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Obesity and Chronic Disease: Understanding the Link

Cindy Davis  

Cindy Davis is the ARS National Program Leader for Human Nutrition.

Welcome Dr. Davis to Under the Microscope.

UM: President Biden launched a Cancer Moonshot initiative to align expertise across the federal government in a unified fight against cancer. What role does nutrition play in the prevention of diseases like cancer, and what is USDA doing that might contribute to our understanding of that relationship?

CD: USDA recently launched the Agricultural Science Center of Excellence for Nutrition and Diet (ASCEND) for Better Health initiative. It is a significant new effort to provide Americans with the information they need to eat a healthier diet that reduces the odds of chronic disease, by bringing together scientists, partner organizations, and communities to deliver science-based solutions.

A lot of our work is focused on reducing the risk of chronic diseases in general. Probably one of the biggest contributors to cancer is obesity, and one of the focus areas of all the Human Nutrition Research Centers is obesity prevention. So, the work that's being done at ARS isn't directly about the link between obesity and cancer, but it's about preventing obesity and working on understanding it. We're doing a lot to try to understand both the mechanisms behind obesity and ways to mitigate it.

UM: What trends have we seen over time with obesity, and what do we know about what might be causing them?

CD: When we look at historical data, we see that there was explosive growth in obesity in the United States from the 1980s through the present (data on the trends can be found on the CDC website: Data & Statistics | Overweight & Obesity | CDC).

There are many, many lines of thinking about why that happened, and we don't have a clear answer. It's a very complex question - that's part of what a lot of our research has been trying to determine. The trajectory looked like it was getting better, and then COVID hit, and COVID made it worse again, because everything changed.

An array of fresh fruits and vegetables
Fruits, vegetables and whole grains are cornerstones of a healthy diet. (Photo by Keith Weller/USDA).

UM: What are we seeing now?

CD: Many studies are just beginning to re-start data collection. Some of the initial results suggest that the trends have worsened since the pandemic. For instance, the CDC reports big increases in obesity among young people: among 432,302 people ages 2–19, the rate of body mass index (BMI) increase approximately doubled during the pandemic, compared to a pre-pandemic period.

UM: What do we know about the mechanisms that connect obesity to cancer?

CD: A number of causal mechanisms have been identified, but there's also a lot that's unknown. With obesity, you have changes in a number of hormones that can be carcinogenic. You also have a pro-inflammatory state in your body that leads to increased risk of cancer. Visceral adipose tissue, or fatty tissue around the organs, is a major source of proinflammatory cytokines and other factors that contribute to systemic low-grade inflammation.  Another factor is that obese people tend to have insulin resistance, which may increase cell differentiation.

There have been a lot of studies in animal models. In the Human Nutrition Centers, we've repeated them somewhat in humans. Caloric restriction has been linked to longevity in animal models, but it hasn't been tested in humans. The closest research has come to examining that relationship is the CALERIE Study (two years of calorie restriction and cardiometabolic risk), which found that moderate calorie restriction has beneficial effects on cardiometabolic risk factors (blood pressure, cholesterol and insulin) in non-obese adults.

UM: What exactly do researchers mean when they talk about "inflammation" as a factor in health and disease development? How is it related to the issues of obesity, disease, and cancer?

CD: Inflammation is the way your body's white cells and the proteins they make protect you from infections and foreign substances such as bacteria and viruses. Acute inflammation is usually beneficial and is typically resolved rapidly. In contrast, chronic inflammation often begins with the same cellular response, but can turn into a persistent problem when the body fails to resolve the underlying issue. Chronic inflammation plays a central role in some of the most challenging diseases of our time, including  obesity, cancer, heart disease,  arthritis, and diabetes.

Female patient with a physician
Obesity is a meaningful risk factor for many chronic illnesses, including cancer. (Photo courtesy of Getty Images)

UM: What are the challenges involved in transitioning to a healthier diet? Are there particular barriers to changing diet that are hard to overcome? How is ARS research addressing these issues?

CD: ARS researchers in Houston, TX,  have been doing a lot in this area. Last year, for instance, they completed a major study on parent feeding styles and child weight status. Knowing that most prevention programs for preschool children have not been effective in preventing weight gain, they examined the relationships among Hispanic mothers and their preschool children and found that an indulgent feeding style (mothers who set few boundaries but are sensitive to their child's individual needs) was associated with an increased child body mass index over time. The lesson there was that educating the mothers about how their responses to their children's weight may impact their feeding directives could help reduce weight problems as children age.

UM: What is precision nutrition, and what role does it have to play?

CD: It's an effort to understand: are the nutritional requirements the same for everyone, or do different groups have different requirements? It represents a shift in focus from past research to get a more granular understanding of how nutritional requirements vary among different ethnic groups, how genetics and a person's microbiome might contribute, to really get a better understanding of variability between people and groups.

The goal is to move away from a "one size fits all" approach to dietary guidance and toward understanding the unique nutritional needs of different subpopulations, include those delineated by age, sex, physiologic status (e.g., pregnancy), ethnicity, health, genetics, epigenetics, and activity level.

UM: Looking at the American diet, are certain foods or nutrients over- or under-consumed?

CD: Americans tend to under-consume fruits, vegetables, and whole grains, and over-consume foods high in added sugar, saturated fat, and sodium. I think that the best recommendations that we can make are just to increase the consumption of fruits, vegetables and whole grains, and limit the consumption of foods that contain empty calories – cake, things like that, that have a lot of calories, but are low in nutrients.

UM: What are the other big questions about obesity that you're looking at now? What developments are you most excited about?

CD: There's a lot of work going on at Baylor University to understand the signaling molecules that are associated both with causing and preventing obesity. They're also doing a lot of work there to try and understand how you get out into the community, particularly the under-served community, for obesity prevention. Other scientists are looking at satiety – that is, the feeling of being "full," or satisfied after eating – and regulation of eating. In Grand Forks, ND, ARS scientists are  looking at the reinforcing value of food choice, which is: if you eat a food, do you then come back the next time either liking it more or disliking it more? In Arkansas, our scientists have done a lot of work on understanding whether, if a mother is obese when they become pregnant, that influences obesity in the children, as well as the interaction between whether the mother is obese or normal weight, and then whether they breastfeed or they feed formula, and how that affects the trajectory of weight gain in the children.

Overall, ARS is shifting away from just studying obesity, and toward identifying the mechanisms whereby food, food components, and physical activity help prevent diet-related chronic diseases such as obesity, diabetes, cardiovascular disease, and cancer. When we do all these studies, we look at risk factors for a number of chronic diseases. A lot of times we might be looking at risk factors for diabetes or risk factors for cardiovascular disease, but a lot of these exact same diets have the same implications for cancer. Even though the mechanisms might be slightly different, it's still the same types of diets having the adverse effects, and when you're recommending the healthy diet, it's the same types of diets that are protective against all these chronic diseases, many times working by similar mechanisms.