Antioxidants: Positive or Negative Actors?
The following is a summary of the stated research mentioned above. The content summarized here, including the figures and tables, all belong to the researchers (unless otherwise indicated). The summary attempts to stay as close to the original paper as much as possible with some adjustments in regards to jargon, length, or to focus on bean to bar aspects.
Note from Geoseph
This is not a chocolate or cacao specific research paper. From time to time I’ll include a research summary which relates to chocolate or cacao. Today, antioxidants are often the number one discussed nutritional aspect of cacao and chocolate. However, most people don’t fully understand how antioxidants work, the mechanisms behind how they work, and also are not aware that there is debate regarding their benefits to our health. I prefer to share information in a more balanced and unbiased view and welcome healthy debate. For this reason, I feel this paper would be beneficial to those who enjoy studying the world of chocolate and cacao. Some of the following may seem a bit technical at first, but as you read along the jargon repeats itself becomes more clear. Take your time with it and enjoy!
Some helpful terms
Antioxidant - a substance that counteracts an ROS and inhibits oxidation
Reactive oxygen species (ROS) - A natural yet highly reactive chemical that contains oxygen, and may react negatively on cells in the body if produced in excess
Oxidative stress - a disruption of the homeostatic balance between oxidants and antioxidants within the cells
Free radicals - Reactive Nitrogen Species (RNS) such as peroxynitrite (ONOO‾) and nitric oxide (NO) as well as superoxide radical (O2‾), hydrogen peroxide (H2O2), and hydroxyl radical (OH) which react with proteins/fats/nucleic acids and may cause damage or death to the cell.
Abstract
Within the realm of biological/medical sciences, the term “antioxidant” can be confusing. In chemistry, “antioxidant” is “a compound that removes reactive species, mainly those oxygen-derived”. In the biological/cell context, the conceptual definition of “antioxidant” is less clearly defined. Many consumers around the world consume antioxidants via antioxidant-rich foods, based on the idea that the cancer, inflammation, and degenerative diseases are triggered by high oxygen levels (or reactive oxygen species). It is believed that blocking reactive species production (by consuming antioxidant-rich foods) various disorders can be prevented or treated. A rise in the popularity of antioxidants is partly due to widespread public mistrust in allopathic (conventional) medicine. The ability for the most common antioxidant supplements (Vitamins C, E, selenium, herbal supplements) to decrease pathologic reactive oxygen is not clearly established. This review aims to provide a nuanced understanding of where current knowledge is and where it should go.
Introduction
Dietary antioxidants are found in many colourful fruits and vegetables, but can also come from other sources such as through exercise. Image by @tombrunberg
Aerobic cells (respiration in the presence of oxygen) produce reactive oxygen species (ROS) as a byproduct of the metabolic process. This is normal. These ROS cause oxidative damage when the body antioxidant defences are overwhelmed. However, even under normal conditions there is a certain amount of oxidative damage, which increases the rate of aging and diseseas processes. Reactive oxygen and nitrogen species (ROS/RNS) are naturally produced, and exist in the cells in homeostasis with antioxidants.
Antioxidants are compounds that inhibit oxidation. Oxidation is a chemical reaction which produces the free radicals, which can lead to a chain of reactions which ultimately may damage cells. Antioxidants such as thiols or ascorbic acid (AKA vitamin C) can end these chain reactions. ROS/RNS production and accumulation is a common denominator in many disorders, and may cause serious cell damage or cell death.
There are systems in the body which regulate the balance between ROS and antioxidants. Some of these systems include superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), lipid-soluble vitamin E, carotenes, and water-soluble vitamin C. Dietary antioxidants (mostly obtained by fruits and vegetables) have been associated with balancing these free radicals and help to minimize oxidative stress and reduce the risk of cancer, cardiovascular diseases, and aging (6). Dietary antioxidants are comprised of a wide group of molecules, and can also act synergistically when mixed. Some antioxidants work better fighting cancer, while others work at fighting degenerative diseases. Thousands of antioxidants are present in dietary patterns and that some of them may have stronger antioxidant effects. Keep in mind the effects vary depending on the person, gender, type of cancer, etc. (7).
