The safety of micronutrients – Part 1: fat-soluble vitamins

Excessive intake of high-energy macronutrients and their potential consequences for people’s health are a problem in many industrial nations. In the case of micronutrients, however, epidemiological data suggests that over-supply is the exception rather than the rule across the globe. Many people are therefore concerned that their dietary habits are failing to provide them with a sufficient supply of essential nutrients such as vitamins and carotenoids. Given that intakes of some nutrients are below officially recommended levels, it might at first sight appear unnecessary to set upper intake levels for micronutrients. However excessive intake of food supplements and fortified foods in addition to normal diets could conceivably lead to intake levels which could potentially be considered harmful. It may be sensible to define an upper daily intake level at which the risk of adverse effects on health is unlikely, particularly in the case of fat-soluble vitamins which are stored in small quantities in the body.

Like recommended daily intakes (RDA), upper intake levels (UL) depend on numerous physiological factors (such as age, sex and health status), genetic peculiarities (gene variants) and lifestyle (general diet, physical activity, nicotine and alcohol consump-tion). There is generally a large gap between the amount of nutrients which the majority of the population must take to prevent deficiency symptoms and the upper level at which no adverse health effects are likely, and so the recommended nutrient levels in the blood can be achieved through diet without the risk of overdosing. For most micronutrients there are therefore considerable safety margins and tolerable upper intake levels are 10 times the recommended intake levels. Targeted intake of micronutrients, for example using food supplements, is considered safe at the officially recommended amounts.

Vitamin A

An adequate intake of vitamin A is essential for vision, cell differentiation, membrane structure and function, reproduction, immune system functions, organ development during embryonic and fetal growth as well as other physiological processes (1). Vitamin A is fat-soluble and can be stored in the liver and other tissues.
50 to 80% of the vitamin A present in the body is stored in the form of retinol in lipid droplets within liver cell cytoplasma. The vitamin A reserve in these droplets is sufficient for several months. High daily doses of the vitamin may lead to levels that are potentially harmful. On the other hand people with long-term vitamin A-deficient diets need occasionally to take these high doses to prevent deficiency symptoms. The safety of vitamin A is thus highly dependent on daily intake, duration of intake and supply status. Calculations of safe vitamin A levels need to take all potential sources into account: the recommended intake level for retinol may already be exceeded if liver or other types of offal are consumed regularly. In addition to appropriate doses of food supplements, medicinal products containing high doses of vitamin A can also generally be obtained subject to medical prescription.

The potential harmful effects of long-term overdoses of preformed vitamin A (retinol and retinyl ester) are well documented. However, there are no known examples of toxic vitamin A effects from excessive intake of vitamin A precursors such as provitamin A carotinoids (e.g., beta-carotene), since the body only converts these precursors into vitamin A when required. Whilst the level of vitamin A in foods and food supplements is given in international units (IU), food scientists normally use milligram or microgram Retinol Equivalents (RE); 1 IU of retinol corresponds to 0.3 micrograms of RE.

The harmful effects of a long-term diet containing high doses of vitamin A include liver damage, extending from reversible increases in liver enzyme concentrations to permanent damage such as fibrosis and cirrhosis of the liver. The latter has been observed in people with daily intakes of 25,000 to 50,000 IU (8,000 to 16,000 micrograms RE) of preformed vitamin A over several months. This is the equivalent of over ten times the recommended daily intake. The reported effects of high dosages can vary depending on how much previous damage has been caused to the liver as a result of other factors (such as alcohol, medication or hepatitis).

Although adequate intake of vitamin A is necessary for normal fetal development, excessive intake of vitamin A (retinol) during the first three months of pregnancy can cause birth defects. The lowest daily intake at which a significant increase in the risk of birth defects was observed was 25,000 IU (8,000 micrograms of RE) (2). A small number of researchers report possible vitamin A-associated deformities at daily doses below 20,000 IU (3, 4), but this is not supported by other studies (5). There are no known cases of an increased risk of birth defects caused by vitamin A formed from beta-carotene.

