Oatmeal, particularly the slow-cooked kind, is generally healthier than Cheerios.  Both are made from whole oats, but the difference comes down to processing. Unprocessed whole oats, like those in steel-cut oatmeal, take a while for the body to digest.  With Cheerios and other processed cereals, “you basically have rapidly digested sugar mixed with bran and germ,” said Dr. Dariush Mozaffarian, dean of the Friedman School of Nutrition Science and Policy at Tufts University. “It provides fiber and minerals, but also digests in the mouth almost immediately.”  That gives you a quick spike in blood sugar, but no energy for later.  One 2013 study, for instance, found that people who ate oatmeal felt fuller and had better appetite control than those who ate the same number of calories of processed cereal.  Both oatmeal and Cheerios are whole grains, which puts them ahead of cereals like Corn Flakes and Special K, in which the bran and germ have been removed, Dr. Mozaffarian said. Whole grains have more fiber and a wider range of vitamins and minerals.  As a practical rule-of-thumb, Dr. Mozaffarian suggests using the total carbohydrate-to-fiber ratio to find more healthful breakfast foods – aiming for a ratio of less than 10 to 1, which is comparable to the ratio in whole wheat flour.  A serving of Corn Flakes, for instance, includes about 24 grams of carbohydrate and 1 gram of fiber, a less-than-ideal ratio. Cheerios achieves the desired ratio of about 10 grams of carbohydrate for every gram of fiber. Instant oatmeals that contain lots of added sugar may be worse than Cheerios using this standard.  For his own breakfast, Dr. Mozaffarian eats Kashi Good Friends cereal along with fruit and full-fat milk. Kashi has more sugar than Cheerios or oats, providing about 42 grams of carbohydrate per serving. But it also has 12 grams of fiber, giving it a better carb-to-fiber ratio than many other cereals, Dr. Mozaffarian said.  The fruit adds even more fiber, and the full-fat milk digests more slowly than low-fat milk.  “If you eat a breakfast of refined cereal and skim milk,” Dr. Mozaffarian said, “your blood sugar is going to crash a few hours later, and you will be hungrier and eat more for lunch.”
How the grain in your morning cereal is processed makes a difference in your body’s digestion process and can affect energy levels later in the day.