Antioxidants and Food: An in depth Guide
As tempting as it is to write a post about what I ate yesterday, or indulge in some sort of paleo quiz, I’m just going to have to write another boring sciencey-type post where I attempt to interpret something from an evolutionary perspective. If that bores people to tears, then I am sure there is an off button around you somewhere. For those of us who have to butt heads with conventional wisdom on a daily basis, boring science posts might still hold some interest… The job isn’t done yet.
One of the constant justifications and marketing points for various incarnations of fruit, vegetables, and grains is their supposed antioxidant capacity. There would be very few products made from these base ingredients that do not make the “rich in antioxidants” claim on the label, giving a halo effect to the total product, and distracting you from the likes of the sugar content also contained within the product.
All living things are prone to oxidation (think of a rusting piece of metal), and as such, all living things have their own endogenous antioxidant defences. What we have been convinced of with regard to fruits and vegetables, is that because they are rich in antioxidants, we inherit their antioxidant capacity upon consuming them. This is true when we look at antioxidants such as vitamins C and E. But things can get a bit sketchy when we look at other compounds, which may well be very active within the source plant, and when that particular compound is isolated, concentrated, and tested in a test tube, but might not hold the same benefits in the human body. Take resveratrol in this example;
The claimed benefits of red wine are so often reported that they may seem to be a proven fact, and the critical voices do not get much attention. As always, we have a possible bias in the form of confounders. Furthermore, in order to get a true protection against cardiovascular disease you may need to drink so much every week that your brain and liver are damaged. In epidemiological studies, the difference in mortality between people who drink 2 glasses of wine per day and those who drink once a month is negligible or non-existent. Enthusiastic reports about resveratrol, a substance in red wine which prolongs life in mice that are fed high-fat/sucrose diets, often forget to mention that the doses used correspond to more than 700 bottles of wine per day.
Fortunately, like plants, we have our very own endogenous antioxidants to stop us going rusty. How clever is that? What was quite exciting for me, when I stumbled up a chapter on endogenous antioxidants in a text-book, was just where some of these endogenous antioxidants come from (clue: not all of them are from plant foods);
Endogenous antioxidants and radical scavengers.
All living organisms are constantly exposed to oxidant agents deriving from both endogenous and exogenous sources capable to modify biomolecules and induce damages. Free radicals generated by oxidative stress exert an important role in the development of tissue damage and aging. Reactive species (RS) derived from oxygen (ROS) and nitrogen (RNS) pertain to free radicals family and are constituted by various forms of activated oxygen or nitrogen. RS are continuously produced during normal physiological events but can be removed by antioxidant defence mechanism: the imbalance between RS and antioxidant defence mechanism leads to modifications in cellular membrane or intracellular molecules. In this chapter only endogenous antioxidant molecules will be critically discussed, such as Glutathione, Alpha-lipoic acid, Coenzyme Q, Ferritin, Uric acid, Bilirubin, Metallothioneine, L-carnitine and Melatonin.
All living organisms, be they plant, animal, or otherwise, are constantly exposed to stressors that can modify biomolecules such as nucleic acids within DNA/RNA, proteins, carbohydrates, and polyunsaturated lipids, all of which might be performing important structural and functional roles at the point they are “modified”. These modifying agents are reactive species known as radicals and may be derived from oxygen, nitrogen, chlorine (chlorinated water anyone?), bromine, and sulphur. When we are talking “antioxidants” we are referring specifically to defences capable off preventing or reducing the impact of a reactive oxygen species (ROS).
ROS is a collective term that includes the oxygen radicals superoxide and hydroxyl, and non-radical oxygen groups that can be easily converted into oxygen radicals, such as hydrogen peroxide and single oxygen molecules themselves. ROS are known factors in cellular injury and the ageing process, including damage to DNA that can lead to premature cell death. The oxidation of amino acids and the alterations to a protein that this might cause might result in the loss of activity of enzymes and metabolic pathways. The oxidation of carbohydrates (leading to the formation of Advanced Glycation End-products [AGEs]), can create highly reactive sugars that can become involved in a range of pathological effects such as increased vascular permeability, oxidation of cholesterol (particularly LDL cholesterol), and oxidative stress.
