The most important is to learn principles of elimination diet.
If you are allergic to some foods you consume, you may react to it already within 5-120 min after eating it, sometimes even after just 10 seconds.
That is the easiest to figure out, as reaction is coming very short time after consuming offending foods.
If you are not allergic to any foods, but instead you are just intolerant to some foods, it may get far more difficult to figure it out, as reaction may come after longer time, and also you may only react when the amount of food consumed exceeds your food tolerance threshold.
In other words, consuming just a small amount of offending foods may not give any negative symptoms.
If your stomach is intolerant to some foods you consume, you may react to those foods within 2-4 hours after eating it.
If your small intestine is intolerant to some foods you consume, you may react to those foods within 3-12 hours after eating it.
If your intolerance to some foods is on the level of large intestine, you may react to those foods within 8-36 hours after eating it.
So it is not always easy to pinpoint what foods you react to.
That is why it is important to write down detail log of everything you eat, and a detail log of all symptoms, with precise date and time when food was eaten, and precise date and time when symptom appeared.
That is one way to figure out what you can tolerate.
Avoid toothpastes and use just salty water for brushing your teeth and washing your mouth.
Avoid all cosmetics.
Avoid contact with animals like cats, dogs, birds ... you may be allergic to those animals. large number of people today are allergic to cat dander, dog dander, bird feathers etc ... and that allergy may be causing skin problems.
If you are not vegetarian, try to consume some animal liver, at least 50 -100 grams of liver per week, or more.
It will provide you with all of the vitamins and minerals you may need (A, B-complex, zinc, iron, iodine, selenium, etc)
Sometimes, you may react to some foods only when those foods are consumed in combination with some other foods.
Women have another problem.
Their food tolerance often depends on the menstrual cycle, so during menstruation they may be far more sensitive than during ovulation ...
Sensitivity to foods also depends on the level of stress and level of physical activity.
More stress, more sensitivity.
More physical activity often means less stress and less sensitivity, but too much physical activity may sometimes have negative effect on digestion .. and may give more sensitivity to some people.