It seems like almost all protein powders use artificial flavouring substances.Many articles on the web,recommend not using the same.Should i stick to natural protein powders?
The criticisms of artificial sweetener are overstated and there's nothing I'm aware of that shows a causal relationship between artificial sweeteners in human beings and real adverse health. Mind you, these substances have been scrutinized more than almost anything else you'll ingest. From the Mayo Clinic:
But according to the National Cancer Institute and other health agencies, there's no sound scientific evidence that any of the artificial sweeteners approved for use in the U.S. cause cancer or other serious health problems. And numerous research studies confirm that artificial sweeteners are generally safe in limited quantities, even for pregnant women. As a result of the newer studies, the warning label for saccharin was dropped.
There is one singular study, revolving around mice, showing a modification of GI bacteria and connection with glucose intolerance for those who consume artificial sweeteners. The study used 7, yes just 7, individual people. Hardly a good sample size.
Contrast that study against the hundreds and possibly thousands who have tried to make connections and failed, or of the ones that were simply thrown out after more information about the study conditions were found.
The other study that gets toted around is showing an increase in weight gain (in rats) with artificial sweeteners over sugar, but even in that fluid retention hasn't been ruled out. This quote is from the study's author, not some Splenda-paid-off lobbyist:
Greater weight gain was promoted by the use of saccharin or aspartame, compared with sucrose, and this weight gain was unrelated to caloric intake. We speculate that a decrease in energy expenditure or increase in fluid retention might be involved.
From Daniel's comment below, there is also this 2009 study showing a 36% relative risk increase (not causal) relationship between diet soda consumption and incident metabolic syndrome. It was a questionnaire based study.
Relative risk (from the 2009 study) is sneaky because it's not absolute risk, and relative risk doesn't really tell you anything about your risk. Relative risk is generally used when the probability of risk is very small.
Also, from the questionnaire-relative-risk 2009 study, there is this below snippet. Boiled down that means that the very associative-link people are grasping for (sweeteners=bad) is speculated to not at all be from artificial sweeteners:
Consequently, the previously observed diet soda–metabolic syndrome associations are generally speculated to be the result of residual confounding by other dietary behaviors, lifestyle factors, or demographic characteristics (1,2). Biological mechanisms possibly explaining these associations are few and largely focus on artificial sweeteners in beverages/foods increasing the desire for (and consumption of) sugar-sweetened, energy-dense beverages/foods (3) or disrupting consumers' ability to accurately estimate energy intake and remaining energy needs (4). Thus, diet soda consumption may result in overconsumption, increased body weight, and consequent metabolic dysfunction.
Keep up on the research, but right now there is no evidence based reason to even slightly consider artificial sweeteners as harmful. If you want to give into anecdotal doubt and general dietary mysticism, you can, but the best that anti-artificial-sweetener folks can drum up is an extremely small handful of non-causal studies that are in the face of mountains of research to suggest safety.