What Is Junk Food?
Junk food refers to highly processed foods that are generally high in calories, refined carbs and unhealthy fats. They’re also low in filling nutrients like protein and fiber.
Some examples include french fries, potato chips, sugary drinks and most pizzas.
Junk food is widely available, cheap and convenient. Also, it’s often heavily marketed, especially to children, and promoted with misleading health claims.
While it is tasty, it is usually not very filling and is easy to overeat.
Interestingly, junk food may also affect your brain in a very powerful way, especially when consumed often and in excessive amounts (4).
It may trigger a massive release of dopamine, a neurotransmitter that helps control your brain’s reward and pleasure center.
When your brain is flooded with dopamine in such unnatural amounts, it can cause food addiction in some people.
It Takes Less Energy to Digest Junk Food
It requires energy to digest, absorb and metabolize the food you eat.
This is referred to as the thermic effect of food (TEF), and it generally accounts for around 10% of your daily energy expenditure (6).
Metabolizing protein in food requires a lot more energy than metabolizing carbs or fat (6, 7).
In fact, eating a high-protein diet may cause your body to burn up to 100 more calories per day (8, 9, 10).
Furthermore, the degree to which foods are processed affects the TEF. It will generally be higher when you consume whole foods made of complex nutrients, compared to refined, processed junk foods.
To investigate this, one small study in 17 healthy people compared two sandwich meals that differed in their level of processing, but not their macronutrient composition or calorie content (11).
The study found those who consumed a whole grain sandwich with cheddar cheese burned twice as many calories digesting and metabolizing the meal than those who ate a sandwich made with refined grains and processed cheese.
While this study was small, the results indicate that processed food requires less energy to digest and metabolize than whole foods. This leads to fewer calories burned throughout the day, making weight loss and maintenance more difficult.