We, the general public, have been told for decades that oatmeal for breakfast is healthful, it would lower your cholesterol, which it does indeed, but that is not the lifesaving change you might have expected as it is outweighed by the following:
Oatmeal, just like any other grain, will spike your blood sugar and make you hungrier and triggers a rollercoaster phenomenon with the rising blood sugar leading to rising insulin levels. Insulin spikes, however, are bad for at least 2 reasons, the first being that it will cause insulin resistance which gets you started on the track to prediabetes and later diabetes, the second being that it acts like miracle-gro for your fat cells, in particular around your waistline.
People with celiac disease or NCGS (None celiac gluten sensitivity) also must consider that most oatmeal sold has been processed with the same shared equipment that was used for wheat etc.; and even oats labeled as gluten-free can “cross react”, which means your body will confuse it with gluten and react the same way as if you ate wheat.
Bottom-line: Don’t eat oatmeal for health reasons, if you must have it, the best time isn’t in the morning, but rather after an intense workout when you’re refilling your muscle glycogen stores and it would be less inclined to end up in growing fat cells.