At one time, the only way to lengthen one's life with diet seemed to be to drastically restrict caloric intake to the point one would be hungry all the time. That worked on virtually all animals tested, from apes down to very simple animals, and so it seemed to indicate some universal metabolic change.Which proved true. When an animal is starving its metabolism changes to run on less energy, and, running on less, the slowed metabolism itself does less damage to the body. But that only works when an animal is caged and forced to starve. Free human beings with plentiful access to food just won't. Unless they're obsessive.
The next idea was to eat high-protein diets (a lot of meat), which seemed to make the person less hungry and reduce the desire for calories. And so it seemed to be an easy way of achieving the same result.
However, new studies are finding that high protein does just the opposite, causing health problems and reduced longevity, and that the only diet that truly promotes health and longevity is a low-protein, low-fat, high carbohydrate diet.
So, if you're the kind of person who believes in the traditional Canadian diet of meat and potatoes, with meat the main staple of every meal, you should think again about what you're doing to your body. You really don't need more than one small piece of meat a day for your protein, and a much better way to get it is from legumes in a vegetarian diet, which also reduces the amount of fat intake, and increases everything else that's necessary for good health. (Not counting the other necessary component of good health, of course, which is exercise.)
Here is one of the studies: http://www.cell.com/cell-metabolism/abstract/S1550-4131%2814%2900065-5