A good diet is essential and whole natural foods should be used over processed foods which lose much of their nutritional value. Fruits, vegetables, whole grains, beans and legumes, are all good sources of vitamins and minerals. However, because of the physical stresses on the cells from the unnatural state of iron overload that is part of thal and the oxygen depletion that comes with lower than normal hemoglobin levels, it becomes essential to supplement certain vitamins beyond what is possible through diet alone. Some prime examples are folic acid, vitamin E, calcium, magnesium and vitamin D. I have visited India twice and been to Dubai three times and also to the Maldives, and in spite of the intense sun in these places, many people, not only thals, become deficient in vitamin D because they get very little exposure to direct sunlight. Long sleeves and long pants prevent sun from reaching most of the body. A study done in UAE found that most women were significantly deficient in vitamin D because they cover most of their skin when outside. Even something that seems as readily available as vitamin D from the sun, is not being taken in sufficient quantities to prevent deficiency. The natural diets in these countries does a lot to provide more than adequate nutrition when followed, but for thals, it still isn't enough in most cases. 2/3 of thal majors studied showed severe deficiency of vitamin E.
A good diet does much but with thal it still isn't enough. Supplementation is necessary in most cases of thal major. Thal minors may be able to get most of what they need through diet but sufficient quantities of folic acid and vitamin E are difficult to obtain from diet alone. If you eat a wholly natural diet you may be fine, but in today's world of over processed foods, it can be quite difficult.