I have been dieting & although i've lost some weight anytime i eat a regular meal i gain 3 or 4 lbs. how can i rectify that at my weight (340 lbs) ?

Gastric bypass: At 350 lbs, you are considered to have morbid obesity. Treatment of choice would be gastric bypass surgery. You should be evaluated for conditions that affect your metabolism such as hypothyroidism. You should also be evaluated for conditions linked to Insulin resistance such as diabetes, hypertension, hyperlipidemia, and cardiovascular disease.

Fast Gaining: Hi, bariatric surgery could be the right answer for you. Please seek a full medical examination including labs. Have you kept a log book to show your physician what and how much you eat in a "regular meal". How have you been "dieting"? Maybe your regular meal has too much food (calories). You did say you lost some weight? How much? Good luck.

Consider surgery: At 340, the likelihood for pills and diet to be effective long term is only 1%. With a normal sized stomach, you could diet all week, but have one modern full meal and consume 2500-4000 calories. Your body can only use about 1800 cal as glucose and glycogen (short storage). The rest must get stored as fat. Hence, you gain weight. You might consider bariatric surg to decrease capacity and hunger.