The fructan problem
Garlic carries fructans, chains of fructose that the small intestine cannot digest. Humans make no enzyme that cuts the fructose-to-fructose bond. The fructans pass through undigested and reach the colon. Gut bacteria ferment them, which produces gas, and the fructans also pull water into the gut. Both things stretch the gut wall at once. In people with irritable bowel syndrome, that stretch registers as pain. About 1 in 20 people worldwide meet the strictest definition of IBS.
Researchers at Monash University in Australia built the low-FODMAP diet and run the lab that tests foods for it. Their food-composition work puts garlic among the most fructan-dense foods ever measured. Gram for gram it outranks wheat by roughly ten times. A single clove can carry more fermentable fructan than a slice of bread.
The 4-to-24-hour delay
Fructans do not act where you would look for them. They have to transit the small intestine first, then ferment in the colon. Symptoms usually land 4 to 24 hours after the meal, sometimes later. A Tuesday-afternoon stomachache is often Monday's dinner, not the lunch an hour earlier.
So people interrogate the wrong meal. They clear the real trigger and convict an innocent one. Do this for a few years and you end up with a long list of foods you think you cannot eat, most of them bystanders, while the garlic walks.
Restaurants are saturated with it
Garlic and onion are cheap flavor, so restaurant food is full of them. They sit in the stock, the marinade, the dressing, and the spice blend. On labels they hide as "natural flavors," "spices," or "seasoning." Garlic powder and onion powder are in most commercial spice mixes, bouillon cubes, and bottled salad dressings.
Ask a kitchen for a dish with no garlic and no onion anywhere, including the stock the rice was cooked in, and you will rarely get one. This is why people who are sensitive to fructans report the same pattern. They are fine all week cooking at home, then feel wrecked after eating out, with no single dish that looks guilty.
The garlic-infused-oil exception
Here is the loophole, and the chemistry is on your side. Fructans are water-soluble. They are not fat-soluble. The aromatic compounds that make garlic taste like garlic dissolve into oil.
Heat whole garlic cloves in oil, take the cloves out, and the oil keeps the flavor with almost none of the fructans. Monash has tested commercial garlic-infused oils and rates them low-FODMAP. The clove itself stays high in fructans, so do not eat the garlic that sat in the oil. Strain it out. A commercially infused oil is also safer than a home batch left on the shelf, which can grow botulism bacteria if stored. Sauté in the oil or finish a pasta with it. Garlic goes back to being flavor instead of a forecast.