The oxidation of lipoproteins such as LDL is an underlying factor in the development of diseases such cardiovascular disease, metabolic diseases, arthritis, and dementia. Within these diseases, the borax is often pointed at high levels of LDL cholesterol being an issue, when in fact the implication of cholesterol refers to its oxidised form. So the idea, with regard to disease prevention, shouldn’t be about reducing cholesterol levels but in protecting this cholesterol from oxidation by ensuring high levels of antioxidant defences – both those obtained by diet and those made endogenously within our body (the base building blocks of which are still likely obtained from diet – just not from all the foods that you would normally think of).
Due to their highly unsaturated nature, polyunsaturated fatty acids, such as the omega 3 and 6 fatty acids, are incredibly prone to oxidative processes. As these fatty acids are often concentrated in the membranes of each cell and are integral to the proper physiological functioning of these cells, any damage that occurs to them as a result of oxidation, may have a profound effect on the structure and function of that cell. Exposing the dietary forms of these fats to light, heat, and air, and then swallowing them in their oxidised (rancid) forms, is really going to stretch your in-built antioxidant defences. Best to stick with the very stable saturated fats for the most part.
There are a couple of key diagrams early within this review chapter that highlight which cells are most prone to oxidative attack and what the results of this might lead to in the long-term;
Main Cell Types Affected By ROS (example tissue)
- Endothelial cells (blood vessels)
- Epithelial cells (airways)
- Neurons (nerve cells)
- Monocytes (immune cells)
- Erythrocytes (red blood cells)
- Fibroblasts (connective tissue)
- Hepatocytes (liver cells)
Diseases Correlated With Oxidative Stress
It is likely that not all oxidative reactions are pathological, with there being a balance between ROS and their inactivation by antioxidant defence systems. But certainly when too many ROS are produced and exceed the body’s ability to control them, the balance shifts toward oxidative stress and the modifications (damage) to cellular membranes and biomolecules that this brings.
Let’s have a look at some specific endogenous antioxidants…
Reduced glutathione (GSH) is a water-soluble tripeptide that is present mainly within cells, although other forms exist extracellularly. Large amounts of GSH is present in the respiratory tract, making sense from the standpoint that air we inhale may contain reactive oxygen and reactive nitrogen species such as ozone and nitrogen dioxide. Therefore, GSH is part of our front line defenses. It is also responsible for providing antioxidant protection to red blood cells. GSH can react directly with a number of ROS and RNS, and it can break disulfide bridges formed inside and between proteins by the action of oxidants.
GSH can also act as an antioxidant indirectly by regenerating vitamin C and vitamin E after both of those antioxidant vitamins have themselves been used in antioxidant reactions. And GSH is part of a group of nutrients which protect mitochondria – our cellular powerhouses – from oxidation. These “mitochondrial nutrients” directly and indirectly protect mitochondrial function, something that is incredibly important for both metabolic and brain health.
At the heart of GSH is the mineral selenium, making GSH a selenoprotein;
Five selenium-containing glutathione peroxidases (GPx) have been identified: cellular or classical GPx, plasma or extracellular GPx, phospholipid hydroperoxide GPx, gastrointestinal GPx, and olfactory GPx. Although each GPx is a distinct selenoprotein, they are all antioxidant enzymes that reduce potentially damaging reactive oxygen species (ROS), such as hydrogen peroxide and lipid hydroperoxides, to harmless products like water and alcohols by coupling their reduction with the oxidation of glutathione (diagram). Sperm mitochondrial capsule selenoprotein, an antioxidant enzyme that protects developing sperm from oxidative damage and later forms a structural protein required by mature sperm, was once thought to be a distinct selenoprotein but now appears to be phospholipid hydroperoxide GPx.
Whilst Brazil nuts are the richest source of selenium, other good paleo sources include fish and seafood, chicken, pork, and beef.
Alpha-Lipoic Acid (LA)
LA is another mitochondrial nutrient that has been noted to assist in the improvement of age-related decline of memory, in mitochondrial structure and function, and in treating diabetes, being involved in glucose utilisation and insulin signalling. It functions to regenerate the antioxidant capacity of other antioxidants and to inhibit oxidative damage from reactive minerals such as copper and iron.
