Eating more calories than you burn causes weight gain, no matter which diet you use.

If you were eating the lowest calorie density foods, like fruits, veggies, and whole grains, all with a calorie density of 600 calories per pound or less, you would have a hard time keeping weight on, let alone gaining weight. I suspect you ate lots of oils and fat (vegetarian does not necessarily equal strict plant based or low fat). Fat is about 4,000 calories per pound. So it would be easy to gain if you eat it.

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