The results of studies into an increase in the risk of bone fragility caused by preformed vitamin A are inconclusive. Whilst many studies report an increased risk of hip fracture from long-term intakes of relatively small doses of vitamin A (from 5,000 IU / 1.500 micrograms RE per day) (6), others did not observe any association between increased levels of retinyl ester in the blood and reduced bone density (7, 8). On the other hand, a considerable number of older people have inadequate intakes of vitamin A, which could also be associated with a reduction in bone density. A study of older men and women established that bone density was optimal within the recommended intake range (9).

Based on the results of studies on the risks of potential birth defects, the US Institute of Medicine (IOM) and the European Commission’s Scientific Committee on Food (EC SCF) recommend a tolerable upper intake level (UL) for vitamin A (retinol) of 10,000 IU (3,000 micrograms of RE) per day in adults (10, 11). The evidence with respect to the risk of bone fragility was found to be insufficient.

Beta-Carotene

Beta-carotene makes a crucial contribution to vitamin A supply as the most widely distributed provitamin A in fruit and vegetables. Once it enters the intestine, some of the beta-carotene is converted to retinal and then reduced to retinol (vitamin A). Around 80% of the carotenoid is stored in subcutaneous fat and around 10% in the liver. When intake is stopped it is only released from the tissue deposits very slowly over several weeks. Alongside its antioxidant properties (12) beta-carotene also appears to be capable of supporting normal communication between cells (13). Since many carcinogenic substances inhibit intercellular commu-nication (14), the carotenoid could help to prevent cancers from developing. Whilst some observational studies have suggested that beta-carotene has potentially preventative effects in cancer and cardiovascular disease (15), clinical trials have not yet produced any clear evidence of this.

Even at very high intake levels, beta-carotene also appears not to contribute via its conversion into vitamin A to the potentially harmful effects of very high vitamin A concentrations in the body (16), nor have large amounts of the carotenoid itself (up to 180 mg per day over several months) been demonstrated in trials – alongside a reversible change in skin color – to be associated with any adverse effects (17).

In order to investigate the possible cancer prevention potential of beta-carotene in more detail, trials were carried out with subjects who were at a very high risk of developing lung cancer: people who had been heavy smokers for many years and people who regularly came into contact with asbestos as part of their work. The results of two randomized controlled trials (18, 19) showed that both long-term smokers consuming more than twenty cigarettes per day and subjects exposed to asbestos who had taken high daily doses of beta-carotene for five to seven years exhibited a considerable increase in the risk of lung cancer versus participants in the placebo group. The causes of these findings are as yet unclear, particularly as these effects were not observed in subjects who were smokers in another trial (20) conducted over 12 years. There were indications in one of the trials that targeted beta-carotene intake could possibly contribute to reducing the risk of lung cancer in former smokers (19). Shorter studies provided no findings to suggest any cancer-promoting or cancer-preventing effect for beta-carotene (21–23). According to one clinical trial the risk of precursors of colorectal cancer (adenomas) recurring in test subjects who neither smoked nor drank alcohol was reduced by administering 25 mg of beta-carotene per day, whilst the risk in smokers and drinkers appeared to be increased (24). A further indication that beta-carotene or its degradation products could have a different effect in people with extremely high levels of oxidant stress due to smoking than in non-smokers. A meta-analysis of randomized controlled trials of beta-carotene and cancer did not reveal any significant preventive effects, but did show an increase in the risk of lung cancer from long-term intakes of 20 to 30 mg per day in smokers and an increased risk of mortality in smokers with pre-existing cardiovas-cular disease (25). No increase in risks was identified from food supplements from 6 to 15 mg beta carotene per day over five to seven years.

Based on the study data the US Institute of Medicine (26) and the European Commission’s Scientific Committee on Food (27) decided not to set a tolerable upper intake level (UL) for beta-carotene. Although the studies indicated an increased risk of developing lung cancer in smokers with a daily intake of 20 mg or more of beta-carotene, it was found that the findings were inconsistent and did not allow a sufficiently precise assessment of a dose-effect relationship. The European Food Safety Authority (EFSA), after evaluating the data situation, came to the conclusion that no harmful effects were to be expected for adults with a daily intake of less than 15 mg of beta-carotene from food supplements and/or fortified foods (28); it found that this applied both to non-smokers and smokers.

