They can but you'd really have to try hard. Here's why:
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Avocados are a high fat food which means they are also high in calories. But to gain weight from them you'd have to eat multiple pieces of the fruit each and everyday, AND have other conditions that help facilitate that in your diet.
Avocados are an accepted food on the Keto diet because they are very low in carbs so people actually eat them to lose weight.
To gain weight eating avocados you'd have to eat about 4 per day consistently on top of what you would usually eat.
Eating 1 a day won't do much to make you fat unless the rest of your diet needs a lot of work and if so, it's not the avocado.
Eat and enjoy.
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