Antioxidant Functional Definition
The term “antioxidants” entered the nutritional vocabulary in the 1990s when researchers were discovering how oxygen-triggered free radical reactions play a role in aging-associated diseases (8). Today, an antioxidant is defined as any substance that can eliminate ROS and derivatives (such as RNS, reactive sulfur species, RSS) directly or indirectly, acting as an antioxidant defense regulator or reactive species production inhibitor (9). ROS are produced via cellular metabolism. RNS and RSS result from the reaction between ROS and nitric oxide and thiols (9). ROS/RNS can cause cell damages by covalent joining with other molecules, and hence stimulate abnormal cell growth or cell death. This may then lead to cell populations that produce inflammatory cytokines (immune secretions) in large amounts. Antioxidants may block the production/deleterious effects of reactive species, and therefore block aging, inflammation, and cancer. Their function can be classified into distinct defense lines according to their mechanism of action:
Preventative agents that supress new radicals from forming. These include enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase PGX, proteins that mind to metals such as ferritin and ceruloplasmin, and minerals such as selenium (Se), copper (Cu), and zinc (Zn)
Radical scavenging agents which inhibit chain initiation and/or propagation. These include glutathione, albumin, vitamins C and E, carotenoids, and flavonoids
Repair and de novo enzymes which repair and reconstitute cell membranes. These include lipases, proteases, DNA repair enzymes, transferases, and methionine-sulfoxide reductases
Adaptation agents which generate appropriate antioxidant enzymes and transfer them to the sites they need to be
Antioxidant Role in Redox Imbalance Prevention: Gaps of Knowledge
Redox imbalance happens when the balance between cell’s antioxidant defence system and the ROS (and derivatives) is off (10). The cells and tissues of our bodies are constantly exposed to ROS through natural cell metabolism but also though external factors such as smoking, pollution, pesticides, microbes, allergens, ultraviolet and gamma radiation, which all generate free radicals and are associated with aging and disease (11). The problems arise when the body can no longer contain or eliminate these. The antioxidants field has been hindered by the following gaps in knowledge:
What are the correct antioxidant doses? There is evidence that more is not necessarily better or may be worse. (12) All antioxidants do not have a classic dose-response, but may have opposite effects at various doses.
How are antioxidants absorbed? There is not enough knowledge in regards to antioxidants gut microbial metabolism and whether antioxidants affect gut microflora in an anti-inflammatory way. It’s poorly understood whether antioxidants are absorbed unchanged or if they metabolize to different compounds (example: ellagic acid which is poorly soluble but is metabolized into soluble metabolites (13,14)).
Naturally occurring antioxidants may be absorbed as complexes of two or more compounds. These complexes may perform different biological activities than when they are isolated compounds alone.
Even with ideal absorption into tumors, the tumors may respond by compensating which may lead to more rapid tumor growth (15)
Adverse Effects of Antioxidants
The most popular forms of antioxidant forms include vitamins such as vitamin A (retinol, retinoic acid), vitamin C (L-ascorbic acid, ascorbic acid, ascorbate), vitamin E (a-tocopherol), B-carotene, and minerals such as Se, and naturally-occurring polyphenols. Each one has a different effect on our cells.
Here we summarize the adverse effects of popular antioxidants which are often consumed as supplements at much higher doses than those found in foodstuffs. While these adverse effects are known in the medical community, they are not so well-known among the general public who sometimes believe natural products cannot be toxic.
Czernichow (16) investigated the effect of antioxidant supplements for 7.5 years on metabolic syndrome (MetS) incidence and epidemiologic association between baseline serum antioxidant concentrations and MetS prospective risk. No beneficial effects of antioxidant supplementation were observed. Baseline serum antioxidant concentrations of B-carotene and vitamin C were negatively associated with MetS risk. Baseline serum zinc concentrations were positively associated with the risk of developing MetS.
Park (17) found no association between dietary intake of vitamins A, C, and E and colon cancer risk in the pooled analysis of 13 prospective cohort studies. However, total vitamins A, C, and E intakes were each inversely associated with colon cancer risk. A low dietary intake of antioxidant vitamins and minerals raises incidence of cardiovascular diseases and cancer.
Hercberg (18) found that after 7.5 years, low-dose antioxidant supplementation lowered total cancer incidence in all-cause mortality in men but not in women. Supplementation may only be effective in men because of their low basal status of certain antioxidants, especially B-carotene.
Researchers (19) reported increased teratogenicity risk (birth defects) among babies born from women who took more than 10,000 IU vitamin A per day. Excessive dietary vitamin A has been associated with birth defects over the past few years (20-22). A controlled clinical trial found that people who took 25,00 IU of vitamin A daily for a median of 3.8 years had an 11% increase in triglycerides, 3% increase in total cholesterol, and 1% decrease in HDL cholesterol as opposed to those who did not take vitamin A (23). A 4-year old boy presented several bone pains due to vitamin A toxicity (600,000 IU daily for over 3 months) (2). It’s been reported that bone loss can occur from excessive vitamin A intake and increase risk of hip fracture and osteoporosis risk (24).
Studies have suggested that vitamin C supplements may increase urinary oxalate concentrations, and double the risk of calcium oxalate kidney stones (25-27). Another study found that high vitamin C intake from supplements was associated with a rise in cardiovascular disease mortality in postmenopausal women with diabetes, but this has never been confirmed (28). In theory, vitamin C may cause too much iron absorption, but this is likely only an issue for those who already have high iron stores or patients with iron overload (hereditary hemochromatosis) where increasing iron toxicity may exist (29).
Pavlotou (30) evaluated free oxygen radicals (FORT) and free oxygen radicals defence (FORD) levels in patients with newly diagnosed type 2 diabetes. They found that diabetic patients had increased levels of FORT and decreased levels of FORD compared to controls.
A study (31) found that dietary vitamin E supplementation significantly increased prostate cancer risk among healthy men. A meta-analysis (32) found evidence of vitamin E supplementation having adverse effects on stroke subtypes, with a 22% increase in hemorrhagic stroke risk and 10% decreased ischemic stroke risk, but the absolute effects are minor. A study on birth weight (33) found that a 22-30 mg/day of vitamin E during human pregnancy saw a decrease in birth weight.
