When Fat and Carbohydrate Arrive Together: Why the Combination Is So Interesting

If you look at the foods people commonly describe as difficult to stop eating, an interesting pattern appears. Cookies, donuts, chocolate, pizza, French fries, pastries, ice cream and many snack foods don’t simply contain fat, carbohydrate, sugar or salt. They combine them.

That distinction matters because appetite doesn’t respond to food as a list of nutrients on a Nutrition Facts label. The brain receives information from taste and smell, from the physical experience of eating, and from signals generated after nutrients enter the gastrointestinal tract. Through experience, it learns relationships between what a food tastes like and what that food delivers.

This allows us to learn that foods have value before digestion is complete. It is an extraordinarily useful system. An organism that can remember which sensory cues predict energy and nutrients doesn’t have to rediscover the nutritional value of every food at every meal.

Modern food production, however, can create nutrient combinations that are much less common in minimally processed foods. One combination in particular has attracted researchers’ attention: fat and carbohydrate together.

Fat isn’t the problem. Carbohydrate isn’t the problem.

Fat is an essential nutrient. It provides essential fatty acids, carries fat-soluble vitamins, contributes to cell membranes and participates in an enormous range of physiological processes. Carbohydrate provides glucose, an important fuel for the body, while carbohydrate-containing foods can also provide fiber, vitamins, minerals and many other compounds important to health.

There is no useful nutritional lesson here that ends with “avoid fat” or “avoid carbohydrates.” The interesting question is what happens to food reward when substantial amounts of the two arrive together.

Fruit generally provides substantial carbohydrate with very little fat. Potatoes and grains are predominantly carbohydrate before we add other ingredients. Nuts and seeds can contain substantial fat but relatively little available carbohydrate. Meat and fish provide protein and varying amounts of fat but essentially no carbohydrate. There are important exceptions, but foods naturally containing high concentrations of both fat and carbohydrate are comparatively unusual.

The modern food environment looks different. Donuts, cookies, French fries, pizza, chocolate, pastries and many desserts routinely combine substantial amounts of fat and carbohydrate. We have become extremely good at putting these nutrients together. The question is whether the nervous system responds differently when we do.

An experiment designed to find out

In 2018, Alexandra DiFeliceantonio, Dana Small and colleagues published an experiment in Cell Metabolism examining exactly this question. Participants viewed familiar snack foods whose calories came predominantly from fat, predominantly from carbohydrate, or from substantial amounts of both. They completed a bidding task using real money while functional MRI was used to examine brain responses.

Participants were willing to pay more for foods containing both fat and carbohydrate than for foods containing primarily one or the other, even after accounting for factors including liking, familiarity and caloric content. The researchers described the effect as supra-additive: the reward associated with combining fat and carbohydrate appeared greater than would be predicted simply by adding together their individual contributions. Brain imaging also showed greater activity in regions involved in reward valuation.

This does not mean researchers discovered a “junk food center” in the brain. Brain imaging does not support claims that simplistic. Combined with the behavioral findings, however, the experiment provides evidence that the brain may assign unusual value to foods containing both fat and carbohydrate.

Why would the combination matter?

Taste tells the brain something about food, but taste is not the whole nutritional information system. After food is swallowed, the gastrointestinal tract and nervous system continue communicating information about what has arrived. The brain can learn associations between a food’s sensory characteristics and its post-ingestive consequences, and those relationships influence what we seek out again.

Fat and carbohydrate appear to generate partly distinct signals communicating nutritional information to the brain. DiFeliceantonio and colleagues proposed that foods containing both nutrients may therefore activate multiple pathways simultaneously. They also found that participants were poorer at estimating the energy density of foods containing both fat and carbohydrate than they were for foods dominated by one nutrient.

That creates an intriguing possibility: some modern foods may generate especially strong signals of reward while simultaneously making their energetic content more difficult for us to estimate accurately. It remains a hypothesis rather than a settled explanation, but the behavioral finding itself is important without exaggeration.

Hyper-palatable foods show a broader pattern

A separate line of research reaches this issue from another direction. Tera Fazzino and colleagues developed an objective definition of hyper-palatable foods based on nutrient combinations that repeatedly appeared in highly palatable foods. Their quantitative definition identified three major clusters: fat and sodium, fat and simple sugars, and carbohydrate and sodium.

