The book value of Warren Buffett's Berkshire Hathaway (BRK.A)(BRK.B) has grown by an average of 19.8% per year for the last 45 years, turning every invested $1 into $3,393. Had that dollar been invested at that rate for only 35 years instead of 45 years, it would have grown to "only" $557 - a remarkable difference!
So how does one increase the number of years in which one is able to invest? By living longer and healthier. It may seem as if you have no control over how long and healthy you live...but you do! A number of fatal diseases are becoming more prevalent in developed countries, even after accounting for the increased lifespans. Evidence increasingly points towards our diets as the causes of these diseases.
But while you may think you know that the reason for this is that we're lazy and therefore simply eat too much, the evidence suggests you may be wrong. It appears increasingly likely that we don't get fat/diabetes/cancer/Alzheimer's because we eat too much, but we eat too much and get fat/diabetes/cancer/Alzheimer's because we eat things our bodies aren't used to. The implications of this are large, as they suggest that cutting our food consumption by reducing portion sizes will not work. This only induces us to expend fewer calories and/or eat more calories later.
For millions of years, our ancestors evolved while eating certain diets. Approximately ten thousand years ago, however, our ancestors started farming. Slowly (at first), this began to change the diets of our ancestors. Today, those dietary changes are accelerating; we eat dramatically different foods today than we did just a generation ago. Our bodies are likely not adapted to this new diet.
If you're interested in learning more, here are a few books on this topic which I have read over the last few months. I've ordered them from most technical (i.e. for those who want to know the historical background, physiological theories and the results of studies) to the practical (i.e. what to eat and not eat if you do subscribe to this line of thinking).
1) Good Calories, Bad Calories by Gary Taubes
2) Why We Get Fat by Gary Taubes
3) Free The Animal by Richard Nikoley
4) The Primal Blueprint by Mark Sisson
For a dissenting view, consider reading The China Study and one of its excellent excellent refutations.
Disclosure: I should note that I've lost 25 lbs. and have more energy than ever since making just a few changes to my diet. It's not about portion sizes, as I have not gone hungry at all; I can eat as much as I want of a surprisingly large number of foods (i.e. I'm not just talking about fruits and vegetables).