products, green vegetables, and meat. RDA for this vitamin is 1.2 mg per day for men and 1.1 mg per day for women [10].

Since vitamin B2 dissolves in water and only gets absorbed in the gut at a limited capacity, any excess vitamin will be removed with urine without any adverse effects [28]. Thus, there are no upper limits set for this vitamin. Studies have reported no toxicity even with a vitamin B2 intake of up to 400 mg per day for three months [29].

Vitamin B3 (Niacin) Toxicity

Vitamin B3, also known as niacin, is another B-complex vitamin that can be found in several foods of both plant and animal origin. It is also available as nicotinic acid or nicotinamide in supplements [30].

A dosage of 16 mg/day of vitamin B1 is recommended for men, while for women, a dosage of 14 mg/day is recommended [10]. The upper limit for this vitamin is 35 mg per day [10].

Although it’s a water-soluble vitamin, taking high-dose supplements for a long time can commonly result in toxicity symptoms such as redness, itching, or tingling of the skin [30]. Other symptoms like nausea, vomiting, and headaches have also been reported [31]. Serious complications such as liver failure can also occur in extreme cases [31].

Vitamin B5 (Pantothenic Acid) Toxicity

Vitamin B5 is an essential vitamin that can be found in almost all food sources [32]. RDA for vitamin B5 is 5mg per day for both men and women [10]. No upper limit has been established for pantothenic acid due to the lack of reported toxicity [32].

However, in some rare instances, supplementation with very high doses (10 g/day) has been associated with symptoms such as stomach disturbances and diarrhea [33].

Vitamin B6 (Pyridoxine) Toxicity

Several animal and plant foods contain vitamin B6 and it’s also found in multivitamin supplements. RDA for vitamin B6 varies depending on age as well as gender. For men and women aged 30-50 years, the RDA is 1.3 mg /d while the recommended dosage is slightly above that value for older people [10].

The upper limit for vitamin B6 is 100 mg per day [10]. You will not most likely ingest high vitamin B6 doses through diet, but you can overdose on this vitamin through supplements. Toxicity symptoms can occur if you take amounts higher than 1000 mg a day for a long time [34]. The most common symptoms include nerve damage in hands and feet, lack of control of body movements, numbness, nausea, and light sensitivity [35]–[37].

Vitamin B7 (Biotin) Toxicity

Biotin (vitamin B7) is a vitamin that can be found in fish, meat, eggs, and vegetables. It’s also popularly sold as a dietary supplement. RDA for vitamin B7 is 30 mcg per day for men and women and an upper limit is not known [10]. Since this is a water-soluble vitamin, it will be removed with urine if taken in excess amounts. However, high doses can result in insomnia, extreme thirst, and frequent urination [38].

Vitamin B9 (Folate) Toxicity

Vitamin B9 is available in the form of folate in various food sources but it’s more bioavailable in the form of folic acid which is found in supplements. Its RDA is 400 mcg/d for men and women and its especially recommended for pregnant women [10].

Folate toxicity is extremely rare but the upper limit for folic acid is established at 1000 mcg/d [39]. High folate doses can temporarily conceal vitamin B12 deficiencies. Thus this upper limit is there to avoid masking a vitamin B12 deficiency rather than to prevent folate toxicity [39]. 

Vitamin B12 (Cyanocobalamin) Toxicity

Vitamin B 12 is naturally present in some foods and is also included in nutritional supplements. RDA for vitamin B12 is 2.4 mcg daily for men and women and an upper limit is not known [10]. Although adverse effects have been rarely reported with higher doses, minor symptoms such as headache, nausea, diarrhea, vomiting, and weakness can occur after ingesting larger doses [40].

Conclusion

Vitamins are essential nutrients to maintain a healthy body. However, it is possible to overdose on them and this can cause various undesired effects which in some cases can be severe. It’s always a good idea to consult a nutritionist or a doctor before taking any vitamin supplements. Before you buy a supplement, check the ingredients list to see which vitamins are included and in which amounts. It’s also important to follow the instructions on the label of the supplement bottle. Moreover, make sure not to mix and match different supplements because you will likely ingest multiple doses of one vitamin that way.

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