I eat 4 bananas, 2 eggs and dry fruits daily and have been going to the gym for the last 2 months, but I haven't gained much muscle/weight. However, I feel the power inside me, as I'm now capable of doing 12kg bicep curls, whereas on the first day, I couldn't even do 5kg. I'm not using any high-protein products and don't plan to for at least 6 months.

Do you have any advice on how to increase my muscles naturally?

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Increase your calorie intake: To gain muscle, you need to consume more calories than your body burns. Make sure you're eating enough throughout the day, including complex carbohydrates like whole grains, potatoes, and oats.
For example, you could add an extra serving of brown rice or a couple more slices of whole wheat bread to your meals.