Whilst LA can be synthesised in the body, it can also be derived from dietary sources, with the richest sources being organ meats, spinach, and broccoli.
CoenzymeQ is the only antioxidant that is made in the body that is active within fatty membranes, exceeding the capacity and efficiency of all other antioxidants. Due to its location (cell membranes), its role is to protect lipids, proteins, and DNA. Like lipoic acid, whilst being made within the body, it can be obtained from the diet, with (sorry vego’s) meat, poultry, and fish being the richest sources. Taking a statin? Perhaps you might want to think about the effect of this drug on your antioxidant capacity;
Coenzyme Q10 shares a common biosynthetic pathway with cholesterol. The synthesis of an intermediary precursor of coenzyme Q10, mevalonate, is inhibited by some beta blockers, blood pressure-lowering medication, and statins, a class of cholesterol-lowering drugs. Statins can reduce serum levels of coenzyme Q10 by up to 40%. Some research suggests the logical option of supplementation with coenzyme Q10 as a routine adjunct to any treatment that may reduce endogenous production of coenzyme Q10, based on a balance of likely benefit against very small risk. However, there are still no conclusive data that support the role of CoQ10 deficiency in the pathogenesis of statin-related myopathy.
This one came as a complete surprise to me, but makes perfect sense. Ferritin is the main intracellular iron storage protein, present in every cell, keeping iron in a soluble and non-toxic form. Whilst iron is essential for cellular functions, excessive iron can be dangerous to the cell because of increased oxidation of DNA, proteins, and cell membranes. The body, therefore, tries to keep tight control on the availability of iron and uses several binding proteins to transport and store iron.
At a guess, I would suggest that the iron contained within meat is also bound to these proteins, minimising the amount of free iron that can roam around the gut and be absorbed. What free iron is set loose could be mopped up by the phytic acid levels contained within vegetable foods eaten concurrently (hence why there is likely a difference between disease rates of those who consume red meat as a “steak and chips” meal, versus those who eat their steaks with some leafy greens). This also underscores the importance of getting your iron from whole food sources, such as meat, rather than thinking iron supplements can take the place of a juicy steak.
Uric acid, considered a waste product, is an important free radical scavenger within plasma, providing nearly a quarter of total antioxidant activity. Elevated levels of uric acid are often associated with being a risk factor for cardiovascular diseases, but the new data suggests that the elevation of uric acid, being an antioxidant, might in fact be a defence mechanism against advanced atherosclerosis. Along similar lines, it has been hypothesised that any cardioprotective effect of red wine might be mediated by uric acid.
Bilirubin is the yellow colour you get on your skin during the bruising process., and is formed as part of the breakdown of the haem component of haemoglobin from red blood cells. Bilirubin has been shown to defend against hydrogen peroxide-mediated damage in neurons ad thus constitutes a defence mechanism against neurotrauma. Bilirubin is also associated with decreased cardiovascular disease and cancer.
Metallothioneins are a protein-rich group that can bind physiological metals such as zinc, copper, and selenium, or toxic heavy metals such as cadmium, mercury, silver, and arsenic. As such, they are heavily involved in zinc homeostasis and in protection against heavy metal toxicity and oxidative stress.
L-Carnitine plays a role in shuttling long-chain fatty acids (such as those derived from animal fats), across the inner mitochondrial membrane for energy (ATP) production. Again, whilst L-carnitine can be made, the important dietary sources are almost exclusively of animal origin. L-carnitine prevents oxidative stress and regulates enzymes involved in the defence against oxidative damage, and works closely with lipoic acid. It can be considered another mitochondrial nutrient.
Melatonin is perhaps better know as the hormone involved with sleep. However, it is thought its original evolutionary role was to act as an antioxidant;
Many biological effects of melatonin are produced through activation of melatonin receptors, while others are due to its role as a pervasive and powerful antioxidant, with a particular role in the protection of nuclear and mitochondrial DNA.
