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Follow the guidelines of the experts - organizations of medical professionals like the American Heart Association. The recommendations have been a Mediterranean or DASH diet for years. If you are concerned about heart health, increase fruits and vegetables and limit foods high in saturated fat. Guidelines recommends mono-unsaturated or poly-unsaturated fats to replace saturated fat.
As for weight, this is a matter of calorie intake. If you want to gain weight, consume more calories than your TDEE. If you have trouble eating enough, choose calorie-dense foods like nuts, fruits and starchy vegetables.
Beware opinions of non-experts (non-medical professionals, non-researchers) like blog writers, internet websites, random people on the internet and journalists.
If you are in the US, google “US dietary guidelines,” AHA guidelines for simple steps to improve your diet with a focus on heart health.
Edit: here’s the most current statement from the AHA, published less than a couple months ago. It answers all questions and provides practical guidance for how to have a heart-healthy diet: https://www.ahajournals.org/doi/10.1161/CIR.0000000000001031
Lots of complex carbohydrates! Just make sure your carbs are loaded with fibre, unbelievably healthy for your immune system bugs in your colon. If you are open to eating healthy fats, you could add avocado or extra virgin olive oil just for the extra calories.
avoid seed oils (high linoleic acid). exercise. drink lots of water and high quality supplements. 16/8 intermittent fast a few times each week. alternate between low carb(keto) and higher carb/protein diet. embrace hormetic stressors. Use Magnesium
Quit junk food and sugar. Eat meat, fish, poultry, veggies, beans and healthy oils and spices. Also some darker rice, bread and potato types. A little butter, cream and plain, full fat yogurt as well. Try it for a year and see how you feel.
Like you I’m thin and when I completely overhauled my diet to very similar to this I actually gained weight. It’s like my body was actually taking the nutrients for the first time because I was eating what my body needs. I’ve gone backwards and have lost weight again but trying to fix that.