High fat foods are a start. Look at a table spoon of extra virgin olive oil. It's very calorically dense. I'm not suggesting you eat nothing but fat, but you could certain stand to add some of the healthy fats to your diet which will simultaneously help with health and a caloric buffer due to its energetic density. 1 gram of fat has more than double the amount of calorie/energy than 1 gram of protein and/or fat.

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