Only More Research Will Prove a Causal Relationship

Jim Prevor - Comments and Analysis

What follows is a response from Jim Prevor pertaining to this month’s research perspective. That article can be found here.


Jim Prevor - Comments and AnalysisIn the many years these analyses of research have been written, there have been countless glimmers of hope citing new and compelling reasons for people to eat more produce. Yet, in the end, only the fewest have come to be “generally accepted.”

The reason is found in one word: associated. When Dr. Haibach explains that, “There is overwhelming research that eating a diet high in produce is associated with better health — physical, mental and possibly even spiritual,” it makes our hearts sink, for the truth is this kind of association leaves us with an overwhelming desire to do a lot more research and see if we can establish a causal relationship.

The dilemma isn’t hard to notice. We study people and find out those who eat a lot more fresh fruits and vegetables are less likely to be depressed than those who do not. Does this mean produce is an effective tool to fight depression? As Ernest Hemingway wrote: “Isn’t it pretty to think so?”

Yet, isn’t it just as likely, maybe more likely, that people who are not depressed will take better care of themselves, and that overall happiness expresses itself in a multitude of ways? These people exercise, stay hydrated, use sunscreen and eat more healthfully — which includes eating more fruits and vegetables. Now researchers can try to adjust for these variables, but we are relying on self-reporting by people as to whether they do these healthy things or conversely, unhealthy things, such as drug use, abuse of alcohol, etc.

The problem is that asking people about these habits is not a neutral question. It is not like asking if they prefer vanilla or chocolate. It is, in fact, like asking them if they are smart people. The temptation to lie is significant.


The problem is that asking people about these habits is not a neutral question. It is not like asking if they prefer vanilla or chocolate. It is, in fact, like asking them if they are smart people. The temptation to lie is significant.


Studies that identify associations are valuable, but they have to be interpreted in the context of understanding that there are many other associations. So, consumption of produce, for example, rises with income and education, which means if a researcher finds something correlates with higher produce consumption, it almost certainly correlates with higher income and educational levels.

The quality of data is also important. Is this a study where the research subjects self-report whether they do drugs, engage in risky sexual behavior and drink excessively — or is this a study where blood tests are frequently given and other steps are taken to monitor actual behavior?

Even produce consumption levels are questionable under self-reporting. Are grocery receipts monitored and other steps taken to ascertain the veracity of these reports? After all, produce consumption is so widely recognized as a good thing — a virtuous behavior — that maybe what we are finding is people who care about how they are perceived, will “gild the lily” about produce consumption. These same people could also be the ones who will care enough about how they are perceived by friends and co-workers that they will quit smoking rather than be thought of poorly.

Then there is always the flip side of the coin. This research points out that due to the fact that eating fruit can promote positive feelings, people suffering from depression or experiencing negative life events might be less likely to reach for a cigarette if their habit is to reach for some fruit. This would be fantastic news. Yet, it is also true that lots of people hesitate to give up smoking because they fear gaining weight if they do. And isn’t eating sweet fruit to squash feelings of inadequacy almost the definition of nervous eating?

Of course, all the benefits of eating produce identified in this research piece could be true just as many other benefits associated with eating produce could be true. Yet, reading these research reports for years has led me to think that a better way of looking at these things is to remember there is no silver bullet. Mostly, people who tout specific benefits from eating this or that are, to be generous, speaking without adequate evidence to prove their cases.

A better way to consider these matters is that life is a seamless web. Just as you can’t spot reduce and get rid of, say, belly fat, by doing lots of sit-ups, so there is rarely one food that magically cures some ailment or alters one’s personality. The mental and physical work together; and if you are open to good ideas, open to the messages that society passes on about who will be healthy and who will be successful in life, then you will do many things right, including eating more fruits and vegetables. If you are deaf to such messages, then the odds you will choose eating more produce as the singular virtue you adopt are slim odds indeed.

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