Nourishing the immune system

If you suspect your immune system is getting flabby, toughen it up with nutrients.

As regular readers of PEAK PERFORMANCE will be aware, taking part in sport – particularly endurance sport – may be at a cost to your immune system. It seems that highly active sports competitors and athletes are more prone to infectious diseases. You may live longer, but you could suffer more than your fair share of colds and flu in the process.

It was over 20 years ago that people started to suspect that exercise could be damping down the immune system. Medics began to notice a pattern in colleges and schools whereby sports team members and athletes were more likely than their couch-potato colleagues to go down with infectious illnesses. In the mid-80s, a large study based in Washington was published showing that respiratory diseases, gastrointestinal infections and skin complaints were at least twice as common among sports competitors than in the public at large.

But with statistics like this it’s hard to tease (out the truth of the matter. Bacteria and viruses might spread quickly between team members because of time spent together in close physical contact; coughs and colds could be a result of all that gulping in of cold (and possibly, polluted) air while training. Experts currently admit that there’s strong suggestive evidence for heavy exercise affecting the immune system, but debate continues over the nitty gritty of the exact effects, and how they come about.

How hard and how long
For example, there’s increasing evidence that the intensity and duration of exercise are key factors determining whether your workouts will increase or decrease your chances of falling ill. For moderate exercise, the evidence points to a beneficial effect on the immune system, particularly for older people. For example, scientists at the University of Newfoundland, Canada, conducted a three-month study into the effects of moderate exercise on people aged 65 and over. Compared with control subjects of the same age, the subjects showed both improved T-cell responsiveness and spent, on average, 40 per cent fewer days in hospital with respiratory infections.

Some experts plump for a figure of 45 minutes of moderate exercise a day as the optimum for the immune system, but this seems overly specific in an area where there’s a lack of precise knowledge.

Intensive, long-duration sport is more likely to be bad news for your immunity. The balance of evidence suggests that heavy exercise tends to damp down T cells. For example, marathon runners have been found to have a white blood cell count at least 25 per cent below average. And natural killer cell count has been found to be depressed for six hours after a race.

If you’re involved in an endurance sport, or one that involves demanding training, you may already be aware that you’re prone to getting ill. Is there anything you can do about this apart from swearing off exercise and chaining yourself to the settee? Yes – you can eat to give your immune system a helping hand. Some nutrients have a negative effect, while others appear to boost immunity.

Micronutrients and immune responses
Over recent years there has been a surge in interest in the relationship between ‘micronutrients’ (ie, vitamins, minerals and other food-derived factors) and the immune system. Much of the early research in this area resulted from attempts to find out how best to treat people and children suffering from malnutrition. Infections are a common cause of death in those suffering from severe food shortage – this is because the immune system is severely compromised by various dietary deficiencies. Work in this area helped to establish which nutrients should be given as a priority to re-establish a healthy immune system.

Knowledge gained from this vital area of work can now be applied to others in less dire circumstances, as deficiencies in key nutrients can have a significant effect on the immune system even when there is no generalised malnutrition. In fact, it’ s been discovered that the immune system is very sensitive to reduced micronutrient intake, with impaired activity being seen at early stages of deficiency. For example, an isolated deficiency of zinc can result in a number of immune-related problems, including slower wound healing and a higher risk of skin infections.

Vitamin A
It has long been known to be vital to immune functioning. Deficiency results in a slight reduction in the weight of the thymus, the boss gland of the T cell system. In experimental animals, deficiency results in decreased numbers of T cells and suppressed production of antibodies by B cells.

B-carotene, found in fruit and vegetables and capable of being converted into vitamin A by the body, has its own independent role in boosting immunity, which may be related to its ability to function as an antioxidant (see below).

Vitamin A is toxic at high doses; B-carotene is relatively safe.

Source: Vitamin A – liver, eggs, milk, butter, margarine.

B-carotene – beetroot, carrots, green leafy vegetables.

B vitamins
Vitamin B6 has been found to be a key vitamin for immunity, being involved in the process of synthesising immune cells. In animals, deficiency causes profound changes in immune responses: the thymus is smaller, and there is impaired cell-mediated immunity. There is decreased antibody formation after immunisation. However, isolated vitamin B6 deficiency is rare in humans, and is generally only seen in the context of generalised malnutrition. Taking large doses (over 50mg) as a supplement is not recommended, as side-effects have been found. Source:
Meat, fish, eggs, wholegrain cereals, nuts and seeds. Folate is also thought to be involved in maintaining a healthy immune system. Sources: Liver, green leafy vegetables.

The other B vitamins have only a minimal influence on the immune response.

