It is widely accepted that increased intake of the preservative sodium nitrite found in processed meats such as deli meats, bacon, and hot dogs is associated with negative health outcomes for certain subgroups of the population (1). These associations have resulted in dietary nitrates and nitrites receiving a “bad name.” However, scientists have been quick to point out that diets proven to be beneficial for cardiovascular health contain high amounts of fruits and vegetables, are low in sodium and contain polyphenols, potassium, fiber, and, you guessed it, nitrate! (2) If dietary nitrates and its reduced form, nitrite, are so harmful, then why do we encourage consumption of fruits and vegetables containing these compounds?
Interestingly, dietary nitrate can reduce blood pressure through conversion to nitric oxide resulting in increased vasodilatation (3). Thus, it stands to reason that enhanced nitrate intake contributes to the cardioprotective effect of dark green leafy vegetables, which are a particularly rich source of nitrate (2). Evidence is emerging that consumption of dietary nitrate not only has positive effects on blood pressure, but that it also has the ability to enhance some parameters of exercise performance (3)! I want to point out, this is
independent of nitric oxide induced vasodilation.
In a recent study, subjects consuming 0.5L of beetroot juice prior to exercise exhibited a reduction in systolic blood pressure, as well as decreased oxygen consumption during walking and running (4). Indeed, beetroot juice is a rich source of dietary nitrate as well as other metabolically active compounds including antioxidants and polyphenols. In order to control for these variables, the authors of this study removed nitrate from the beetroot juice to use as a control. It became apparent that the positive effects on exercise observed in this study were due to the nitrate content of beetroot juice. Importantly, results from this study as well as others indicate dietary nitrate increased time to exhaustion in constant speed severe intensity running and cycling as well as decreasing the oxygen cost of exercise, indicating improved exercise economy (3, 5). While the mechanisms by which dietary nitrate improves these parameters remains to be fully understood, by use of a nitrate free beetroot juice, the authors definitively show that dietary nitrate has a positive physiological response to exercise.
These studies provide a valuable dietary modification of exercise performance. By reducing the oxygen cost at moderate and severe intensity running, one is reducing the energy cost of running. According to the authors, 4 days of dietary nitrate supplementation induced improvements equivalent to those observed following 6-9 weeks of physical training (4). Ideally enhanced running economy translates to enhanced endurance; however this parameter was not assessed during this study.
So how do we actually apply this knowledge to our sport, you ask? I am certainly not going to run around with beetroot juice in my bottles. What it boils down to is that there are not a lot of nitrate/nitrite supplements available - mostly because they are associated with the generation of N-Nitrosocompounds, a carcinogen thought to increase the risk of developing colon cancer. I think for this reason, you will not see a lot of companys (Clif, Gu, Powerbar, Hammer, etc.) pursue utilizing this interesting nutrient. However, as Hord et al. point out, the generation of these carcinogens is highly dependent upon the presence of vitamin C and lipids (fat). This is likely to occur during the consumption of salami for example (due to the fact it is preserved with nitrates and vitamin C and also has a high fat content) but is less likely to take place when eating lean deli meats, or during the consumption of a meal rich in dietary nitrates like can be found in spinach or beets. Dietary nitrates are thought to have a half life of 5-8 hours in circulation, thus we can try to incorperate these into our meal the night before a race, or if we have a supplement, immediately before, and during. This would also suggest we need a fresh bolus half way through a 24 hour endurance event. Using reference 2 I have provided a table classifying nitrate concentrations in commonly consumed vegetables. To me, it is interesting to think that there are some elite athletes who are known for their diets, think Scott Jurek, Dave Scott, Devon Crosby-Helms, and others who are open with their vegetable rich diets, may actually be benefiting from dietary nitrate. After having a conversation with Dr. Norm Hord, the author of an excellent review on this subject, he pointed out that V8 (the vegetable kind, not the sugary juice) is actually based in beetroot juice and is an excellent source of nitrate. My stomach rolls at the thought of drinking a V8 during a long run, but heck with all that salt, it might not be so bad.
TABLE 3
Classification of vegetables according to nitrate content
| Nitrate content (mg/100 g) |
|
| Very low, <20 | Artichoke, asparagus, broad bean, eggplant, garlic, onion, green bean, mushroom, pea, pepper, potato, summer squash, sweet potato, tomato, watermelon |
| Low, 20 to <50 | Broccoli, carrot, cauliflower, cucumber, pumpkin, chicory |
| Middle, 50 to <100 | Cabbage, dill, turnip, savoy cabbage |
| High, 100 to <250 | Celeriac, Chinese cabbage, endive, fennel, kohlrabi, leek, parsley |
| Very high, >250 | Celery, cress, chervil, lettuce, red beetroot, spinach, rocket (rucola) |
Works Cited
1. Gilchrist M, Winyard PG, Benjamin N. Dietary nitrate--good or bad? Nitric Oxide. Feb 15;22:104-9.
2. Hord NG, Tang Y, Bryan NS. Food sources of nitrates and nitrites: the physiologic context for potential health benefits. Am J Clin Nutr. 2009 Jul;90:1-10.
3. Ferreira LF, Behnke BJ. A toast to health and performance! Beetroot juice lowers blood pressure and the O2 cost of exercise. J Appl Physiol. Mar;110:585-6.
4. Lansley KE, Winyard PG, Fulford J, Vanhatalo A, Bailey SJ, Blackwell JR, DiMenna FJ, Gilchrist M, Benjamin N, Jones AM. Dietary nitrate supplementation reduces the O2 cost of walking and running: a placebo-controlled study. J Appl Physiol. Mar;110:591-600.
5. Bailey SJ, Winyard P, Vanhatalo A, Blackwell JR, Dimenna FJ, Wilkerson DP, Tarr J, Benjamin N, Jones AM. Dietary nitrate supplementation reduces the O2 cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans. J Appl Physiol. 2009 Oct;107:1144-55.
