This week, a new single-study is taking over the airwaves. Everywhere from the Today Show to Glamour magazine is featuring headlines touting serious panic, saying that carbs like white bread, potatoes, and rice ‘give you lung cancer’ or ‘are the new cigarettes.’ (The study is available here). The comments sections on these reports are already telling us that this new panicked report isn’t passing the ‘sniff test’ for readers. Here’s why their instincts are right on:
Does that study really tell us enough about risk?
This new study divided people into five groups based on intake. The study determined that people with the highest intake (group 5) for glycemic index (GI) were about 49% more likely to have lung cancer than people in the lowest intake (group 1). Unfortunately, it is impossible to give examples of specific foods that people in group 5 were actually eating (e.g. we don’t know if it’s twice the amount of carbs, or 10 times). It’s pretty difficult to draw conclusions from the data, because the risk level wasn’t different for the people in Groups 2 – 4. Even aside from the fact that this kind of study can’t show causation (more on that later), the results leave some big questions unanswered.
We also don’t want to lose sight of much more significant factors that influence cancer rates, including lung cancer. Cigarette smoke is the leading cause of developing lung cancer. The US Surgeon General Report says that smoking cigarettes increases a person’s risk for developing lung cancer by about 25 TIMES. A number of dietary factors like low fruit and vegetable intake and being overweight are also well-established (check out the Cancer Institute’s sum-up on the right).
So what about the actual study methodology? Information about what each person ate came from a food frequency questionnaire (FFQ), which are not among the most accurate methods to objectively measure dietary intake. (Case in point, check out FiveThirtyEight’s breakdown of FFQs, to the left)
Big inaccuracies are linked to FFQs, since people may not be able to remember what they ate yesterday, let alone the past several months. Additionally, the psychology of dealing with a major illness can make FFQ reporting drastically different between healthy people and individuals with lung cancer.
For example, the authors of the study state “healthy controls are more likely to recall healthy dietary habits than patients, leading to biased effect estimates.” Basically, that means that if you think you’re pretty healthy, you might remember the healthiest things that you ate and did. If you see your own health as poor, your mind may focus on your least healthy habits.
And finally, there’s the whole focus on the glycemic index. The Glycemic Index is a complex measure with various factors that affect its validity. GI doesn’t take the effect of fiber, fat or protein into account. Nor does it assess intake in mixed meals (you know, the way we actually eat). Another example of how the GI may not always be the most sound measurement is when you look at fruit: GI value of fruit depends on how ripe it is—more ripe fruit has a higher value. Similarly, GI values of pasta depend on how much they’re cooked—al dente pasta having a lower GI.
Kris Sollid, RD, emphasizes that “the glycemic index should not be used in isolation or replace sound nutritional advice. It’s largely a tool available for certain populations that require tighter glycemic control, like those with Type 2 diabetes. The totality of current scientific evidence does not support its widespread use among the general population as an isolated and definitive tool to improve overall health.”
Other Major Red Flags
The study was designed as a case-control study, which aims to identify risk factors for specific medical conditions by looking at a group of patients who already have those health issues, versus a group of patients who don’t have that specific condition. In this instance, the condition was lung cancer. Although the study included people with and without lung cancer, it did not look back at any previous data on factors that may have influenced cancer risk. Researchers measured diet only at one specific moment in time, so we can’t tell if the dietary reports are indicative of long-term eating patterns.
We were also troubled that the study only included a specific population of Caucasian adults from Texas. That makes it pretty inappropriate to apply to a diverse population without any additional research.
So What Does this Mean for YOU?
According to the Report of the Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2010, “When selecting carbohydrate foods, there is no need for concern with their glycemic index or glycemic load. What is important to heed is their calories, caloric density, and fiber content.”
The 2015 Dietary Guidelines also did not do not make a recommendation or statement on the usefulness or validity of the glycemic index. What do the dietary guidelines recommend? Make half your grains whole. Whole grains come with a ton of benefits like reducing your risk of heart disease and improving your digestive health. This recommendation also means that the other half of your grains can be refined grains like non-whole wheat breads, cereals, and pasta. According to the Dietary Guidelines, “individuals who eat refined grains should choose enriched grains” which include important micronutrients such as B-vitamins, iron, and folic acid. Since most refined grains, such as flour, are enriched before being added to foods, you are getting important nutrients when consuming these foods as well as providing valuable energy for your body and brain. Keep on track with a diet that’s filled with a variety of foods, and don’t let the news flash around a single study scare you.
This blog includes contributions from Sarah Romotsky, RD.