What not to do Vitamin A and Beta Carotene
Preclinical and observational evidence through the 1970’s, 80’s, and into the 90’s saw beta carotene and vitamin A emerge as agents of tremendous research interest; the two agents in isolation and in combination appeared to be capable of dramatically altering the course of chronic degenerative disease, most notably heart disease and cancer. The stage was set for large, long- term placebo- controlled human intervention trials. Outcomes of these trials however have proven to be among the greatest embarrassments of modern nutritional science. Vitamin A and beta carotene supplementation have been reproducibly demonstrated to deliver an array of detrimental outcomes including but not limited to increased all cause mortality, cancer incidence, and perinatal morbidity. Despite a well- developed evidence- base to this effect, harmful levels of vitamin A and beta carotene remain mainstay components of over- the- counter natural health product formulations, most notably multivitamins. We revisit the vitamin A and beta carotene story in hopes of having integrated healthcare providers become the leaders in a united voice against the widespread inclusion of vitamin A and beta carotene in products offered by the natural health products industry, and to remind our colleagues of the immensely strong evidencebase advocating the complete avoidance of these agents from their clinical practices.
Introduction
Vitamin A (retinol) is a fat-soluble antioxidant and vitamin; and beta carotene is its primary plantderived precursor. Vitamin A has a long history of use as a pharmacologic agent, dating from the 1970s (Fritz 2011, Goodman 2008, Micksche 1977). Over the past 10-15 years, a large and complex body of evidence has accrued, which overwhelmingly indicates a range of specific harmful effects associated with supplementation of this nutrient; these effects range from increased risk of overall mortality, increased risk of lung cancer among high risk populations, poorer perinatal health outcomes among a group of African children, and possible adverse effects on bone (Bjelakovic 2008, Fritz 2011, McGrath 2006, Omenn 1996, Penniston 2006). While it may be difficult for some to conceive of harms associated with a naturally occurring, essential micronutrient, the quite substantial body of research that has reproducibly demonstrated these effects in large; well controlled RCTs among diverse populations increasingly makes this point irrefutable. The evidence can no longer be ignored: vitamin A and beta carotene are two single agents that deserve to be treated with a very high degree of caution with respect to nutrient supplementation. We argue that given the series of seriously harmful effects which have been clearly demonstrated, and considering the range of highly safe agents at our disposal, vitamin A and beta carotene are two substances that should never be utilized as a supplemental intervention in the course of a North American integrative healthcare practice.
I. A Global Perspective Historically, vitamin A supplementation has been used in developing countries to prevent blindness due to rampant vitamin A deficiency. Vitamin A deficiency is considered to be a public health problem in 45 of 122 countries in these parts of the world, based on the prevalence of night blindness and biochemical vitamin A deficiency (serum retinol concentration <0.70 μmol/l) in preschoolage children (WHO 2009). A recent meta analysis published in the British Medical Journal including 43 trials found that vitamin A supplementation among impoverished children under the age of five significantly reduced the risk of mortality by 24% in 17 trials (rate ratio 0.76, 95% CI 0.69-0.83), and reduced the prevalence of vision problems including night blindness by 68% (RR0.32, 0.21- 0.50) (Mayo-Wilson 2011).
Nonetheless, although this is undoubtedly a crucial therapy regionally, the generalizability of this practice to well nourished North American populations remains minimal. Among Western populations, vitamin A deficiency is virtually nonexistent, with the possible exceptions of select cases among very limited and specific subgroups such as patients who have undergone bariatric surgery or who are severely anorexic (Ballew 2001, Braunstein 2010, Fok 2012). In an excellent review of vitamin A’s safety published in the American Journal of Clinical Nutrition, Penniston argues that hypervitaminosis A is a growing problem in Western populations, in part due to an increasing number of sources containing preformed vitamin A that have become a part of our diet (2006). These include many fortified sources: multivitamins, cod liver oil, and the fortification of common foods such as milk, butter, margarine, breakfast cereals, and some snack foods (Penniston 2006).
