Ultra-Processed Foods Might Sneak Fat in Your Thighs

Ultra-processed foods, or UPFs, are foods built mostly from industrial ingredients instead of whole foods. They often come with flavorings, colorings, emulsifiers, and other additives whose job is to make them taste better, look better, and last longer on the shelf.

Think packaged snacks, sugary breakfast cereals, instant noodles, fast food, and many frozen ready meals. In the U.S., these foods now make up more than half of the average person’s daily calories, which is a lot of “food” that spent most of its life in a factory meeting.

“These products are cheap, convenient, and everywhere, but they tend to be low in fiber and key nutrients and high in salt, sugar, and unhealthy fats,” says study author Dr. Zehra Akkaya.

Osteoarthritis of the knee is one of the most common causes of pain and disability in older adults. Extra body weight is a major risk factor, but the story is not just about what the scale says after breakfast.

Your thigh muscles help keep your knees steady and support almost everything you do while standing and moving. When fat builds up inside those muscles, called muscle fat infiltration, the muscle gets weaker and less efficient, like a helper who keeps taking smoke breaks.

That can make it harder to walk, climb stairs, or get out of a chair, and it may speed up joint damage over time. “Muscle quality is just as important as muscle size when it comes to staying mobile,” says Akkaya.

The new research looked at 615 adults from the large Osteoarthritis Initiative study. People were about 60 years old on average, and all were at risk for knee osteoarthritis but did not yet have clear X-ray signs of disease or regular knee or hip pain.

Everyone filled out a detailed food questionnaire about what they ate over the past year. Researchers used the NOVA system, a common research tool, to calculate what percentage of each person’s diet came from ultra-processed foods.

Participants also had MRI scans of both thighs. Radiologists graded how much fat was mixed into key muscle groups, including the front (extensors), back (flexors), and inner thigh (adductors).

On average, ultra-processed foods made up about 41% of people’s daily diets. Some people ate much more, and some ate less.

After adjusting for age, sex, total calories, smoking, physical activity, and depression, higher UPF intake was linked to more fat inside all thigh muscles. This was true for the flexors and adductors in particular, even when body mass index (BMI) was taken into account.

When the team adjusted for abdominal circumference, a marker of belly fat, the link was even stronger. That suggests central fat and diet quality may work together to harm muscle quality.

The relationship looked similar in men and women. “Regardless of sex, people who ate more ultra-processed foods had fattier thigh muscles on MRI,” says Akkaya.

You do not have to ban every packaged food from your kitchen, but lowering your UPF intake can support healthier muscles and joints over time. Aim to build most meals from minimally processed foods, and let the ultra-processed stuff stop acting like the main character.

• Replace sugary breakfast cereal with oatmeal topped with fruit and nuts.
• Trade chips and crackers for nuts, yogurt, fruit, or cut veggies and hummus.
• Choose plain yogurt and add your own berries instead of pre-sweetened cups.
• Cook extra chicken, beans, or whole grains on weekends for quick weeknight meals.
• Limit fast food to an occasional treat, not a weekly habit.

Reading ingredient lists can help. If you see a long list of additives you do not recognize, it is likely an ultra-processed product.

This study cannot prove that ultra-processed foods directly cause fat to build up in your muscles, but the link is strong and showed up even before knee arthritis and pain.

Cutting back on UPFs and eating more whole or minimally processed foods may help protect your thigh muscles, support your knees, and keep you moving comfortably as you age.

If you are not sure where to start, a registered dietitian or your health care provider can help you make a plan that fits your life and budget.