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Visa fullständig version : Dietärt nitrat minskar maximal syreförbrukning medan arbetskapaciteten upprätthålls


King Grub
2009-11-17, 09:50
Background: The anion nitrate - abundant in our diet - has recently emerged as a major pool of nitric oxide (NO) synthase-independent NO production. Nitrate is reduced stepwise in vivo to nitrite and then NO and possibly other bioactive nitrogen oxides. This reductive pathway is enhanced during low oxygen tension and acidosis. A recent study shows a reduction in oxygen consumption during submaximal exercise by dietary nitrate. We now went on to study the effects of dietary nitrate on various physiological and biochemical parameters during maximal exercise. Methods: Nine healthy, non smoking volunteers (age: 30 +/- 2.3 years, VO(2)max 3.72 +/- 0.33 l/min) participated in this study which had a randomized, double-blind crossover design. Subjects received dietary supplementation with sodium nitrate (0.1 mmol/kg/day) or placebo (NaCl) for 2 days prior to the test. This dose corresponds to the amount found in 100-300 g of a nitrate-rich vegetable such as spinach or beetroot. The maximal exercise tests consisted of an incremental exercise to exhaustion with combined arm and leg cranking on two separate ergometers. Results: Dietary nitrate reduced VO(2)max from 3.72 +/- 0.33 to 3.62 +/- 0.31 l/min, p<0.05. Despite the reduction in VO(2)max the time to exhaustion trended to increase after nitrate supplementation (524 +/-31 vs 563 +/-30 sec, p=0.13). There was a correlation between the change in time to exhaustion and the change in VO(2)max (R(2)=0.47, p=0.04). Conclusion: A moderate dietary dose of nitrate significantly reduces VO(2)max during maximal exercise with a large active muscle mass. This reduction occurred with a trend towards increased time to exhaustion implying that two separate mechanisms are involved; one that reduces VO(2)max and another that improves the energetic function of the working muscles.

Free Radic Biol Med. 2009 Nov 11. Dietary nitrate reduces maximal oxygen consumption while maintaining work performance at maximal exercise.

Guddi
2009-11-17, 12:39
Två intressanta inlägg i debatten kring nitrat som kosttillskott i senaste numret av Journal of applied physiology:

Bitarna är utvalda av mig. Vill ni läsa hela så krävs det tillgång till tidskriften.
These data confirm and expand on the surprising and original findings by Larsen et al. (4) of nitrate-induced improvement of exercise efficiency in humans. This newly discovered ergogenic potential of dietary nitrate will certainly inspire athletes to boost their performance and nutriceutical companies to boost their sales. But what are the safety concerns of nitrate supplementation?

What health risks could possibly be expected from the nitrate content of a—on first thought—normal dose of vegetables, such as beetroot or spinach? The concern about nitrate is possible carcinogenicity. Nitrate and nitrite are probably not carcinogenic, but nitrite formed from dietary nitrate might react with dietary amines from proteins to form carcinogenic nitrosamines in the acidic gastric environment and is generally associated with intake of pickled or preserved food. The ADI guidelines have a large safety margin, so the health risk of the used nitrate intake in the above-mentioned studies is probably very limited. However, the problem may enlarge when nitrate is combined with other nutritional supplements, for there is a growing and troubling tendency among supplement-selling companies to create "ergogenic cocktails" without sufficient scientific support for their efficacy or safety.

It can be anticipated that the combined intake of creatine and nitrate will be promoted by companies and adopted by athletes. Creatine in itself has been allegedly linked to cancer risk because creatine can react with nitrite to form the highly carcinogenic N-nitrososarcosine (NSAR) in vitro (1). Urinary NSAR levels correlate positively with esophageal cancer mortality rates (6). However, in vivo nitrosation of creatine is probably negligible, as urinary NSAR levels do not increase in people daily ingesting 20 g of creatine monohydrate (3). However, this does not exclude that the toxicological hazard becomes realistic when not only the nitrosation-prone substrate (creatine or protein) but also the nitrosation agent (nitrite) is increased by elevated dietary nitrate consumption above ADI, leading to elevated gastric and circulating nitrite levels (2).

Moreover, when considering vegetables as a nitrate source, vitamin C or other antioxidants are also present in the vegetables, which could partially limit the formation of nitrosamines in the gastrointestinal tract (5). Dietary nitrate supplementation as sodium nitrate, in absence of this "natural" antioxidant protection, could prove to be more harmful for human health, especially in conditions of enhanced protein or creatine intake.

The recent findings on nitrate (2, 4) are valuable in promoting our understanding of oxygen consumption regulation in contracting muscles. Yet, progress in our field can often have relevance to performance enhancement. The safety of new ergogenic supplements falls under our responsibility and should be subject to further investigation.
http://jap.physiology.org/cgi/content/full/107/5/1677?rss=1
In truth we do not know the relative risk/benefits of giving large amounts of inorganic nitrates to humans. We know that vegetarians (who eat larger amounts of nitrate) have reduced incidence of cancer and vascular disease, and on average live longer than those who eat meat. We believe that nitrate may be a major reason for the beneficial effects of a high vegetable intake, such as reducing blood pressure, protecting the stomach from ulceration, preventing vascular damage, and preventing infection (4). Bryan and colleagues (3) have recently pointed out that the influential "Dietary Approaches to Stop Hypertension" (DASH) diet recommended to hypertensive patients exceeds the WHO ADI for inorganic nitrate by 550%. At the same time, as Derave and Taes (2) point out, it is possible that certain combinations of inorganic nitrate and nitrosatable compounds could be harmful, and that certain individuals, such as those with Barrett's esophagus (8), may be particularly at risk.

We should be aware that there is some slight evidence of risk, but there is also increasing evidence that more nitrate in our diet may be good for us.
http://jap.physiology.org/cgi/content/full/107/5/1667?rss=1

cyclist_90
2009-11-19, 10:20
intressant för oss sprinters på cykelsidan;) ingen mer rödbetsjuice för oss