Vitamin D

The body needs far less vitamin D then the other fat-soluble vitamins. In addition to the vitamin D3 which the body produces in the skin, induced by sunlight (UV B radiation), the vitamin can also be introduced into the body through food, food supplements and fortified foods. Intake levels depend on the extent to which it is synthesized in the body, which itself is affected by the length of the skin’s exposure to solar radiation, the season (UV intensity, extent of clothing), latitude, skin pigmentation and use of sunscreen products. Vitamin D3 and D2 are activated by its conversion to the “storage” form 25-hydroxy-vitamin D (25[OH] D or calcidiol), blood plasma levels of which are used as a measure of the supply status of the vitamin, and subsequently to 1,25-dihydroxy-vitamin-D (1,25-di[OH]D or calcitriol), the active (hormonal) form. Some of the vitamin D is stored in fat, from where it is slowly released when there is a deficiency. The half-life of
25-hydroxy-vitamin-D is three to six weeks and so the most stable vitamin D blood levels are achieved if it is taken daily, weekly or monthly. Calcitriol regulates intake of calcium in the intestine and its levels in the blood. Vitamin D is essential for the formation of stable bones, affects muscle strength and is involved in numerous other metabolism processes in the body. Vitamin D is stored mainly in fat and muscle tissue. The level of vitamin D can be expressed in international units (IU) or micrograms; 1 microgram is equivalent to 40 IU/ 0.025 micrograms are equivalent to 1 IU.

Since the amount of vitamin D produced in the skin reduces once sufficient blood levels of the active form of the vitamin are reached, it is not possible to achieve excessive, potentially harmful levels in the body through solar radiation alone (29). However if large amounts of vitamin D are ingested via food over a prolonged period, this can theoretically lead to an excessive intake of calcium via the intestine, increased release of the mineral from bones resulting in high levels of calcium in the blood (hypercalcemia) with mineral deposition in muscles, heart and kidneys (30). Intake levels of vitamin D resulting in these harmful effects are estimated at 50,000 IU (1.25 mg) per day in the majority of adults (depending on body weight) (31). Longitudinal clinical trials conducted in the last ten years have not reported any indications of hyper-calcemia in test subjects at daily doses of 10,000 IU (250 micrograms). One exception applies in the case of patients with idiopathic hypercalcemia, for whom vitamin D doses at these levels may lead to a deterioration in their condition (32). There have been reports of adverse effects in children at 2,000 to 4,000 IU (50 to 100 micrograms) per day and at 1,800 IU (45 micrograms) in infants.

The US Institute of Medicine (33) and the European Food Safety Authority (34) have set the daily tolerable upper intake level (UL) for vitamin D at 4,000 IU (100 micrograms) in adults. These estimates were based mainly on the findings from two trials in which healthy young men were given 234 to 275 micrograms of vitamin D3 daily for eight weeks to five months without developing hypercalcemia (35, 36).

Vitamin E

Vitamin E consists of a family of eight related substances: 4 tocopherols and 4 tocotrienols. Of these, alpha-tocopherol is the most active form in humans (37). Vitamin E’s main role appears to consist of protecting cell components – particularly polyunsaturated fatty acids as an integral part of cell membranes – from oxidative damage. In addition to maintaining the integrity of cell membranes, vitamin E also prevents oxidation of LDL cholesterol, which is implicated in the genesis of atherosclerosis. Clinical trials investigating the vitamin's role in preventing cardiovascular disease and other chronic illnesses did not lead to any conclusive findings. However, later research has shown that a person’s genotype may be crucial in determining the extent to which targeted intake of vitamin E, for example using food supplements, influences cardiovascular health (38). Examples of functions of alpha-tocopherol that go beyond its antioxidant properties include regulation of cell signalling, gene expressions as part of immune and inflammatory responses and influence on blood clotting (37). Once the vitamin has neutralized free radicals it loses its antioxidant capacity and can be regenerated by other antioxidants such as vitamin C.

