High-fat diets stimulate overeating and lead to obesity, a new study finds

05 Mar 2024 --- A team of UK and China-based researchers reveal that a hedonic overdrive model best explains male mice’s weight gain after a switch to a high-fat diet. The researchers found that when fed calories made up of more than 40% fats, the males of many strains of mice are susceptible to fat gain and obesity.

“The hedonic overdrive model is that there are rewarding properties in food that cause individuals to overconsume calories beyond their metabolic needs,” John Speakman, co-author of the study, director of the Shenzhen Key Laboratory of Metabolic Health at the Chinese Academy of Sciences, and professor of zoology at the University of Aberdeen, UK, tells Nutrition Insight.

“That extra energy has to go somewhere; the main place it goes is into fat stores. We might overconsume calories for other reasons — like passive overconsumption and reverse causality. Passive overconsumption means we eat about the same weight of food, and so if the energy density increases by adding fat, that leads to us consuming more energy even though the weight we eat is the same. Reverse causality is an idea that the deposition of fat drives the intake, not the intake drives the deposition of fat,” he explains.

While mice ate smaller volumes of food on a high-fat diet, they ate more calories and gained weight.“Our work tested these two ideas. We found the increase in intake did not involve eating a constant weight of food, disproving passive consumption, and also showed intake increases ahead of fat deposition, disproving reversed causality. Hence, the hedonic overdrive model fits the data best (but not perfectly). So it is likely there are other things going on that we don’t understand yet.”

 High-fat/high-carb diets 
The study, conducted by researchers based in China and Scotland and published in Obesity, The Obesity Society’s flagship journal, measured the food intake and body weight of 12 groups of individually housed 12-week-old male mice exposed to 12 different high-fat diets. 

The researchers explored why male mice get fatter when fed a high-fat diet by considering hedonic overdrive, reverse causality, and passive overconsumption, concluding that the hedonic overdrive model provides the best fit. 

The weight of the high-fat foods presented to the mice decreased on average by 14.4 g in comparison to their previous diets; however, simultaneously, they consumed 357 kJ more energy on average, which led to weight gain.

“The mass balance model is that the etiology of obesity is due to chronic overconsumption of the weight of food rather than chronic energy overconsumption,” details Speakman. “Our mice actually ate a lower weight of food than controls at the same time as eating more calories (due to the energy density being higher), so that is incompatible with the mass balance idea.”

Based on the findings, the researchers propose that if the data pertains to humans, it can be argued that people overeat because the hedonic qualities of high-fat diets override the homeostatic intake regulation and recommend reducing the rewarding qualities of high-fat food to curb overeating. 

“We should always be cautious about making that extrapolation, but there is data showing the food composition that mice like best also correspond to the composition of foods humans like most. So it is certainly possible that this also applies to humans,” Speakman continues.

Optimizing fat consumption
Speakman argues that for humans, there is an optimal fat-protein-carbs consumption pattern.

According to the study author, the diet most likely to result in weight gain is that with 10-20% protein, 40-60% fat and 20-40% carbs.“Sustaining a lower intake of fat (around 10% by calories) and a higher intake of unrefined carbohydrates (around 70% of intake by calories) is probably the most protective against obesity and most healthful.” 

He points out that high-fat diets can be healthier and less likely to result in obesity when combined with low-carb eating while still cautioning against some aspects of such a diet pattern. 

“The most dangerous composition that seems most rewarding to eat involves about 10 — 20% protein, 40-60% fat and 20-40% carbs (think pizza, cakes, ice cream, etc),” he underscores.

“If you reduce the carbs from these values and replace them with fat, that food doesn’t seem to be as rewarding to eat and leads to less fat deposition. Whether it is healthier to do that or not is a hotly debated question, but at least in some people, moving to high fat and low carbohydrates leads to undesirable increases in LDL (low-density lipoprotein) cholesterol,” he explains. 

 “The protective effect of eating more fat and reducing carbs once you move above the dangerous combination mentioned above has been classically interpreted by a model called the carbohydrate-insulin model (CIM). That predicts this effect is due to reverse causality.”

Speakman explains that the data gathered in this new study is “superficially consistent” with the CIM — “in the sense that in this zone reducing carbs and increasing fat is protective against obesity — but it clearly does not support reverse causality.

“Reverse causality predicts they increase in fat first and increase intake second, while our data show the opposite,” he asserts. “What this work shows is that the effect of lowering carbs probably doesn’t work in the way the CIM suggests but is much more related to food reward properties.” 

Another recent study conducted on rodents similarly found issues with the health outcomes associated with the long-term consumption of a high-fat diet. 

Another team of researchers found that high-fat diets may result in a dysregulation of the genes of the intestines responsible for immunity and metabolism.

By Milana Nikolova