Having obesity increases the risk of health conditionsTrusted Source such as heart disease, stroke, type 2 diabetes, and certain types of cancer.
Although scientists have been studying obesity for years, the underlying causes of this complex disease are not fully understand.
Many factorsTrusted Source contribute to excess weight gain, including patterns of eating, physical activity levels, and sleep routines.
Genetics are also involved, as are social determinants of health, which are the conditions at home, work, and in a person’s community. Even racial discrimination may increase a person’s risk of developing obesity.
Because dietary patterns play a key role in a person’s risk of obesity, several scientific models have been proposed for how food intake leads to weight gain.
Some incriminate specific food groups, such as fats and sugars, as fueling obesity. Another suggests that obesity is driven by ingesting more calories than what the body burns during its daily activities.
While each model, or hypothesis, has research to back it up, there is still debate about which one best explains the rise in obesity — in the U.S. and worldwide — and the optimal approach for addressing this public health problem.
A paper published Oct. 17 in the journal ObesityTrusted Source suggests that these models are, in fact, compatible with each other, and can be tied together with one key nutrient: fructose.
Fructose is a natural sugar that occurs in fruits, fruit juices, certain vegetables, and honey.
It is present in table sugar (along with sucrose) and high-fructose corn syrup, which is made from corn starch and added to many manufactured foods. The body can also produce fructoseTrusted Source from other carbohydrates, such as glucose.
Dr. Richard Johnson, a researcher at the University of Colorado Anschutz Medical Campus, and his colleagues, write that when the body is in a starved state, ingested fructose acts like other nutrients and restores the active energy in the cells, which is known as ATP or adenosine triphosphate.
However, in a fed state, when fructose is metabolized, it lowers the ATP level in the cell, while at the same time blocking the replenishment of ATP from the body’s fat stores.
As ATP levels drop, it signals that the cell is running low on active energy, which stimulates a number of biological responses, including hunger, thirst, increased food intake, insulin resistance and reduced resting metabolism.
These changes can lead to weight gain, especially when there is easy access to energy-dense and tasty foods like fast food, candy, chips, baked goods and ice cream.
This “fructose survival hypothesis,” as Johnson and his colleagues call it, views obesity as a low-energy state — in particular, active energy, or ATP — and ties together the other dietary hypotheses.
This includes the “energy balance hypothesis,” which suggests that obesity is driven by consuming more energy than is burned off; and the carbohydrate-insulin model, which proposes that sugars and other carbohydrates are the primary factor behind excess weight gain.
“Fructose is what triggers our metabolism to go into low-power mode and lose our control of appetite, but fatty foods become the major source of calories that drive weight gain,” Johnson said in a news release.
He and his colleagues point out in the paper that most of the studies investigating the role of fructose in obesity involve animals studies. More research is needed to know if the fructose survival hypothesis also applies to people, and if it can lead to effective interventions for preventing obesity.
Joanne Slavin, PhD, a professor in the Department of Food Science and Nutrition at the University of Minnesota in St. Paul, cautions against blaming one specific nutrient when talking about obesity.
Slavin was not involved in the study.
“Foods are very complicated, and eating is very complicated,” she told Healthline.
While she recommends removing extra calories from the diet, excess energy intake in any form — whether it’s protein, fat, or carbohydrates — can result in weight gain.
In addition, “I don’t think labeling fructose as a ‘villain’ will help people improve their diets,” she said. Neither will laying blame solely on added sugars, saturated fats, or ultra-processed foodsTrusted Source, she said.
By focusing on one nutrient, “we forget everything else we know about nutrition,” she said, such as “the importance of being connected to the land, eating foods as a family, and being respectful of individual people’s differences and the foods that grow in different places.”
For example, athletes, and even growing teenagers, have different nutritional needs from adults who get little physical activity. Likewise, someone with high blood pressure or diabetes will need to adjust their meals to help manage those conditions.
Slavin said a person’s culture also shapes their dietary preferences. In some cultures, people prefer whole milk over low-fat milk, the latter of which may be recommended by certain nutrition guidelines.
In others, a slice of apple pie each week during the fall is a tradition, especially when the apples are grown locally. This dessert contains more calories — in the form of sugar and fat — than eating an apple by itself.
But Slavin said what’s important is how these foods fit into a person’s overall eating patterns.
“Rather than beating up on foods that are important in people’s culture or their traditions, let’s realize that we can put together diets that meet nutritional rules, without being disrespectful or going after one nutrient,” she said.
One thing that Slavin does recommend is that people make sure they are getting enough protein in their diet, with athletes and other active people needing more protein than someone who gets little activity.
However, “you have a lot more flexibility with the fat and carbohydrate balance,” she said. “So if cultures are going to only eat high-fat dairy products, we should be respectful of that. They can take out some of those calories from somewhere else in their diet.”
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