Vitamin C
Ever since Nobel prizewinner Linus Pauling championed the downing of megadoses of vitamin C, the debate has raged on whether vitamin C can help to prevent infections. It’ s clear that vitamin C is involved in the workings of certain branches of the immune system. In particular, it plays a role in keeping macrophages nippy and able to ingest bacteria. It’s not thought to be so involved in cell-mediated immunity or the production of antibodies. Source: Fresh/minimally cooked fruit and vegetables

It has been the focus of much research in the field of immunity, especially where the treatment of malnutrition is concerned. When this mineral is lacking, Iymph glands become smaller, there are delayed skin hypersensitivity responses (this indicates an impaired T cell response), and there is lower thymus hormone activity. In animals the same effects are seen, together with reduced natural killer cell activity. There is a lot of recent interest in the role of zinc in macrophage activity. Wound healing is impaired.

Male and female athletes have been reported to have lower plasma zinc concentrations than people who are untrained, and their diets typically contain less zinc per calorie (‘Hypozincemia in Runners’, Physician & Sportsmed 8, pp97-100). What’s more, exercise may increase losses of zinc from the body. For example, higher excretion of zinc in the urine has been found in runners compared to sedentary subjects. Thus, a decreased body pool of zinc could be a factor in the impaired immunity of some athletes.

Don’t take huge doses – high amounts can actually suppress the immune system. Also, you should be aware that iron supplements can hinder zinc absorption. Source: Meat, nuts and seeds, wholegrains, milk.

Iron deficiency is the most common nutritional problem worldwide. Some athletes may be at additional risk of having a poor iron credit – for instance, distance runners are thought to be prone to higher losses of iron, and to be susceptible to exercise- induced anaemia.

It seems to be common folk wisdom that taking some extra iron will sort you out if you’re under the weather and maybe going down with colds with uncomfortable frequency. But the iron/immune system equation is not that simple. On the one hand, iron is needed by natural killer cells, neutrophils and Iymphocytes for optimal function. But on the other, free iron is actually necessary for bacterial growth. Researchers have found that if iron is removed chemically from a solution in a test tube, bacterial multiplication is slowed down. This has led to some experts suggesting that iron deficiency could be a smart way of limiting bacterial infections. However, the clinical information available at present tends not to support this idea (‘Nutrition and Immunity- lessons from the past and new insights into the future’, Am J Clin Nut, 53, pplO87-1101).

So, if you feel your immune system could do with a boost, should you consider upping your iron intake or not? One thing’s for sure – it definitely won’t do you any good to take iron supplements if you don’t know you’re deficient in the first place. If you think you may be anaemic, ask your doctor to do a blood test. Iron is one of those nutrients where more than the optimum is definitely not better. Source: Red meat, whole grains, pulses and beans.

Amino acids
Amino acids affect immune responses in many different ways. Deficiencies of selected amino acids decrease antibody responses; in other states of amino acid imbalance, the overall response may be enhanced, indicating that changes in suppressor T cells may have occurred. In particular, there is evidence for the immunostimulatory and anti-infective role of glutamine and arginine. For example, in a study in a Boston hospital, patients suffering from immune suppression who were given extra glutamine for about a month significantly improved their resistance to infectious diseases.

There is some evidence that endurance athletes may have a need for extra glutamine. For example, biochemist Eric Newsholme and fellow researchers at Oxford University found that during a marathon race, the concentration of glutamine fell by up to 25 per cent in the blood of the runners. The researchers also found that the concentration of glutamine was low in blood samples from a group of athletes suffering from the overtraining syndrome.

The free radical connection
Free radicals are highly reactive molecules which are made inside the body, mainly as part of the process of making energy from oxygen. We can also take in extra doses by eating food in which the fat has turned rancid, by smoking or drinking alcohol. Left to their own devices, they can leave a trail of destruction around the body – and of most relevance to immune function, they will attack and disrupt cell membranes. In particular, free radicals have been shown to affect the cell membranes of white blood cells, which in turn adversely affects immune responses.

If you’re involved in a lot of physical activity, you need extra energy, which means you need to process more oxygen to get it. Unfortunately, this also means that you’ll generate more free radicals as an unwelcome side-line. End result? Athletes are more vulnerable to the problems caused by free rads, including damage to immune cells. But all is not lost; this damage can be minimised by increasing your intake of ‘antioxidants’ – substances used by the body to neutralise these destructive molecules. Substances which function as antioxidants include vitamins C and E, B-carotene and selenium.

In addition, a recent study investigated the effects of zinc supplementation on exercise-related changes in the immune system. Five male runners were randomly assigned to take a zinc supplement or placebotwice daily for six days. The clearest finding was that zinc supplementation blocked an exercise- induced increase in free radical generation by certain immune cells. The authors speculated, ‘perhaps decreasing free radical generation by Zn supplements will confer benefit to individuals exposed to chronic physical stress for prolonged periods’.

There is considerable evidence that eating too much fat impairs your immune system. Diets high in total fats (fat= greater than 30 per cent of calories) have been associated with decreased immune responses, including decreased cell-mediated immunity and natural killer cell function. Yet another reason to cut your fat intake and fill up with carbohydrates !

Janet Pidcock