II. A Historical Perspective In North America during the 1970s and 80s, observational and preclinical research findings sparked an interest in vitamin A as a potential anticancer agent (Kurata 1977, Salonen 1985, Smith 1972, Wald 1980). Studies emerged showing that individuals with higher blood levels of vitamin A and beta carotene (as well as other antioxidants) had a significantly lower risk of developing cancer, including lung cancer (Salonen 1985, Wald 1980). In preclinical models, vitamin A was shown to have antiproliferative effects on epithelial cells, and was able to prevent progression to lung cancer in animals exposed to carcinogens (Kurata 1977, Smith 1972). Early phase I and II trials were initiated investigating vitamin A as an adjunctive treatment for lung cancer, as well as head and neck cancers (Micksche 1977, Thatcher 1980). However, despite the promising early findings, intervention with various forms of vitamin A showed minimal benefits on lung cancer outcomes, based on five RCTS and 26 Phase I and II trials reviewed by Fritz et al (2011). As a result, further research in the area has not been pursued in recent years.
In the 1990s, the focus of vitamin A and beta carotene research turned to their use as possible cancer preventive agents. To this end, two large chemoprevention trials were designed, the findings of which are discussed below. Today, vitamin A remains an agent that is sometimes utilized as a single agent or in combination formulas to stimulate immune function and as an anticancer agent; the appropriateness of such interventions will emerge from the following discussion of the evidence that has emerged over the past 20 years with respect to vitamin A and beta carotene.
Pharmacology
Vitamin A is also known as retinol or retinylpalmitate, which is the form that occurs in foods and is used in supplements; however the term is also used to encompass a number of retinoid derivatives, including all-trans-retinoic-acid (ATRA) and 13-cis-retinoic acid (Accutane) (Fritz 2011). Since the focus of this paper is on supplemental use of vitamin A, these pharmaceutical retinoids are not reviewed here.
The body obtains vitamin A from two sources: preformed vitamin A, present in cod liver oil, butter, eggs, animal products, and fortified foods (ie. grain products), as well provitamin A carotenoids, most notably beta carotene but also inclusive of alpha carotene and beta cryptoxanthin, which are converted to retinol and retinyl esters in the body (Higdon 2009). It has been estimated that up to 88% of beta carotene is converted to retinyl esters in the intestinal wall, while up to 30% enters lymphatic circulation unchanged (Higdon 2009, Stahl 2005, Tang 2010). Beta carotene possesses singlet oxygen quenching activity, and administration has been shown to increase levels in lung epithelial tissue (Fiedor 2005, Obermueller-Jevic 2002, Patrick 2000). Thus beta carotene’s proposed anticancer effects were thought to be a result of its combined antioxidant activity and known concentration in the lungs.
As a fat-soluble nutrient, vitamin A has long been recognized for its potential toxicity at higher dosages. According to Penniston, “daily intakes of >25,000 IU for >6 y and>100,000 IU for >6 mo are considered toxic, but there is wide interindividual variability for the lowest intake required to elicit toxicity” (2006). Classic toxicity symptoms include impaired liver function, with elevated liver enzymes, hypertriglyceridemia, skin rashes, dry eyes, myalgia, and headaches (Fritz 2011). Vitamin A is a recognized teratogen, and can cause congenital malformations such as spina bifida and cleft palate (Ackermans 2011). The Recommended Dietary Allowance for vitamin A in pregnancy is approximately 2500 IU (Higdon 2009). According to the Teratology Society, a balanced North American diet contains between 7000-8000 IU per day, and 8000 IU should be considered the safe upper limit for pregnancy (1987). Currently the upper limit for pregnancy as set by the Food and Nutrition Board of the Institute of Medicine is 10,000 IU, however this includes both food and supplement sources (NIH 2012).The only known toxicity associated with beta carotene is an orange discoloration of the skin.
