Just have a look at the various nutritional studies taken in the past 40 years, and the corresponding diet plans, you'll be able to find the hidden truth about long life. Most of our ancestors followed these diet plans which helped them live a healthy long life. These are some of the best diets for long term good health:

• Carbohydrate free diet – A diet which is low in carbohydrate consumption helps in keeping the glucose and insulin levels within normal range. Studies show that a diet that is less than 80 carbohydrate grams per day help people remain more active and healthy. Such diet also promote fatty acids as cellular fuel. This factor plays an important role especially for diabetic patients as the body starts storing fat instead of releasing it. Hence, this process releases high levels of free radicals which can damage cellular function. In short, it will help in keeping blood level low. One of the prominent experts on longetivity, Ron Rosedale says that, "Health and life span is determined by the proportion of fat versus
  sugar people burn throughout their lifetimes. The more fat you burn as fuel, the healthier you will be."
• Diet rich in nutrient and free of chemicals – A diet is considered best if it is rich in nutritional values and avoid chemicals and additives in any form. Such diet plans will ensure that the body has all of building blocks it needs to repair damage, grow new cells, make energy for all the activities.

However, most of the modern diet plans are full of chemicals and polluted. Today, most of the commercial products like meat, chicken, and milk that we buy from grocery stores come from animals, which have been fed on genetically engineered grains and injected with antibiotics and growth hormones. Even, grains and vegetables are sprayed heavily with pesticides, or genetically modified to increase yields.

Therefore, organic foods are much cleaner and safer to eat. Make sure to eat whole food that are packed with nutrition. If you want to live longer and healthier then you must avoid processed and oily food. A diet which is low on carbs and high on nutritions is ideal for better health.