How Does Eating Junk Food Slow Your Metabolism? An RD Explains

When you eat a donut, has it ever occurred to you that it stays in your system for 3 days? Or that it could slow down your metabolism? It's true: Processed foods give us a quick jolt of sugar or salt to satisfy a craving, but all the fat, sugar, and processed ingredients can slow your metabolism, studies show, and also impact your long-term health.

Once food enters your mouth, the digestive process begins. The only control you have throughout the course of digestion is when you’re chewing and breaking the food down. As soon as you swallow it, your stomach, small intestine, and large intestine take over and do the rest.

When you digest food your body starts the job of taking the macronutrients (protein, fat, and carbohydrates) and breaking them down into smaller pieces. This allows them to be absorbed in the bloodstream and utilized by any part of the body that needs it—also called metabolism. Typical digestion time can range anywhere from 24 to 72 hours, depending on what you eat. Your metabolism is measured by your basal metabolic rate (BMR) which is how many calories your body needs to perform basic functions such as breathing and circulation. These processes can vary depending on the type of food you’re eating, and what you're drinking.

The term “junk food” refers to foods that are highly processed and contain large amounts of added sugar, fat, and calories, while being low in fiber and healthy nutrients. This includes foods like candy, sugary drinks, fast food, and packaged food like potato chips.

Fat takes the longest to leave your system, according to research. Fat is a complex molecule that takes the most time to be broken down in order for the body to utilize it. Most of its digestion occurs in the small intestine and requires numerous enzymes to make it water-soluble, therefore certain conditions that impact the small intestine could delay digestion and absorption time. Depending on the types of food you eat, and how much water you drink, you can help your metabolism speed up this process.

Solid fats, like butter, are harder to digest compared to fat droplets, according to a 2018 study published in the Journal of the American Oil Chemists’ Society. The study used a model of the human digestive system and found that solid fats took almost two times as long to break down, but more research still needs to be conducted.

Sugar, on the other hand, is quick to digest, which is why you may be hungry within an hour of a sugary snack. This is specific to “simple sugars” which are often added to foods including soda, fruit juice concentrates, breakfast cereal, and pre-packaged baked goods. Although quick to pass through the digestive tract, sugar has been found to actually slow your metabolism.

A study published in the European Journal of Clinical Nutrition found that individuals who drank 25 percent of their daily calorie intake from beverages sweetened with fructose had a large drop in their metabolic rate.

This type of eating pattern has also been shown to impact the gut microbiome and encourages inflammation. The research, published in the journal Gut, found that processed and animal-derived foods formed bacteria that were pro-inflammatory where plant foods were anti-inflammatory. Inflammation in the gut may cause conditions such as inflammatory bowel disease (IBD) or Crohn’s disease, which can impair your digestion and your body’s absorption of nutrients.

Even after food has gone through the entire process of digestion and metabolism, it can still have long-term effects on your health. What you eat, and how often, can potentially increase your risk of serious chronic conditions, such as diabetes and cardiovascular disease.

Consuming certain fats, such as saturated and trans fat, along with having high blood sugar has been linked with damage to the inner layer of arteries (atherosclerosis). Saturated fat and trans fat, which is found in animal products like dairy and meat, baked goods, and fried foods, can raise “bad” LDL cholesterol levels and lower “good” HDL cholesterol. When LDL levels get high, it can start to build up in your arteries and narrow that pathway blood needs to travel to reach your heart, brain, and other vital organs.

Insulin resistance is another outcome that can occur when you eat a diet high in fat and added sugar. Insulin is a hormone that helps feed your cells glucose from the food you eat. When your body becomes resistant to insulin, it doesn’t respond as it should, and this causes glucose to stay in your bloodstream and leads to high blood sugar. Although eating sugary or fatty foods once won’t cause you to become resistant with one stop at the donut store, over time it can contribute to weight gain, which is one of the causes of insulin resistance and, eventually, diabetes.

The same diet that speeds metabolism is the one recommended for a heart-healthy lifestyle, which is high in fiber from plant-based foods like salad greens, vegetables, fruit, legumes, and whole grains and low in processed or junk food. Allowing sat fat and high-fat foods to stick around in the body also raises the risk of high cholesterol, plaque deposits, and what is ultimately heart disease in the form of clogged arteries, or atherosclerosis.

According to the National Institute of Health, following a diet that is heart-healthy can prevent or delay atherosclerosis. This includes eating fruits, vegetables, and whole grains while also limiting sodium, added sugar, and solid fats. This type of eating pattern can also help prevent insulin resistance by promoting weight loss.

Ensuring that you get enough fiber in your diet is the most important factor in maintaining digestive health, and it can also control “bad” cholesterol levels and blood sugar levels. Fiber-rich foods include whole-grain foods, fruits, veggies, beans, legumes, nuts, and seeds.
Takeaway: Junk food slows down your metabolism, while fatty foods take longer to digest than other foods. Although an occasional donut or other high-fat-filled “junk food” treat likely won't stay in your body that much longer than other junk foods, making a habit of it does have a lasting effect on your health.