III. A Scientific Perspective: The Evidence Vitamin A at a Glance: Cochrane Meta Analytic Review A 2008 Cochrane meta analysis combined data from 67 randomized trials assessing antioxidants, including 232,550 participants. Trials investigating any of vitamin A, vitamin C, vitamin E, beta carotene, and selenium were included. Among the studies assessing vitamin A and beta carotene, supplementation was associated with a significant increase in all cause mortality (death from any cause): (RR 1.16, 95% CI 1.10 to 1.24) and (RR 1.07, 95% CI 1.02 to 1.11) respectively (Bjelakovic 2008).
Chemoprevention Trials
Following upon the early studies of the 1970s and promising animal data, two large chemoprevention trials were conducted in the 90s to investigate the ability of vitamin A and/ or beta carotene to reduce risk of lung cancer among smokers and asbestos workers. The expected outcome was a reduction in risk due to supplementation with these antioxidants, which seemed to have an affinity for the lung epithelial tissue.
The Carotene and Retinol Efficacy Trial (CARET) was a multicenter study conducted in the United States. A total of 18,314 smokers and asbestos workers were randomized to receive a combination of 25,000 IU retinyl palmitate plus 30mg beta carotene, or placebo, for four years (Omenn 1996). Instead of a decrease, however, there was a significant increase in the incidence of lung cancer, (RR 1.28; 95% CI 1.04-1.57). Subgroup analysis showed higher risk in asbestos workers (RR 1.40, 95% CI: 0.95–2.07), and current heavy smokers (RR 1.42, 95% CI: 1.07–1.87), while there was a non significant reduction in risk amongst smokers who had already quit at randomization (RR 0.80, 95% CI: 0.48–1.31) (Omenn 1996). Risk of death from all causes was increased 18% (RR 1.18, 95% CI: 1.02–1.37); death from lung cancer was increased 46% (RR 1.46, 95% CI 1.07–2.00); and death from cardiovascular disease was increased by 26% (RR 1.26, 95% CI 0.99–1.61) (Omenn 1996). There was no evidence of increased risk of other cancer types.
In addition to the CARET trial, two smaller lung cancer prevention trials exist investigating retinyl palmitate; of these, one showed no significant effects (Kamangar 2006), and one shows a reduction in risk of mesothelioma, an asbestos-related lung cancer (de Klerk 1998, Fritz 2011). This last study, the Western Perth Australia study, found a significantly reduced risk of mesothelioma associated with vitamin A supplementation (RR 0.24, 95% CI 0.07-0.86), however, its findings are hampered by an important limitation, namely the lack of a placebo- or inactive- control group (de Klerk 1998). Instead, the beta-carotene arm was used as the comparator. Since beta carotene has in fact been shown to be detrimental, it is quite likely that these apparently supportive findings are in fact falsely inflated.
The Alpha Tocopherol Beta Carotene (ATBC) trial was a second large cancer prevention study conducted among Finnish male smokers. A total of 29,133 men were given either 20mg beta carotene, 50mg alpha tocopherol, both, or placebo for between 5-8 years. Investigators found that supplementation of 20mg beta carotene (in this case without vitamin A) increased incidence of lung cancers by 16% compared to those not receiving beta carotene, RR 1.16 (95% CI 1.02 – 1.33) (Albanes 1996).
Conditional Pro-oxidant Theory To explain these unanticipated results, the conditional pro-oxidant theory was developed (Omenn 1998). Under certain circumstances, in particular situations of high oxidative stress, an antioxidant may act as a conditional pro-oxidant. It is well established in the field of chemistry that each ion possesses a relative redox potential, which determines whether it is reduced or oxidized in reaction with other substances. Similarly, a weak antioxidant may be oxidized by a stronger prooxidant or vice versa. The resultant new free radical may perpetuate the chain of oxidative damage on other targets. Carotenoids are particularly vulnerable to such oxidation due to their long chains of conjugated double bonds (Omenn 1998), and animal studies have since confirmed that in the presence of cigarette smoke, vitamin A and beta carotene do indeed act as pro-oxidants, with the potential for pro-carcinogenic effects in the body (van Helden 2009, Wang 1999).