Melatonin has the ability to not only act as an antioxidant itself, but to activate other antioxidant enzymes and induce their expression (make more of them). It is effective in protecting DNA, membrane lipids, and intracellular proteins from oxidative damage and increased membrane rigidity. Within the brain, melatonin concentration is only 5% of what is seen in the serum, and as such, melatonin is not likely to be a major antioxidant in this tissue compared with the likes of glutathione and vitamin E.
Melatonin plays a key role in regenerating the antioxidant capacity of glutathione and protecting mitochondria. Mitochondria produce high amounts of reactive oxygen species which is mopped up by glutathione. After oxidative stress in mitochondria, virtually all glutathione is oxidised and the total antioxidant activity of glutathione is reduced to practically zero. Melatonin restores the normal activity of glutathione is this situation.
Reductions in melatonin concentration, particularly nocturnal levels, has been seen in pineal gland, plasma, cerebral spinal fluid, and in the urine, though this may be more a factor of lifestyle changes as we age than simply a factor of ageing itself. Melatonin has been shown to have neuroprotective effects in situations involving neurotoxicity and/or excitotoxicity. And in pathologies in which the high production of free radicals is the primary cause of the disease, melatonin is also protective. The common feature of these diseases is the existence of mitochondrial damage due to oxidative stress.
Is all oxidation bad?
It would be easy to believe, from all the hype around antioxidants, that all oxidation in the body needs to be suppressed. However, like many processes in the body, a distinction needs to be made between what is physiological and what is pathological. At a physiological level, reactive oxygen species are part of normal cellular signals and induce cellular differentiation and cell death (you don’t want damaged and aged cells living on in perpetuity). In fact, a cascade toward cancer development that begins with decreased levels of reactive oxygen species has been hypothesised.
Antioxidant supplementation has recently been questioned with regard to muscle damage and adaptation to athletic training;
The high forces undergone during repetitive eccentric, or lengthening, contractions place skeletal muscle under considerable stress, in particular if unaccustomed. Although muscle is highly adaptive, the responses to stress may not be optimally regulated by the body. Reactive oxygen species (ROS) are one component of the stress response that may contribute to muscle damage after eccentric exercise. Antioxidants may in turn scavenge ROS, thereby preventing or attenuating muscle damage. The antioxidant vitamins C (ascorbic acid) and E (tocopherol) are among the most commonly used sport supplements, and are often taken in large doses by athletes and other sports persons because of their potential protective effect against muscle damage. This review assesses studies that have investigated the effects of these two antioxidants, alone or in combination, on muscle damage and oxidative stress. Studies have used a variety of supplementation strategies, with variations in dosage, timing and duration of supplementation. Although there is some evidence to show that both antioxidants can reduce indices of oxidative stress, there is little evidence to support a role for vitamin C and/or vitamin E in protecting against muscle damage. Indeed, antioxidant supplementation may actually interfere with the cellular signalling functions of ROS, thereby adversely affecting muscle performance. Furthermore, recent studies have cast doubt on the benign effects of long-term, high-dosage antioxidant supplementation. High doses of vitamin E, in particular, may increase all-cause mortality. Although some equivocation remains in the extant literature regarding the beneficial effects of antioxidant vitamin supplementation on muscle damage, there is little evidence to support such a role. Since the potential for long-term harm does exist, the casual use of high doses of antioxidants by athletes and others should perhaps be curtailed.
Similarly, antioxidants and the suppression of reactive oxygen species may cause issues with fertility in women. If a degree of reactive oxygen species is required for normal physiological function, then the aim in dealing with ROS and oxidation is regulation rather than eradication.
Without a doubt we can improve our antioxidant capacity via our diet, particularly obtaining the basic building blocks for our primary endogenous antioxidants, and by obtaining some of these endogenous antioxidants pre-formed, e.g. L-Carnitine. And of course, we can obtain the likes of vitamins C and E from our diet. For sure, vegetables and fruit can provide many of these building blocks, but they are by no means the sole source. I remain unconvinced that there is much to the marketing hype that often goes with extolling the virtues of antioxidants in many plant foods and the juices (loosely) derived from them. And let us not forget that many of the endogenous antioxidants that have been covered here can be built from or augmented by food derived from animal sources.