The point is not that foods crossing a numerical threshold suddenly become dangerous. Biology does not work that way, and the definition was not intended to create another system of forbidden foods. What matters is the pattern: highly palatable foods frequently do not maximize one appetitive property. They combine several.

This is why reducing the conversation to “sugar is addictive” or “fat makes you overeat” misses something important. The food environment is not built around isolated nutrients. It is built around foods, and foods can deliver multiple rewarding properties at once.

The whole food matters

Nutrition education often deconstructs food into its chemical components. A cookie contains carbohydrate, fat and perhaps a little protein. That information is useful for understanding metabolism, but it does not completely describe the experience of eating a cookie.

The ingredients have been processed and recombined into something with a particular sweetness, fat content, aroma, texture, energy density and rate of consumption. Those properties arrive together and interact with sensory systems, learned associations, physiological signals and reward circuitry. The whole eating experience therefore cannot necessarily be predicted from the individual nutrients considered separately.

That is essentially what the fat-plus-carbohydrate experiment demonstrates: the combination matters.

The food supply isn’t a random collection of foods

Food companies operate in competitive markets. A product that consumers enjoy, purchase repeatedly and choose over competing products is commercially valuable. That creates an obvious incentive to make foods people strongly want to eat, optimize them, make the experience reliable, and make them convenient and widely available.

This does not require imagining food scientists secretly plotting to override human biology. Ordinary product development is enough: test formulations, measure consumer preferences, modify ingredients and textures, test again, and sell the versions people prefer. Products that do not appeal to consumers disappear; products people repeatedly buy survive.

The foods surrounding us are therefore not a random sample of every combination humans could manufacture. Successful products have been repeatedly selected because consumers respond positively to them. That matters when the characteristics being optimized overlap with biological systems that evolved to motivate food seeking.

Self-control is an incomplete explanation

If someone can keep plain potatoes in the kitchen for a week without thinking much about them but finds an opened bag of potato chips difficult to ignore, it is tempting to interpret the difference as a personal failure. Yet those are not equivalent stimuli. The potato has been sliced, cooked with added fat, salted, dehydrated to varying degrees and transformed into a food with a different texture, energy density, nutrient composition and eating rate.

The person has not mysteriously developed weaker character between the potato and the potato chip. The food changed.

That does not tell us exactly which property caused the difference for a particular person, and people do not all respond identically. It does give us a more useful starting point than assuming the response reflects a deficiency in the eater.

Understanding this shouldn’t create another food rule

There is an unfortunate pattern in nutrition: we discover that some property of food influences eating behavior and almost immediately turn the discovery into another prohibition. Don’t eat sugar. Don’t eat fat. Don’t eat carbohydrates. Don’t eat processed foods. Adding “don’t eat fat and carbohydrate together” would completely miss the point.

The purpose of understanding food reward is not to create increasingly elaborate rules for resisting food. The more useful goal is awareness. You can notice that different foods affect your appetite differently without assigning moral value to either the food or your response.

Those observations become information about the interaction between your body, the food and the environment in which you are eating it. That is a much richer source of information than “I need more willpower.”

The research on fat and carbohydrate gives us one particularly elegant demonstration of the larger idea behind this series: change the characteristics of food and you can change its reward value. Once that is true, eating behavior can no longer reasonably be treated as a pure measure of character, discipline or motivation.

The environment participates in the behavior. And nutrient combinations are only one way it does so. A food can also determine how quickly you are physically capable of consuming it. That is where we go next.

References

DiFeliceantonio AG, Coppin G, Rigoux L, et al. Supra-additive effects of combining fat and carbohydrate on food reward. Cell Metabolism. 2018;28(1):33–44.e3.

Fazzino TL, Rohde K, Sullivan DK. Hyper-palatable foods: development of a quantitative definition and application to the US food system database. Obesity. 2019;27(11):1761–1768.

Related reading

Educational information only. This article is not a diagnosis or individualized medical or nutrition treatment. New, severe, persistent, or concerning symptoms should be evaluated by an appropriate healthcare professional.