If you want to turn this into a health/SEO blog headline, the claim should be completed carefully because onions do not generally cause a serious health problem in healthy people.
Better headline options
Doctors Reveal That Eating Onions May Cause These 7 Unexpected Effects
or
Doctors Reveal What Eating Onions Can Do to Your Body
or, for a more curiosity-driven title:
Doctors Reveal Why Eating Onions May Cause Bloating, Heartburn, and Other Digestive Symptoms
Blog-style introduction
Onions are a nutritious ingredient found in kitchens around the world. They provide antioxidants and beneficial plant compounds, but they can also cause digestive discomfort in some people. Eating raw or large amounts of onion may lead to bloating, gas, abdominal discomfort, or heartburn, particularly in people who are sensitive to FODMAPs or already experience digestive problems.
For most people, onions can be safely included in a balanced diet. The effects can vary depending on the amount consumed, whether the onion is raw or cooked, and an individual’s digestive sensitivity.
Important: Onion does not generally need to be avoided unless it causes you symptoms or a healthcare professional has advised you to limit it.