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A real food allergy shows itself on the body: a rash, a swollen or inflamed throat, something you can see. Behaviour on its own does not identify one, although behaviour often improves once a genuine allergy is dealt with.
One child's attention and mood went back to her usual level once berries came out of her diet, and what made that findable was the rash and the sore throat rather than the behaviour. So if there are visible signs, follow them. If there are none, a food is unlikely to be the hidden explanation and there are usually more useful places to look.
Qur’an and Sunnah
With its fruits, its palm trees with sheathed clusters (Quran 55:11). The list of what the earth gives is long, so leaving out the one thing that harms you is not much of a loss.
فِيهَا فَٰكِهَةࣱ وَٱلنَّخۡلُ ذَاتُ ٱلۡأَكۡمَامِ
with its fruits, its palm trees with sheathed clusters
Qur’an 55:11read it in the library
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.
What the research says
That feeling better physically shows up mentally has some direct support. In the SMILES trial, adults with major depression given twelve weeks of dietary support improved substantially more than a group given social support instead. The trial was small at 67 people and participants knew which group they were in, so the size of the effect is uncertain, but the direction is the point here.
Jacka FN, O'Neil A, Opie R, Itsiopoulos C, Cotton S, Mohebbi M, Castle D, Dash S, Mihalopoulos C, Chatterton ML, Brazionis L, Dean OM, Hodge AM, Berk M. (2017). A randomised controlled trial of dietary improvement for adults with major depression (the 'SMILES' trial).. BMC medicine doi
Strength of evidence: one line of research. A single study, or a theoretical account.
Why it works
Anyone who feels physically unwell seems less able to concentrate and less able to cope, whatever the cause.
When not to
Suspected allergy with any breathing difficulty or swelling is an emergency and needs medical care, not a home experiment.
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
Low mood that arrives with bloating, fatigue and a foggy head is worth looking at from the food side too. An immune reaction to something you eat regularly can produce all three, and it gets labelled depression far more often than it gets tested.
The same brand of sweets is often made with plant based colours in one country and artificial ones in another. If a company can make it without the dyes elsewhere, the dyes are not essential to the food.
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.