Hooray! It is great to hear an epidemiologist speak in defense of observational science.
This is particularly important where food is concerned as it is so difficult to reduce complex food issues down to a simple randomized controlled study.
The effect of diet on hyperactivity is a very good example particularly as what looked simple proved to be a very complex clinical condition. At different times some researchers have tried to reduce the question to provide a double-blind-placebo-controlled (DBPC) trial that would prove or disprove the connection. Here I will comment on all that has been found by careful observational science which is what clinical research is. It includes the many observations of the clinicians working with families, separate from any DBPC trials. Their findings have added greatly to our understanding of food chemical sensitivity over the last 35 years. This clinician followed up 1000 families over 10 years and a group of 112 in even more detail.
What was found?
In the hyperactive group diet had its main effect on mood: reducing irritable, touchy and cranky behaviour, and it also reduced difficult to reason with, poor concentration, impulsivity, restlessness, tantrums, and difficult to control, more than it changed hyperactivity. So it was not a placebo effect. Diet was found to be an aggravating factor not a causative one, and the amount of shift was different in different children such that some found diet necessary but not sufficient treatment, while the symptoms in others reduced to within the normal range. Clinical researchers also found that the threshold of suspect substances required to produce a reaction varied in individuals, and with age.
There were many other interesting findings. Diet changed more than behaviour in the presenting child and in siblings and close relatives. The range of symptoms that reduced with diet formed a cluster that now appreciated as typical in diet responding families. These include allergy symptoms particularly eczema, as well as a variety of other symptoms including migraine, chronic headaches, irritable bowel syndrome, sleep problems, mouth ulcers, and car sickness. The tendency to have these symptoms runs in the diet responsive families. To add to the complexity the symptoms were reported to increase or decrease over the lifetime in family members.
The suspect chemicals included additive artificial and natural colours, flavours, some preservatives, as well as foods containing natural salicylates, amines, and monosodium glutamate. These include most highly flavoured foods, and, interestingly, the quality of flavour affects reactions too. An important addition was any strong smells, including perfumes, petrol, paint or flowers. Additionally these food chemicals interacted with other factors such as inhalants, hormone changes, stress, and the presence of infections. These all added up to a concept of the “total body load” of factors that affect reactions.
This is rather a dense summary but it does show just how complex research can become and how thinking and knowledge can develop as more is learned and various researchers add to the knowledge. For further interesting reading see the various Articles on this website especially an article called ‘Why after 30 years is diet and hyperactivity still controversial? https://foodintolerancepro.com/category/adhd/adhd-controversy/