This bloke said he saw Elvis down the chip shop. What? You mean he was mistaken or lying? Surely not!
Regularly, you read or hear:- "Aspartame gives you cancer!" "Meat gives you cancer!" "Crisps give you cancer!" "Coffee gives you cancer!" "Coffee doesn't give you cancer!" "Coffee gives you cancer!" and so on and so forth...... What are we to believe?
In terms of Diet & Nutritional information, there's a hierarchy of credibility. Here's a rough list of credibility, starting with the least credible and ending with the most credible.
Inexpert Opinion. Basically, anyone can say anything (especially on the internet) but that doesn't make it true. This includes anything that I say, which is why I try to back up what I say with evidence from higher up the pile.
Anecdote/Testimonial. Just because it worked for Joe or Josephine doesn't mean that it'll work for anyone else. There's also the placebo effect e.g. Nothing acts faster than Anadin (so I use nothing!)
Expert opinion. Even "experts" with lots of letters after their names get things wrong and have personal biases, hence the controversies over MMR, HIV/AIDS, Cholesterol etc.
In Vitro (in glass) studies. What happens in Petri dishes/test-tubes etc doesn't necessarily happen in humans.
In Vivo (in life) Animal studies. What happens in rats/monkeys etc doesn't necessarily happen in humans.
Epidemiological (population) Human studies. A being associated with B doesn't necessarily mean that A caused B, as the association may have been due to random chance or due to both being caused by C, D, E........Z etc.
Case studies. The number of subjects is usually quite small and often just 1.
Small and/or non-randomised and/or unblinded and/or non-placebo-controlled and/or non-crossover trials.
Large, Randomised, Double-blinded, Placebo-controlled Crossover trials with a decent Washout.
Meta-studies of large, randomised, double-blinded, placebo-controlled crossover trials.
Even the last two in the list aren't perfect. Study outcomes can be manipulated by tweaking the methodology (e.g. using pre-trial screening) or statistical jiggery-pokery. If a trial is funded by (a) drugs company(ies), the methods used & the results obtained should be scrutinised very carefully.
EDIT: Systematic Reviews & Meta-studies can be manipulated by including studies with completely different input & output ranges, so as to dilute the data and force a null result. See Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease and Low Carbohydrate versus Isoenergetic Balanced Diets for Reducing Weight and Cardiovascular Risk: A Systematic Review and Meta-Analysis.
Abstracts can misrepresent the data. See Intensive lipid lowering with atorvastatin in patients with coronary heart disease and chronic kidney disease: the TNT (Treating to New Targets) study.
"Conclusion: Aggressive lipid lowering with atorvastatin 80 mg was both safe and effective in reducing the excess of cardiovascular events in a high-risk population with CKD and CHD."
There was a 15-32% reduction in major cardiovascular events. Wow, that's impressive! What the abstract failed to mention is the fact that there were 26 more deaths in the 80mg group, than in the 10mg group. What's worse? Having a major cardiovascular event or being dead?
In conclusion, when you read something in the media, do some research of your own before accepting it as fact. Use Google, Wikipedia, PubMed and the Cochrane Library.