Nutrition is an incredibly complicated subject but a good understanding of it is essential to healthy living. Most of us these days have a basic understanding of what we should be eating. We know about five (or seven) fruit and vegetables. We know we should cut down on the sugar and the chemicals. We also often notice reactions that we have to the food that we eat - dairy bloats us, gluten makes us tired and so on.
There are general rules that should be used as a guideline, but there are often individual reactions to foods as well. Everybody is different and reacts differently. It is important to notice your individual reaction to foods. If something gives you a tummy ache it is probably having other effects on your body and even though you are willing to put up with a bit of a tummy ache to have your favourite food, it may be having other effects on your body that you find harder to cope with.