We do not always need a definitive RCT to know what we know; and I make a living running such trials. Suppose you wanted to know with something nearing certainty what specific dietary pattern was best for human health. How would you proceed?

First, you would need to define best in an operational (i.e., measurable) way. Does best mean lowers LDL in the short term, or does it mean raises HDL, or both? Does it mean it lowers inflammatory markers, or insulin, or blood glucose, or blood pressure? Does it mean all of these, or does it mean something else? Is the short term one month, or three, or a year?

I dont think any of these, or anything like them, really satisfies what we think we mean when we say best for health. I think the intended meaning of that is actually rather clear: the combination of longevity, and vitality. Years in life, and life in years, if you will. I think a diet is best for health and yes, I have wrestled with this very issue before if it fuels a long, robust life free of preventable chronic diseases (e.g., heart disease, cancer, stroke, diabetes, dementia, etc.) and obesity, and endows us with the energy both mental and physical- to do all we want and aspire to do. That, I think, is a robust definition of best for health.

We are obligated to wrestle comparably with the operational definition of a specific diet. Low fat, or low carb dont mean much. A low fat diet could be rich in beans and lentils, or made up exclusively of lollipops. A low carb diet could cut out refined starch and added sugar, or exclude all fruits and vegetables. Lets not belabor this, and simply concede that the relevant test to prove that one, specific dietary prescription (e.g., the Ornish diet, or the South Beach diet, or the DASH diet, etc.) is best is to establish optimized versions of the various contenders, from vegan to Paleo, and put them up against one another directly.

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And now our tribulations begin. As we noted at the start, our outcome is the combination of longevity and vitality. To get at longevity, we need a very long trial; in fact, our trial needs to last a lifetime. So, just to get started, we are toying with the notion of a randomized trial running for 80 to 100 years.

Since we are comparing optimal versions of diets reasonably under consideration for best diet laurels, we may anticipate that our study participants are apt to be healthier, and longer-lived in general than the population at large, consuming the lamentable typical American diet.

Thats a problem too. If our entire study sample does well, it raises the bar to show that one of our diets is truly, meaningfully better than another. The smaller the difference we are seeking, the larger the sample size we need to find it. That now means we need not only a RCT unprecedented in length, but unprecedented in size, too. We need to randomize tens of thousands, if not hundreds of thousands to study the effects of competing diets on vitality and longevity at a cost that is staggering to contemplate, and would certainly run into the billions of dollars.

This study has not been done. This study will not be done. But, so what?

Lets contrast our ostensible need for this RCT to how we know what we know about putting out house fires. There has never been, to the best of my knowledge, a RCT to show that water is a better choice than gasoline. Do you think we need such a trial, to establish the legitimacy of the basic theme (i.e., use water) of the right approach? Would you, and your home, be willing to participate in such a trial when you call 911 knowing you might randomly be assigned to the gasoline arm of the study?

I trust we agree that observation, experience, and sense serve to establish beyond the realm of reasonable (or, even, any) doubt that water is generally good for putting out house fires, and gasolinenot so much.

My friends diet is the same. The want of a RCT addressing this kind of water versus that does not mire us in perpetual cluelessness about the basic approach to putting out fires. Sure, we could do RCTs to add to what we know, but the want of such studies does not expunge what we already know based on empirical evidence, long experience, observation, and sense.

A diet comprised principally of minimally processed vegetables, fruits, whole grains, beans, lentils and pulses, nuts, seeds, with plain water preferentially for thirst is the best theme for human and planetary health alike, and runs commonly through all the legitimate, specific contestants, just as water is the best theme when aiming a fire hose. To conclude otherwise is to misconstrue the utility of randomized trials, succumb to their tyranny, and lose our way in a bog of tribulations.

Dr. David L. Katz, http://www.davidkatzmd.com; is founder, True Health Initiative.

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Dr. David Katz, Preventative Medicine: Diet Trial Tribulations - New Haven Register