Although findings of the large CARET and ATBC trials have been the most influential, other chemoprevention trials investigating beta carotene have also been conducted. A 2008 meta analysis of these confirmed the lack of benefit from beta carotene supplementation (Gallicchio 2008). Among studies comparing beta carotene supplements to placebo, the relative risk of cancer was 1.10 (95% CI 0.89 – 1.36). The six trials included in this review were:
1) the Carotene and Retinol Efficacy Trial (CARET) (Omenn 1996);
2) the Alpha Tocopherol Beta Carotene (ATBC) study (Albanes 1996);
3) the Physicians’ Health Study (PHS) (Cook 2000);
4) the Western Perth, Australia study (de Klerk 1998);
5) the Women’s Health Study (Lee 1999); 6) the Linxian General Population trial (Kamangar 2006).
African HIV Studies When evaluating intervention trials with any nutritional agent, the concept of baseline status of the population must be taken into consideration. For example, supplementation among individuals on the brink of scurvy with vitamin C would be expected to deliver an important magnitude of benefit to an array of outcome measures, while the same intervention administered to a population of citrus farmers is far less likely to yield benefit, given the expectation that citrus farmers enjoy a relatively high dietary intake of the vitamin.
HIV positive, pregnant women in Tanzania Africa were recruited and assigned to one of four groups; multivitamin, vitamin A (5000IU) and beta carotene (30mg), multivitamin free of vitamin A and beta carotene, and placebo. Outcomes were presented in a series of papers, following the women throughout their pregnancy, as well as following the resulting offspring until age 18 months (Fawzi 2004, McGrath 2006, Merchant 2005, Villamor 2002).
The multivitamin free from vitamin A and beta carotene achieved significant benefit to the following outcomes relative to placebo; improved weight gain during pregnancy, reduced risk of low weight gain (<100g/wk) (Villamor 2002), reduced risk of progression to stage IV AIDS or reduced risk of death from AIDS related causes, reduced risk of the following AIDS- related complications (thrush, gingival erythema, angular cheilitis, oral ulcer, reported mouth and throat ulcer, painful tongue and mouth, difficult or painful swallowing, nausea and vomiting, dysentery, fatigue, rash, and acute URTI), increased CD4+ counts, reduced viral load (Fawzi 2004), reduced risk of development of hypertension during pregnancy (Merchant 2005), improved psychomotor development index score in resulting offspring, and reduced risk of developmental delay on the motor scale (McGrath 2006).
The multivitamin containing vitamin A and beta carotene achieved significant benefit to the following outcomes relative to placebo; improved weight gain during pregnancy, reduced risk of low weight gain (<100g/wk) (Villamor 2002), reduced risk of the following AIDS- related complications (thrush, angular cheilitis, rash) (Fawzi 2004), reduced risk of development of hypertension during pregnancy (Merchant 2005), and improved the psychomotor development index score in resulting offspring (McGrath 2006).
Vitamin A and beta carotene supplementation on their own produced no outcomes significantly different from placebo. The authors highlight that the relative magnitude of benefit for outcomes impacted by multivitamins with or without vitamin A and beta carotene was greater across the board for the group receiving the multi free from vitamin A and beta carotene. Also, as described above, the multi free from vitamin A and beta carotene achieved benefit to a far broader and clinically relevant range of outcomes relative to the group receiving a multi containing vitamin A and beta carotene. Although not indicated by the data of the trial, one may make the stretch to suggest that it is as though the inclusion of vitamin A and beta carotene in the multi reversed the benefit expected from delivery of a multi to this malnourished population. The team followed this trial up with a trial among 8468 pregnant, HIV- negative women in Tanzania, Africa. In this follow- up trial, subjects were assigned to one of two groups; placebo or multi free of vitamin A and beta carotene. It was deemed unethical to include a group that received vitamin A and beta carotene (Fawzi 2007).
If vitamin A and beta carotene are incapable of materially benefiting this obviously malnourished population, what benefit can possibly be hoped for when supplementing relatively well- nourished, freeliving North American populations?
