Why does nutrition advice change?

Nutrition information seems to change daily. That can leave some of us a little confused and often wondering about the right diet to follow. Can sugar harm your heart? Are eggs in or out? Is caffeine good or bad?  There are a few reasons why the outcomes of nutrition research can be misunderstood or flawed.

We cannot isolate a nutrient’s effect.

In the past, nutrition research focused on the role of a single nutrient acting as a magic bullet to prevent disease or as the only thing responsible for the development of a disease.

We can get so stuck on the health benefits of a certain vitamin or phytochemical that we miss an important point: Different components in a single food can work together to benefit our health, and so can components in different foods eaten together. This is known as “food synergy.”

One example of how different nutrients and components in food work together is the pairing of broccoli with tomatoes. In a study published in 2004 in the Journal of Nutrition, prostate tumors grew much less in rats fed tomatoes and broccoli than in rats who ate diets containing only broccoli or only tomatoes and in diets that contained cancer-fighting substances (like lycopene) that had been isolated from tomatoes or broccoli. 

The bottom line is that a lycopene supplement may not hurt, but the whole tomato will probably help more. And a tomato eaten with broccoli may be even better. Nutritional relationships are complex but the natural state is usually the best package.

READ MORE: Nutrition myths: Hype or help?

Data is flawed.

Data collection for nutrition studies is flawed.  We tend to underestimate what we eat, but even more importantly, we don’t remember what we ate to report it back.

Additionally, when we eat out, we don’t know what ingredients are used or how our food is prepared. As nutritionist Marion Nestle says in her book, Food Politics, “The most intellectually demanding challenge in the field of nutrition is to determine food intake.”

In studies comparing one diet to another, it is not always possible to control what people eat. For example, in a study comparing a group eating a low-fat diet to a group eating a higher-fat diet, those in the low-fat group cheated and ended up eating a higher-fat diet than the researchers recorded, while those in the control group (or normal diet) lowered their fat intake voluntary, perhaps because the study made them aware of it. Without the researchers knowing about this, the two groups were not far apart in the amount of fat they were eating. This is why it’s not surprising the “low-fat diet” group didn’t show any health benefits. 

Genetics play a role.

Something else that’s confusing about nutritional research is the genetic difference among the participants studied. These differences can impact how individuals digest and use nutrients in food. Sometimes, when a study of a nutrient shows a difference only in a small number of people, it could be due to individual metabolic differences. We cannot separate the metabolism of a food from the genetic makeup of the person eating it.

While individual studies may give different data about a particular nutrient, the value of a healthy diet is not in doubt. More often than not, studies show good food choices have a positive impact on health and poor diets have negative and long-term effects. 

Let food be your medicine to deliver the nutrients you need to perform, maintain function and fight disease.  American diets that mimic the U.S. Department of Agriculture’s “My Plate” have a lower incidence of major chronic diseases. To look for nutrition information, visit www.nih.gov and www.eatright.org.

Originally published June 3, 2019. Last updated March 2, 2020.