In less than a century, human diets have transformed in ways that our distant ancestors could not have imagined. Although it is doubtful that humankind had an identifiable “ancestral” diet, the transition of our diets from whole and minimally processed foods to energy abundance and highly refined foods represents a dramatic shift in eating patterns.
Until recently, modern dietary advice has largely ignored this massive shift in food characteristics, relying instead on nutritionism, a reductionist approach that separates foods down to their constituent nutrients such as carbohydrates, fats, and proteins. For example, a comparison of a pastry having 6 grams of sugar with an apple that has 25 grams of sugar could be distorted if the goal is to be “low sugar.” While the nutrient compositions of our foods are of the highest importance to well-designed dietary planning, focusing on them alone misses one of the greatest biological shifts in human history — our transition to highly processed foods.
Today, a new discussion is beginning to take place regarding the “novel” foods that have entered our supply. “Ultra-processed” foods — derived by intensive processing of their natural components — now account for over half of all calories consumed in the United States. These foods undergo industrial processing, such as refinement, dehydration, extraction, or hydrogenation, and the addition of sugar, high fructose corn syrup, salt, oils, or stabilizers. Examples of these foods include soft drinks, many breakfast cereals, candies, chips, packaged snacks, instant noodles or oats, many frozen meals, and fast foods. On the timeline of human history, foods of this nature are truly extraordinary, having only existed in the last 100–150 years, and being absent from dietary intake for approximately 99.99% of our 1.5–2-million-year history.
Until recently, the paucity of direct evidence causally linking ultra-processed foods to disease was interpreted as evidence supporting a benign role. Conventional wisdom held that highly processed foods were effectively equivalent to the sums of their constituent nutrients. Today, the tide is turning, and new evidence has begun to challenge this notion. It has become evident that ultra-processed foods could have effects on physiology that change eating behaviors and, potentially, metabolism.
Over the last several decades, the best evidence linking highly processed foods with human disease were prone to confounding factors, due to their observational nature. Higher intake of ultra-processed foods, for example, was found to be associated with increased risk of metabolic syndrome and diabetes, even after attempting to account for sociological factors and energy intake. Perhaps the most concerning epidemiologic data were from studies of children showing increasing proportions of highly processed food intake associated with possible risk of future disease. In a recent study of children across multiple countries, consumption of ultra-processed foods was associated with increased energy density and free sugars and decreases in fiber. Similarly, a study spanning over two decades in the United Kingdom found that consumption of ultra-processed foods in childhood was associated with increased risk of developing obesity.
How do ultra-processed foods affect our biology? These foods tend to be hyper-palatable, providing high reward signaling to the brain through tastes, textures, and gut-mediated signals. Because of this, their influence on physiology likely extends beyond basic metabolism and reaches into the neural circuitry driving behavior. Animal data, for example, suggest that certain characteristics of highly processed foods, such as the addition of sugar, activates brain reward signaling in the ventral tegmental area, leading to behavior changes like bingeing, withdrawal, and cravings. The recent discovery of “neuropod” cells in the gastrointestinal tract that are activated by glucose ingestion shed light on why foods high in sugar have reward mechanisms beyond taste, texture, and satiety.
In 2019, however, a research group at the National Institutes of Health, led by Kevin Hall, PhD, offered some of the causal evidence linking ultra-processed foods with human disease. In a highly controlled metabolic ward setting, 20 human participants spent 14 days on either an unprocessed or ultra-processed diet, followed by 14 days on the opposite diet. The two diets were matched for macronutrient composition (the carb, fat, and protein content), sugar, sodium, and fiber content. The scientists monitored changes in body composition via dual x-ray absorptiometry and found that while participants were on the ultra-processed diet, they gained weight, confirmed to be fat mass. Conversely, participants lost fat mass on the unprocessed diet. Despite matching the two diets for macronutrients and calories presented, the participants tended to have increased caloric intake while on the ultra-processed diet, leading to energy surplus and weight gain. This study shed light on a possible causal role of ultra-processed foods in weight gain, but longer follow-up studies are needed to confirm these findings.
Today, we find ourselves in food environments where the predominant sources of calories are energy-dense, highly processed, relatively inexpensive, and far removed from foods that were historically consumed by humans. What can we learn from recent studies about how to be healthy in these new food environments?
First, being exposed to ultra-processed foods seems to make people more likely to consume more calories, at least in the short term, possibly via effects on central nervous system reward pathways and brain appetite regulation centers. Accordingly, limiting highly processed foods in the household may be important to reduce exposure.
Second, increasing the proportion of whole and minimally processed foods in our diets can be a key step to reduce dietary proportions of ultra-processed foods. Whole foods like fruits, vegetables, and nuts provide a range of beneficial nutrients and fiber. High-protein foods can be helpful to increase satiety (the sensation of being full) and require more energy to absorb and utilize compared to carbohydrates and fats. In contrast, ultra-processed foods tend to be energy-dense and low in nutrients and protein, leading to imbalanced diets and potentially poor health outcomes. By incorporating more whole foods into our diets, we can increase satiety and reduce the amount of highly processed foods we consume. It is important to consult with your doctor and dietician before making any dietary changes.
Third, it is important to be aware that food marketing often emphasizes convenience, taste, and low cost, but may not truly reflect the nutritional value of these foods. Until recently, there has been little regulatory scrutiny about what food products can and cannot be labeled “healthy.” Carefully inspecting food labels to understand the ingredients can help to reduce our intake of foods with excess sugars and additives. In the future, we may hope that food labels include information regarding processing.
The progression of science and technology has, paradoxically, led to novel foods and relatively new diseases of metabolism, and enables us to investigate them. We are just beginning to understand the long-term effects of highly processed foods, and it seems likely that future scientific studies may focus on elucidating the exact mechanisms through which they exert their effects on human health.
References
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- Srour, Bernard, Léopold K. Fezeu, Emmanuelle Kesse-Guyot, Benjamin Allès, Charlotte Debras, Nathalie Druesne-Pecollo, Eloi Chazelas, et al. 2020. “Ultraprocessed Food Consumption and Risk of Type 2 Diabetes Among Participants of the NutriNet-Santé Prospective Cohort.” JAMA Internal Medicine 180 (2): 283–91.
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