Healthy Foods
Which food groups are healthy and which unhealthy? A meta analysis was published in 2017 that looked at 12 food groups: whole grains, refined grains, vegetables, fruits, nuts, legumes, eggs, dairy, fish, red meat, processed meat, and sugar-sweetened beverages. They evaluated each food group by looking at its effect on all cause mortality. Some decreased all cause mortality, others increased, and some had no effect.
Whole grains, vegetables, fruits, legumes, nuts, and fish decreased all cause mortality and thus are healthy foods. It's not surprising that foods are the backbone of the Mediterranean diet most often recommended by doctors. Red meat, processed meat, eggs, and sugar sweetened beverages increased all cause mortality and thus should be consumed infrequently. Refined grains and dairy had no effect on all cause mortality. There was no difference between low fat dairy and full fat dairy.
An optimal diet of 3 servings of whole grains, 3 servings of vegetables, 3 servings of fruit, 1 serving of nuts, 1 serving of legumes, and 2 servings of fish results in a 56% reduction in death compared to non-consumption. The study is from the American Journal of Clinical Nutrition and you can read the full text online