You’ve just eaten 10 Taco Bell tacos and a frozen Mountain Dew and you feel... suspiciously still hungry. You’ve consumed about 1,880 calories, but your body isn’t satisfied. What’s happening?
Studies show that satiety, the mechanism that stops us from eating more than what we need, has less to do with caloric intake than it does with the intake of certain macronutrients ― types of protein, carbohydrates and fat ― and the physical volume of food.
We’re getting plenty of calories when we eat a full sleeve of Oreos, but we’re not getting the nutrients that our bodies need for high-quality, sustainable energy. And even though it may feel like a large volume of food, it moves through us quickly ― meaning the feeling of fullness fades soon after we eat.
The satiety level of a food is partially due to its nutrient density, which refers to the ratio of nutrients to calories. Though highly caloric, junk foods supply a much lower amount of nutrients compared to the volume of food.
In other words, calories aren’t created equally.
For 100 calories, we can eat about 15 cups of spinach or two Oreos. The spinach will physically fill our stomachs with more food, plus provide dietary fiber and vital nutrients like beta-carotene and iron. The Oreos, on the other hand, provide little more than intense levels of simple carbohydrates, which give us quick bursts of energy that don’t last.
“Frequently, the failure of foods to produce satiety is that they are deficient in fiber, that they are too easily digested, or that they do not provide a steady supply of calories into the body during digestion,” said Jeremy Furtado, a senior research scientist at Harvard University.
Protein and dietary fiber are critical for slowing digestion and releasing energy over a prolonged period of time, and vitamins and minerals can impact satiety as well: If your body doesn’t receive the nutrients it needs, it may continue to send hunger signals to encourage you to eat other foods that may contain them.
Junk foods are generally made up of highly processed ingredients that lack both satiating macronutrients and naturally occurring vitamins and minerals.
It’s called ‘junk food’ for a reason
Processed food manufacturers use cheap, low-quality ingredients that minimize nutrition and maximize profitability.
Most of our favorite junk foods fall under the category “ultra-processed foods.”
That’s according to NOVA, a food classification system used by the World Public Health and Nutrition Association. The system categorizes food into four groups: unprocessed or minimally processed foods (e.g. olives), processed culinary ingredients (e.g. olive oil), processed foods (e.g. whole grain bread, canned vegetables) and ultra-processed foods (e.g. all our favorite packaged chips and cookies, plus many fast foods and frozen meals).
“Ultra-processed foods contain ingredients that are exclusively used industrially, which primarily means food extrusions and cosmetic additives,” said Carlos Monteiro, a professor of nutrition and public health at the University of San Paulo, Brazil.
Any form of processing can affect the nutrition of a food ― milling grains or blanching vegetables can destabilize vitamins like folate, thiamine and vitamin C ― but ultra-processed foods go through a much more complex system of industrial processes.
Food manufacturers use “extrusion” to change the physical makeup of a food and isolate specific nutrients. Proteins, carbohydrates and lipids are isolated from low-cost commodities like corn, soy and peas and later recombined to make the final product.
Cosmetic additives, such as flavor enhancers, emulsifiers and artificial colors, are then used to replace the texture, color and flavor lost to high-intensity processing. These are the ingredients found at the tail end of the food label, such as maltodextrin or Yellow 5.
A fast-food chicken nugget, for example, contains “a slurry that’s ‘mechanically recovered’ from remnants of the animals that otherwise would be discarded, by use of high-pressure grinders and centrifuges,” wrote Monteiro in a 2010 paper for World Nutrition. “The animal-source material becomes an ingredient much like the refined starches, oils and other substrate of the product, reconstituted to look, smell and taste like a juicy battered slice of chicken.”
While ultra-processed food can resemble whole or minimally processed foods, they lack their satiety effect and nutritional value. Breaking a food into parts means it’s “predigested,” so our bodies exert much less energy during digestion. And the nutritional value suffers substantially too.
In an attempt to match the nutrition of whole foods, manufacturers might enrich their products with chemically formulated vitamins and minerals. But research says it doesn’t compare.
“Even when nutrition labels show similar levels of important vitamins and minerals comparing a highly processed food to a whole food, the whole food will be more nutritious,” Furtado told HuffPost. In the example of processed white bread, “the processing of the flour removes nearly all of the naturally occurring vitamins and minerals, as well as the fibers essential to maintaining a healthy gut microbe population,” Furtado said. “The artificial return of a select group of nutrients only addresses some of the problems with refined white bread.”
This might have to do with something called food synergy, which suggests that certain nutrients work better together. For example, our bodies can only digest the antioxidants in cereal grains in combination with the dietary fiber from the bran.
Ultra-processed foods mess with our hormones
Another reason junk food can’t make us full?
Ultra-processed foods can contain as much as 8 times the amount of sugar as whole foods, which can substantially raise the triglycerides in our bloodstream. While a certain level of triglycerides is healthy, too much can inhibit our ability to recognize when we’re full. That’s due to a hormone called leptin ― also called the “satiety hormone,” which communicates to our brain when we’ve had enough to eat ― and triglycerides can block leptin from passing through the blood-brain barrier.
One last thing to keep in mind: Low-sugar and sugar-free junk foods are just as nutritionally empty as their high-sugar counterparts. In fact, a 2013 study found that sugar substitutes like aspartame can adversely affect our metabolisms ― leading us to actually eat more. Diet soda, for example, can increase your appetite, confusing your system by supplying the “sweetness” factor without delivering the energy your body expects.