NEW YORK (CBSNewYork) – You’ve probably heard the old adage “you are what you eat.”

As CBS2’s Alex Denis reports, one New Jersey doctors says that’s not entirely true.

If you feel tired, sluggish and can’t lose weight, it may be the healthy foods in your diet that are causing the problems.

A simple blood test can show which foods, even the healthy ones, are wreaking havoc on your day.

Tim O’Donnell is a law enforcement officer who found himself struggling with his normal routine.

“Kind of tired and sluggish, and I felt that my work was beginning to suffer,” he said.

He’s not alone. Dr. Ali Maz, clinical director of Medwell Functional Medicine in Bergen County, says he hears the same complaints far too often.

Surprisingly, what’s causing the symptoms may be the very thing that also fuels the body – food.

“Many people they are under the impression when they eat, everything gets absorbed, which is a fallacy,” said Dr. Maz. “If you eat something that your body cannot absorb, guess what happens? It goes to your blood stream, and your body can not digest it.”

That can trigger inflammation in the body, causing fatigue, weight gain, headaches and digestion issues.

So how many people have a food sensitivity and don’t even realize it?

“Actually 83 percent of health conditions are food related,” Dr. Maz said.

A simple test can tell you everything you need to know. A prick of the finger and a scan of the blood breaks down each individual’s response to 96 common foods.

The test reveals an individual’s delayed immune reaction to certain foods, versus an allergy test, which is the immediate reaction. Food sensitivity symptoms can take three days to show, which makes it difficult to diagnose without a test.

“They have chicken on Monday, on Wednesday they have (a) migraine, they have low energy, they are fatigued and cannot relate their symptoms on Wednesday to what they had on Monday,” said Dr. Maz.

Some of the common culprits are milk, eggs, butter, tomatoes, bananas, strawberries, chicken, pork, oat, barley and rye.

“A lot of people consume those on a daily basis, which could be the reason they are chronically ill,” Dr. Maz said.

That was the case for Health Rizzo, a busy mother of four who spent years trying to manage her symptoms.

“Everything inside of me was screaming like something’s wrong,” she said.

The test indicated that she was sensitive to wheat and dairy, so she immediately cut out the foods from her diet. She told Denis she began feeling better “within two days.”

The same goes for O’Donnell, who now has more energy and an added bonus.

“I lost about 25 pounds. And not to boast, I really didn’t try,” he said.

Most insurances will cover the food sensitivity test.