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When researchers ran DNA testing on 13 herbal products, only 2 contained the plant on the label. Three had none of it at all, and several were padded out with rice, soy or wheat that nobody had declared.
The obvious problem is paying for something you did not get. The quieter problem is the undeclared filler, which matters a great deal if you avoid wheat or soy for medical reasons, and matters in a different way if you care about what a capsule is made from. Testing the actual contents rather than the paperwork is the only way any of this comes to light.
Qur’an and Sunnah
He has only forbidden you carrion, blood, pig's meat (Quran 2:173). Knowing what is genuinely in a thing has always mattered here for more than one reason, and an undeclared filler takes that knowledge away from you.
إِنَّمَا حَرَّمَ عَلَيۡكُمُ ٱلۡمَيۡتَةَ وَٱلدَّمَ وَلَحۡمَ ٱلۡخِنزِيرِ وَمَآ أُهِلَّ بِهِۦ لِغَيۡرِ ٱللَّهِۖ فَمَنِ ٱضۡطُرَّ غَيۡرَ بَاغࣲ وَلَا عَادࣲ فَلَآ إِثۡمَ عَلَيۡهِۚ إِنَّ ٱللَّهَ غَفُورࣱ رَّحِيمٌ
He has only forbidden you carrion, blood, pig’s meat, and animals over which any name other than God’s has been invoked. But if anyone is forced to eat such things by hunger, rather than desire or excess, he commits no sin: God is most merciful and forgiving
Qur’an 2:173read it in the library
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.
What the research says
Identity really does decide effect, which is why substitution is not a technicality. A meta-analysis of randomised trials concluded that EPA rather than DHA explains the antidepressant signal in omega-3 supplementation. Two closely related compounds from the same source, and only one carries the benefit. Something else entirely in the capsule is not a weaker version of the remedy, it is a different substance.
Martins JG. (2009). EPA but not DHA appears to be responsible for the efficacy of omega-3 long chain polyunsaturated fatty acid supplementation in depression: evidence from a meta-analysis of randomized controlled trials.. Journal of the American College of Nutrition doi
Strength of evidence: one line of research. A single study, or a theoretical account.
Why it works
Nothing about a sealed capsule tells you what is inside it, so only testing the contents can answer the question.
This one is still a draft. It is waiting on a scholar and a clinician to read it before it is published.
Near this one
Working out what you have comes before choosing what to take. The right remedy for the wrong condition is still the wrong remedy, however good the remedy happens to be.
Irritable bowel symptoms turn up more often in people with ADHD, and they tend to track stress. If your stomach is at its worst in the weeks when everything else is at its worst, that is information rather than coincidence.
If you are testing whether a food affects you or your child, the test has to be complete. Among children with suspected sensitivity to artificial colouring, most reacted to as little as 100 mg, so a bit left in is enough to blur the answer.