Your metabolism refers to all the chemical reactions that occur within your body.
Having a fast metabolism means that your body burns more calories.
On the other hand, having a slow metabolism means that your body burns fewer calories, making it more difficult to maintain or lose weight.
Some foods may increase your metabolism. But how does junk food affect it?
This article explores whether processed foods slow down your metabolism.
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.
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.
Summary: Junk food is inexpensive, low in nutrients and high in calories. It affects the reward center in your brain and may cause addictive behaviors in some people.
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).
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.
Summary: Metabolizing food requires energy, which is referred to as the thermic effect of food. Processed junk food requires less energy from your body to digest because it's high in refined ingredients.
Insulin resistance is when your body's cells stop responding to the hormone insulin.
This can lead to higher blood sugar levels.
The consumption of processed foods has been associated with an increased risk of insulin resistance.
A small study in 12 healthy men reported changes in the ability of skeletal muscle to process glucose after only five days on a diet rich in fatty processed foods (15).
The researchers concluded that a diet comprised of high-fat junk foods may lead to insulin resistance in the long term.
Furthermore, the results of a 15-year study indicate that your risk of developing insulin resistance may double when you visit a fast food restaurant more than twice per week, compared to less frequently (16).
This implies that eating junk food on a regular basis may promote insulin resistance.
Summary: Consuming lots of processed junk food has been linked to an increased risk of insulin resistance, a condition characterized by high blood sugar levels.
Of all the junk foods out there, sugary drinks may very well be the worst for your body.
These issues are mainly attributed to their high levels of fructose, a simple sugar primarily metabolized by the liver.
When you consume a lot of fructose, the liver may become overloaded and turn some of it into fat.
Sugar-based sweeteners like table sugar (sucrose) and high-fructose corn syrup are around 50% fructose and commonly found in sugary drinks.
When consumed in large amounts in the form of added sugars, fructose may alter fullness signals, impair the response of the "hunger hormone" ghrelin after meals and promote fat storage around the belly (21, 22, 23).
Additionally, it may slow down your metabolism.
In one study, overweight and obese people consumed drinks that were sweetened with fructose and provided 25% of their daily calorie intakes. During a 10-week period, they experienced a significant drop in resting energy expenditure (24).
This suggests that the fructose in sugary drinks may decrease the number of calories you burn, at least when consumed in excess.
Summary: In addition to increasing your risk of all sorts of health problems, beverages high in sugar may also slow down your metabolism. These effects are attributed to their high fructose levels.
However, the calorie content of your food isn't the only thing that matters (25).
The quality of the foods you eat is just as important.
For example, eating 100 calories of french fries can have vastly different effects on your body than 100 calories of quinoa.
Most commercial french fries are high in unhealthy fats, refined carbs and salt, while quinoa is rich in protein, fiber and many vitamins (26).
First of all, you burn more calories metabolizing whole foods than junk foods. Also, you burn more calories by consuming high-protein foods, compared to foods high in unhealthy fats and refined carbs.
Moreover, high-protein foods may reduce your appetite, curb your cravings and impact hormones that regulate your weight (27).
Therefore, calories from whole foods like quinoa are usually more satiating than calories from processed junk foods like french fries.
Before you start restricting your calorie intake to lose weight, consider making better food choices and choosing more nutritious, high-quality foods.
Summary: A calorie isn't a calorie. It's important to focus on the quality of the calories you're consuming, as some calories can reduce the number of calories you burn and negatively impact your hunger and hormone levels.
Consuming large amounts of junk food has metabolic consequences.
In fact, it may increase your risk of insulin resistance and reduce the number of calories you burn every day.
If you want to boost your metabolism, several strategies can help you do that.
But most importantly, choose whole, single-ingredient foods whenever possible.