Weight gain is only three to five pounds during this time, while the baby is being sustained mostly on your wife's lifetime of good nutrition. The best thing to do is eat several small meals instead of three large ones. If she smokes, she must stop immediately. If you feel there is really a problem with her getting the essential nutrients, you could ask for a referral to a dietician. They are a wealth of information.
Many good books discuss antepartal nutrition.
Her appetite will undoubtedly return just when it's needed. Trust her to take good care of your baby. Watch kids controlling behavior of breath holding. Most children do not get enough water in their diet. You should not rely on this information as a substitute for personal medical attention, diagnosis or hands-on treatment.