Scientists reported that B-carotene supplementation (not from intake of vegetables) has an increase risk of death from lung cancer or heart disease in smokers, rather than reducing the cancer incidence (34-37).
We believe that other antioxidant side effects may not be reported and other ones will be discovered in the future.
Are Antioxidants Benefits More Apparent Than Real?
One of the issues regarding antioxidants is imposing its chemical definition on a biochemical system. For example, in ancient times, man used carbon as an antioxidant to reduce iron ore to iron by removing the oxygen in iron ore with carbon and driving off oxygen and carbon dioxide. However, this definition while applicable to metal refining does not consider the complexities of biological systems.
The most studied antioxidant system in biology is nuclear factor erythroid 2-like 2 (Nrf2) transcription. It upregulates the antioxidant response elements (AREs) through the expression of genes involved in oxidative stress response and drug detoxification. Cells treated with Nrf2 inducers leads to Nrf2 nuclear translocation (39,40). As well, reduced levels of glutathione (which is the main small antioxidant molecule in mammalian cells) is a product of Nrf2 target genes (44). The question is whether the antioxidant effect of glutathione synthesis exerts antioxidant activity beyond correcting the original oxidative stressor (44). In glutathione, free sulfhydryl groups (-SH groups) could have antioxidant effects, but may also bind to chemotherapy reactive intermediates and block effective killing of tumor cells (45-47). This may explain why glutathione supplementation has not be an effective strategy against cancer as originally thought.
Supplemental antioxidant administration during chemo and radiation therapy is currently controversial (9,10,12,56,57). It has been difficult to determine which antioxidants are beneficial to cancer treatment, and which may contribute to preventing treatment. For this reason, an antioxidant prescription is confusing and should consider the type of cancer, background and state of patient, antitumor therapy, drugs mechanism of action and drugs used in treatment, as well as the antioxidant type and dosage (9,57).
Redox Imbalance Positive and Antioxidant Effect Negative
Although severe redox imbalance may lead to oxidative and nitrosative damage and cell death, a moderate redox imbalance level can yield beneficial effects on adaptive cellular responses (such as endogenous antioxidant defence systems levels (12, 58). Hormesis is the cellular adaptive response to stressors where low-dose stimulation results in a beneficial adaptation, but high-dose results in a toxic effect (59). With this in mind, low ROS doses, such as those produced during exercise, are required for exercise-induced training response in skeletal muscle. ROS are required for exercise adaptive response, and are essential to enhancing sports performance (59). Therefore, low cell stressor doses such as chemicals, toxins, radiation, and moderate exercise results in an adaptive response, and increases the antioxidant capacity of cells. Exercise itself can even be considered an antioxidant as it releases classical antioxidant enzymes such as SOD and GPX. Therefore, antioxidant supplements may not be a good strategy when training as they would block the ROS production which acts to stimulate endogenous antioxidant enzymes (60).
It is believed that antioxidants can prevent cancer development affecting the cell cycle, inflammation, tumor proliferation and invasiveness, apoptosis (cell death) and detox mechanisms. The antitumor effects of many antioxidants (catechins, isoflavones, lignans, flavanones, resveratrol, ellagic acid, quercetin, and curcumin) (9). However, antioxidant supplementation may block natural antioxidant production and other cell mechanisms. A basal redox imbalance is crucial for cell adaptation. The question then is which reactive species concentrations is beneficial and which is harmful.
What Antioxidants Can and Cannot do
Billions of dollars are spent on antioxidant supplements worldwide every year in order to modify chronic disease. However, whether they actually benefit is still the question, and most studies have been based on murine (mice) models, but not yet translated to human disease. There is enough clinical evidence to suggest that antioxidant supplements have at best little value in preventing or modifying a chronic disease course. The failure to suggest otherwise is likely based on the differences between rodent and human biology, high-dose treatments in rodents, and non-delivery of human tissues. However, there is hope the more we come to understand chronic disease biology. As stated earlier, some antioxidant supplements may counteract the cells ability to balance itself and overdosing on some antioxidants may have a counter effect, such as with tumors. What is needed is a “smart antioxidant” which can cause a redox imbalance in tumor cells but not in normal tissues. A potential category of compounds could be sirtuin 3 activators (a major mitochondrial NAD+-dependant deacetylase that plays a critical role in mitochondrial proteins activation, involved in energy metabolism, and changes in its expression associated with excessive ROS production (61). There is evidence that some natural polyphenolic compounds could act as “smart antioxidants” via sirtuin 3 activation (62-64).
Conclusion
In order to minimize chronic redox imbalance damages, it’s best to follow a a balanced and varied diet along with healthy habits such as regular exercise to avoid obesity, not smoking, and reducing alcoholic beverage intake. The consumption of antioxidant supplements would only make sense in cases where one is suffering form deficiencies in order to normalize levels. Antioxidant therapeutic usefulness against cancer still requires more research and investigation.