Potential Bone Toxicity Penniston has outlined the somewhat weaker but still important evidence suggesting that high intake of preformed vitamin A may be associated with poorer bone health, in particular osteoporosis (2006). In human observational studies, higher intake of preformed vitamin A has been associated with increased risk of osteoporosis, up to three fold greater compared to those with the lowest levels of serum retinol (Mata- Granados 2010). Increased vitamin A intake has also been associated with increased risk of hip and wrist fracture (Opotowsky2004, White 2006). In animal studies, feeding a retinoic acid enriched diet compared to a standard diet resulted in significantly lower bone mineral content and bone mineral density after two and four weeks (Hotchkiss 2006, Xue 2011). Yet other studies have found no significant effects in animals or humans (Ambrosini 2012, Wray 2011). Although the evidence in this area is not yet conclusive, it is an important topic to follow and be aware of.
Biomarker Hypothesis Intervention trials have established a very strong case against supplementation with vitamin A and/ or beta carotene. However, a very large body of observational evidence has, and continues to show that determination of dietary beta carotene intake, through food frequency questionnaire or through determination of plasma beta carotene levels, positively and powerfully predicts reduced risk of heart disease and cancer. What explains the apparent discrepancy between expected benefit from elevating plasma beta carotene levels as demonstrated through observational evidence to the detriment witnessed from supplementation with beta carotene?
One must take into account the concept of biomarker. The very nature of observational evidence is that it does not include an intervention. When beta carotene intake is estimated, or better yet, when plasma beta carotene levels are determined, where is the beta carotene coming from? It is most certainly not coming from supplementation. Hindsight has taught us that plasma beta carotene is the single best available biomarker of exposure to fruit and vegetables. It is erroneous to conclude “plasma beta carotene of X reduced risk of chronic disease”. It is far more appropriate to conclude “fruit and vegetable consumption of X, objectively confirmed by assessment of plasma beta carotene, protected against risk of chronic degenerative disease”.
A landmark investigation, the BASEL study, prospectively followed 4858 men for 12 years. Participants were divided into quintiles based on baseline determination of plasma beta carotene Individuals in the lowest quintile of plasma beta carotene were found to be at a 49% elevated risk of developing cancer (Stahelin 1991). Similarly, 1720 men were divided into quartiles based on baseline determination of plasma beta carotene. Individuals in the highest quartile were found to be at a 48% reduced risk of all cause mortality and a 43% reduced risk of death from cardiovascular disease (Greenberg 1996). The above observational study also included an intervention arm; across each quartile, subjects were randomized to receive a beta carotene supplement or placebo. As could be anticipated, individuals in the highest quartile of baseline plasma beta carotene received no benefit from the intervention. Surprisingly, yet completely in- line with the biomarker hypothesis of plasma beta carotene status, subjects in the lowest quartile of baseline plasma beta carotene also achieved no benefit from intervention with a beta carotene supplement (Greenberg 1996).
Hindsight has indeed proven 20/20. The community of nutritional scientists as a whole has recognized the importance of plasma beta carotene as an accurate marker of fruit and vegetable consumption, and likewise recognized its lack of importance as a “nutrient unto itself” for impact to risk of chronic degenerative disease. Modern observational trials demonstrating beta carotene as protective against chronic degenerative disease conclude that their outcomes strengthen recommendations for the public to consume more fruit and vegetables (Hak 2004). Modern intervention trials that include dietary modification as part of the intervention utilize determination of plasma beta carotene to assess compliance; if participants comply with recommendations for increased consumption of fruit and vegetables, plasma beta carotene of the intervention group is elevated relative to the control group (Pierce 2007, vanBreda 2004).
Conclusion
Over the past 20 years, an eloquent and well developed body of research has emerged that clearly delineates the detrimental effects associated with the use of supplemental vitamin A and beta carotene. Concrete harm has been reproducibly documented with respect to a number of important clinical outcomes, including all cause mortality, cancer incidence, perinatal morbidity, and potentially bone density. Given the range of safe and effective interventions available, we recommend against the routine use of vitamin A and beta carotene in North American integrative healthcare practice.
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