Most protein bars are made from milk or soy proteins, which can cause stomach aches if you have an allergy or intolerance towards these proteins. Stop eating protein bars if you experience stomach aches every time you eat them. If you've been using the same protein bar in your diet, take it with you when you go to your doctor's appointment. Your doctor may perform certain blood and stool tests to determine the cause of your stomach ache.
You may be allergic to the proteins used in the protein bar. Whey, a protein found in milk and soy proteins are the most common proteins used in protein bars. Both milk and soy are considered highly allergic foods. If you are allergic to the protein in the protein bar, you will develop symptoms within a few minutes, such as nausea, vomiting, diarrhea, stomach pain, cramping and bloating. Protein allergies will cause other symptoms to develop alongside the stomach ache, such as asthma, skin rashes, sinus congestion and facial swelling, according to MayoClinic.com.
Protein intolerance to soy or whey can also cause a stomach ache, but will not cause any other symptoms to develop aside form digestive systems. If your digestive system cannot break down the proteins used in the proteins bar, you're considered to have an intolerance, according to the American College of Gastroenterology. Food protein intolerance causes swelling, irritation and inflammation to occur in your intestines because the proteins go undigested. When the proteins enter the colon, bacteria attack them, leading to gas, bloating and stomach pain.
If the protein bar is made from whey protein, it will contain lactose. Lactose intolerance is one of the most common food intolerances that can lead to stomach aches, gas, bloating and diarrhea. Lactose is a sugar found in dairy products. Ingesting lactose if you're lactose intolerant will not cause any permanent damage, but it will cause unpleasant symptoms. Depending on how much lactose you can handle, you may or may not develop symptoms from eating a single serving of a protein bar. You can treat and prevent lactose intolerant symptoms by taking a lactase enzyme supplement.
If your stomach ache progresses and gets worse or develops other gastrointestinal symptoms that do not improve, you may have food poisoning. Food poisoning occurs when you eat a protein bar that is contaminated with an infectious organism, such as bacteria, toxin or virus.