Intake of vitamin E via diet (the vitamin is usually present in the diet as multiple forms of vitamin E from plants) is relatively low as compared with intake via supplements. The vitamin E forms are absorbed from the intestinal lumen, packaged into large lipoproteins and then transported throughout the body via the circulatory system. Once in the liver alpha-tocopherol is packaged into Very Low Density Lipoproteins (VLDL) and excreted back into the circulatory system, while other vitamin E forms are metabolized by enzymes in the liver and excreted (37). The fat-soluble vitamin does not appear to accumulate in potentially harmful amounts in the liver or other types of tissue. As levels of vitamin E intake increase, the absorption rate in the intestine falls (39). At high vitamin E intake levels (e.g. 400 IU of vitamin E from food supplements) liver secretion of alpha-tocopherol is reduced and elimination/excretion rates increase; plasma concentrations do not increase more than 2–4 fold (40). Very high intakes achieved with supplementation only succeed in doubling the tissue levels of vitamin E, which is not harmful.

Thus, it was quite surprising that meta-analyses reported that consumption of vitamin E supplements (400 IU or more) by humans were associated with increased risk of dying (41–43), although the accuracy of these statistical analyses does remain in dispute (44, 45). Nor was this reported in other trials at daily intake levels of 800 to 2000 IU (46-48). Indeed, when toxicologists searched for evidence of adverse effects of excess alpha-tocopherol, the only consistent finding was the observation that vitamin E increased tendencies towards hemorraging in certain persons, likely as a result of interference with vitamin K status, particularly where anticlotting agents were concomitantly administered (49, 50). But no research has found this poses a health risk. A large trial of patients taking the anticlotting agent warfarin in combination with 800 to 1,200 mg vitamin E also showed no impairment in blood clotting parameters (51).

The US Institute of Medicine therefore set the permitted tolerable upper intake level (UL) in adults at 1,000 mg alpha-tocopherol per day in any form (natural or synthetic) (equivalent to up to 1,500 IU of RRR-alpha-tocopherol or 1,100 IU of all-rac-alpha-tocopherol) to prevent the risk of hemorrhaging (52).

Vitamin K

Vitamin K refers to a group of fat-soluble vitamins of which vitamin K1 (phyllochinon) from plant-sourced foods meets most of the requirement. Vitamin K2 is produced by bacteria in the human large intestine (menaquinone and its various forms MK-4 to MK-14) and plays a minor role in the provision of vitamin K since it is only taken up by the body to a limited extent. Vitamin E is involved as a coenzyme in the formation of clotting factors and is thus essential for the regulation of blood coagulation. Vitamin K is also important for the activation of certain enzymes which, together with vitamin D, regulate calcium metabolism and could therefore help in the prevention of calcification of blood vessels (atherosclerosis) and decalcification of bones (osteoporosis). It also appears to influence cell division and cell differentiation in numerous soft tissues (53). Vitamin K is absorbed via the intestine and is found in high levels predominantly in the liver. The total amount stored is relatively small and is only sufficient to compensate for a deficiency in vitamin K supply for around 1–2 weeks (54).

Although high doses of vitamin K1 and K2 have been used in some trials, there have been no reports of harmful effects (55). No adverse effects were observed in a clinical trial with daily intakes of 10 mg of vitamin K1 (56). However, high doses of forms of vitamin K that do not occur in nature (K3, K4 and K5) were associated with allergic reactions, liver toxicity, jaundice and hemolytic anemia (54). The anticlotting effects of vitamin K antagonists (e.g., warfarin) may be impaired by a very high dietary or supplement-based intake of vitamin K. It is therefore generally recommented that people taking warfarin do not take more than the established daily dose of vitamin K and maintain a constant daily intake of vitamin K (57).

Based on published study data the European Food Safety Authority (58) and the US Institute of Medicine (59) decided not to define any tolerable upper intake levels.