Diet affects performance: this is such a well-researched and documented fact that nothing more need be said on it here. In contrast, there has been relatively little research on whether diet can be a factor in the prevention of sports injuries. Nevertheless, in recent years there has been enough discussion of theoretical links between nutrition and potential injury causes, along with a few empirical studies, for every athlete to realise that diet is as vital for injury prevention as it is for maximum competition and training results.
Carbohydrate and protein:
First of all, it's important to recognise that the correct carbohydrate intake is essential for avoiding injury just as it is for energy. Theoretically, the level of carbohydrate intake could influence injury-risk status. If glycogen levels are low, then muscle-protein breakdown may increase to compensate for the lack of the preferred fuel supply. Chronic glycogen depletion may lead to decreases in strength and possible soft-tissue damage.
While glycogen is the main fuel for exercise, recent research has emphasised that muscle protein is definitely broken down in both strength and endurance training (Brukner, 1993). To compensate, protein synthesis is increased post-exercise. Thus the athlete's diet must include an increased amount of protein to allow for this synthesis. As with a lack of glycogen, insufficient protein may lead to decreases in strength and possible muscle damage due to decreased muscle mass. Tarnopolsky et al (1988) measured the nitrogen balance of bodybuilders and endurance athletes who were training at a high level. Their findings led to recommendations of 1.2g of protein per kg of body weight per day for the bodybuilders and 1.6g/kg/day for the endurance athletes. Other research confirms these findings, recommending 1.4g/kg/day for regular training. This is significantly greater than the US RDA of 0.8g/kg/day.
The message here is that, with regular training and especially endurance training, you will be unable to support your muscle mass without sufficient carbohydrate and protein. This may lead to long-term strength loss, which is a potential injury cause.
Calcium and iron:
At the micronutrient level, the lack of certain vitamins and minerals has been linked with injuries. For instance, calcium intake is a factor contributing to bone density. Research with animals has shown that calcium deficiency can cause osteoporosis, which is reversed when calcium levels are restored. If calcium is essential for healthy bone density, then any deficiency may increase the risk of stress factors.
This theoretical link is supported by some empirical research by Myburgh and colleagues (1988). They analysed 25 athletes suffering from lower-leg stress fractures and compared them with control subjects who had no shin pain. The controls were matched for weight, height and performance levels. The researchers found that the stress-fracture group had significantly lower bone-mineral densities than the control group. They also had significantly lower calcium intakes and fewer dairy products in their diets. Although the research suggested that some biomechanical factors were also implicated in the injury rates, it clearly showed that the athletes' diet, specifically their calcium status, was a potential cause of their stress fractures.
Iron deficiency can also be a potential cause of injury, Peter Brukner (1993) describes how low blood-iron levels will reduce the oxidative potential in the muscles. This will shift the energy production towards producing more lactate, which may contribute to muscle injuries. Frank Horwill, the BAF coach, cites a study involving 164 cross-country female runners which found that a quarter of the athletes had low serum ferritin level (blood iron count) and that this low-iron group was three times more likely to be injured during the rest of the season than the other runners.
The message here is that athletes who want to train regularly must ensure that their diets contain sufficient calcium and iron. This will help promote healthy bones and muscles with full oxidative function. Thus the bones will withstand repetitive stresses and the muscles won't fatigue too early.
There is strong evidence to suggest that vitamins C and E may have an important role to play in injury prevention, acting as antioxidants to combat the muscle damage caused by exercise.
When we exercise, the body uses oxygen to form ATP, which allows the muscles to contract. This is known as the aerobic energy pathway. However, the downside of this pathway is that the breakdown of oxygen is not 100 per cent clean. Inevitably, free radicals such as superoxides, hyperoxides and hydroxyls are produced. The more training an athletes does, the more oxygen is used and the more oxygen free radicals are formed. These free radicals are dirty and cause damage to the muscle cells via a process called lipid peroxidation. This then triggers off an inflammatory response in the muscle to mop up the damaged cells. It is this inflammatory response that is associated with muscle soreness after vigorous training. Dekkers and colleagues (1996) and Michael Colgan, the sports nutrition guru (1993), both review conclusive research which shows that this lipid peroxidation process is increased post-exercise and is a cause of muscle-cell damage.
Fortunately, the body has a defence mechanism against the unwanted free radicals in the form of antioxidants. These neutralise the radicals, inhibiting the damaging lipid peroxidation process. The body has endogenous antioxidants, which it can form itself and which combat the formation of free radicals. In fact, with training the body is able to increase the endogenous antioxidant activity post-exercise (Dekkers et al, 1996). In addition, the body can also use nutrient antioxidants, such as vitamin E (tocopherol) and vitamin C to act against free radicals.
Michael Colgan describes how vitamin E is fat-soluble, and therefore it can absorb the radicals inside the fatty membranes, breaking down the lipid peroxidation reaction. This leaves behind tocopherol radicals which are then neutralised by vitamin C to regenerate vitamin E. Dekkers et al (1996) support this theory with reviews of various studies done with humans which show that vitamin E and vitamin C supplementation inhibits lipid peroxidation. This suggests that athletes in regular training who supplement their diet with these antioxidants will reduce their injury risk and enable them to train harder without suffering as much soreness.
To support this, Dekkers et al describe a study which examined the relationship between exhaustive stepping exercise and delayed onset muscle soreness (DOMS). The study compared two groups, one of which was given vitamin C and the other not. The vitamin C group showed significantly less DOMS than the control group. This suggests that extra antioxidants can reduce the muscle damage caused by exercise. Dekkers recommends antioxidant supplementation for anyone doing heavy, frequent exercise.
Michael Colgan argues that because of the extra antioxidant requirements due to the increased lipid peroxidation post-exercise, athletes will be unable to get sufficient vitamin E and vitamin C in their regular diet. Therefore, supplementation is crucial if an athlete wants to recover properly from heavy training and stay injury-free. He also recommends selenium and co-enzyme Q10 as two additional antioxidant supplements.
I've summarised some of what's been written on the nutrition-injury links which is supported by research using humans. While this is a new research area, and much more is needed, it should be clear that there are established relationships between nutrition and injury risks which athletes should be aware of. First, sufficient carbohydrate and protein is essential to maintain muscle strength with regular heavy exercise, especially endurance exercise. Second, sufficient calcium and iron are needed to ensure healthy bone density and full muscle oxidative function. Studies have shown that any calcium or iron depletion will increase injury risks significantly, since bones are weaker and muscles tire faster. Sufficient quantities of these minerals are especially important for women athletes. Finally, there is a specific biomechanical mechanism called lipid peroxidation which causes damage to muscles. Studies have shown that antioxidant supplementation, specifically vitamins E and C, will inhibit this process, thus allowing better recovery from hard training which in turn lowers injury risks.