We’ve all been tempted after a long workday by the convenience and comfort of a frozen dinner, bag of chips or can of soda. While those choices probably seem harmless in the moment, mounting research shows that such ultraprocessed foods can quietly interfere with how our bodies burn and store fuel, regulate blood sugar and maintain a healthy fat balance.

But if concepts like “metabolic flexibility” or “fuel regulation” sound abstract, you’re not alone — most people have only a vague idea of what metabolism actually is.

So here’s what it means, why it matters, and which processed foods may be doing the most harm to your metabolism and beyond.

What Is Metabolism, and Why Does It Matter?

Metabolism refers to the “biological machinery” that helps your body convert food and drink into energy, repair tissues, regulate hormones and glucose, and decide whether to burn or store fuel, Dr. Walter Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health, tells TODAY.com.

A healthy metabolism “keeps energy steady, supports muscle mass, regulates weight and blood sugar levels, and helps you avoid constant hunger or crashes,” adds Lisa Young, Ph.D., a registered dietitian and adjunct professor of nutrition at New York University.

But when that machinery gets disrupted — often through chronic inflammation, insulin resistance, excess visceral fat or poor dietary choices — your body becomes less efficient. “Without the right mix of fuel in your diet, your biological machinery will not run well,” Willett explains. Over time, this can mean burning fewer calories, storing more fat, and slowly increasing your risk of developing a wide range of problems and conditions.

“Multiple large studies show that the greater the number of ultraprocessed foods in your diet, the stronger the association with higher risk of cardiovascular disease, Type 2 diabetes, obesity, mental health disorders and mortality,” Jen Messer, a registered dietitian and president of the New Hampshire Academy of Nutrition and Dietetics, tells TODAY.com.

But she adds that “not all ultraprocessed foods are created equal, and only some are harmful to human health.” Here are six of the biggest culprits.

Sugar-Sweetened Beverages

Soda, sweetened fruit drinks and sugary energy drinks deliver a high glycemic load without fiber or protein to slow absorption. The result is repeated blood sugar spikes that promote insulin resistance, increase visceral fat, and chip away at your body’s ability to easily switch between burning carbs and fats.

“These beverages cause large spikes in blood sugar levels, and that increases risk of diabetes, heart disease, and some cancers,” says Willett.

A major meta-analysis found that people in the highest category of sugary beverage intake had a 26% higher risk of developing Type 2 diabetes and a 20% higher risk of metabolic syndrome compared to those in the lowest category.

Regularly drinking even one or two sweetened beverages a day doesn’t just add extra calories — it can nudge your metabolism toward insulin resistance, increased fat storage, higher liver stress, and, as Messer notes, “a significantly increased risk of mortality.”

Processed Meats

Processed meats pack a troubling combination of saturated fat, sodium, preservatives and compounds formed during high-heat processing. Together, these can drive inflammation, impair insulin sensitivity and encourage fat buildup in the liver.

“Processed meat also contains large amounts of salt that increase blood pressure,” adds Willett.

A recent Lancet study found that eating just 50 grams of processed meat per day, roughly two slices of ham, is associated with a 15% higher risk of developing Type 2 diabetes.

Other research shows that high intake of red and processed meat is independently associated with non-alcoholic fatty liver disease and insulin resistance, even after the researchers adjusted for saturated-fat intake.

“Processed meats are also consistently associated with higher risks of multiple chronic diseases and higher all-cause mortality,” Messer adds, noting links to colorectal and other cancers, as well as stroke.

Packaged Sweets

Packaged sweets such as candy, cookies, pastries, cakes and doughnuts “are typically made with refined starches, oils and flavor additives that make them easy to overeat and leave you hungry again soon after,” Young tells TODAY.com.

Many of these treats also still contain trans fats or similar ingredients that “can contribute to elevated blood pressure, worsen cholesterol, trigger inflammation and increase insulin resistance,” Allison Stowell, a Maine-based registered dietitian for supermarket chain Hannaford, tells TODAY.com.

A 2024 Diabetes, Obesity, and Metabolism study found that diets dominated by such ultraprocessed foods lead to health issues — even when calories were matched to a less processed diet.

And because these foods spike blood sugar quickly, they also trigger bigger crashes and more intense cravings, nudging your metabolism toward fat-storage mode.

“They create a double hit that stresses blood sugar and slows metabolic efficiency,” Amy Goodson, a registered dietitian based in Dallas, tells TODAY.com.

Packaged Snack Foods

Consuming refined-grain snack foods such as chips, crackers and other savory snacks “is associated with a modest increased risk of overweight, obesity, abdominal fat and adverse cardiometabolic outcomes, especially when eaten in large amounts,” says Messer.

These snacks tend to be high in sodium, saturated fats and refined carbs, while offering very little fiber or other nutritional value. In fact, “it’s their combination of refined carbohydrates, sugars and unhealthy fats that can disrupt blood sugar control and promote abdominal fat over time,” Goodson explains.

Such foods are engineered to be “hyper-palatable,” Goodson adds, meaning they’re designed to be hard to stop eating once you begin.

Another concern, Messer notes, is that chips, crackers and similar foods now make up an especially large share of daily calorie intake in the U.S.— especially among youth.

That’s worrisome because these are the years when metabolic patterns, body-fat distribution and long-term dietary habits begin to take shape.

Ready-to-Eat or Heat Meals

Frozen entrées or microwaveable dinners may feel like lifesavers on a busy night, but many fall into the highest food processing tier and are associated with negative health outcomes.

A BMC study published last month found that diets high in such foods were associated with poorer glucose regulation and insulin resistance in young adults. And research published in Cell Metabolism also shows that eating these meals can harm metabolic outcomes, including increases in fat mass and shifts in hunger hormones, in a short time.

Microwave dinners may also lead to higher sodium intake, increased blood pressure and reduced diet quality overall due to their refined starches, sodium, saturated fat and additives. They also to displace nutrient-rich foods like vegetables, legumes and whole grains in the diet.

Fast Food

“Burgers, fries and shakes are a triple threat because they’re each high in some of these most worrisome ingredients,” Young says. Think: lots of salt, highly refined starches, and large amounts of oil and added sugars.

These ingredients are why “regularly eating fast food can impair insulin sensitivity and promote inflammation, two key drivers of metabolic dysfunction,” Young adds.

Fequent consumption of fast food has also been linked to increased risk of obesity, Type 2 diabetes, hypertension, coronary artery disease, major cardiovascular events and all-cause mortality, says Messer.

High-temperature frying also produces harmful compounds linked to increased risk of cancer, inflammation and metabolic diseases, she adds.

While we often think metabolism is predetermined or based solely on calories in versus calories out, the foods we choose can play a major role in how our bodies handle fuel, store fat and manage energy. And when ultraprocessed foods dominate the diet, your metabolism begins to pay the price.

The good news is that change is possible and often brings about near-immediate effects. Prioritizing vegetables, legumes, whole grains and lean proteins cooked at home will not only help you feel and look healthier, but it can also shift your metabolic path.

And when you do need to opt for packaged food or drinks, Stowell says, “simple swaps, like choosing low-sodium deli meats, no-salt-added or low-sodium snacks, higher-fiber foods, or no-sugar-added beverages, can make a meaningful difference.”