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How to gain weight from eating almost exclusively bread and potatoes?
For calorie dense you can't do better than fat. Oil might be 'expensive' but it is one of the cheapest things to buy on a per calorie basis and you only need a teaspoon to fry an egg or make mashed potatoes taste half-decent. You would be surprised how much better potatoes and eggs can be with a little fresh chill pepper or coriander if you can find it. The better it tastes the easier it is to eat. One more tip... Canned condensed milk. It's cheap, calorie dense and delicious. A spoon a day will last weeks. Delicious in tea and coffee. Good luck!