oh..... well if you want to refer to genetic disorders, it gets a little more complex. Of course you have to be realistic when someone makes a statement. You are what you eat, is a broadspectrum statement that would imply the importance of a healthy and balanced diet. If you stray from that disease awaits.
Eating rubbish will lead to CVD, diabetes (type2), bone disorders, blood disorders.... you name it, it will come. However its silly to think that this can be applied (or is it?) that this will be a result of genetic disorders which where developed in the womb. Now it gets a little more complex than that. This would mean your diet is still important at this point. Remember genetics is a blueprint. You can have alpha genes but still come out diseased why? poor diet .... from your mother.
So you are built to what she ate and you are maintained on what you eat. In terms of genetic diseases, like autoimmune diseases, lets look at lupus (SLE). Nobody knows why people get autoimmune diseases, you can have the genetic code for it but it never shows itself, in other words you a carrier of the gene. Others have the genes and they have SLE, why?
Its because in autoimmune it requires a trigger, which can be food. Few dietary triggers have been isolated and removed from foods. Tartrazine for asthmatics, you wont develop asthma from it, but if you are asthmatic it will trigger it. Some kids their asthma doesnt show itself until its been triggered by their environment.
Lets go back to your diabetes if im understanding what your are saying. I assume you refering to type 1 diabetes - which is insulin dependant. There are many ways you can get type one diabetes, but we not interested when you diagnosed. The primary concern is to treat, its too late to worry why this patient has type 1 diabetes. In research its a big concern. There are many ways to get type one diabetes. You can lack enzymes, a deformed pancreas, lack of c-proteins, poor beta-cells. The point is these can occur from the diet your mother had. During the development of the foetus, if your nutrients are poor you going to get a poor body. Its the same with workmanship, poor materials result in a low grade product.
So diet is more important than most people think, that is why when a women pregnant, they are given a lot of minerals, vitamins and trace elements in specialised formulations used for pregnacy supplements.