Glycemic index refers to the rate at which foods increase blood sugar levels. Foods with a high glycemic index increase blood sugar levels quickly, and those with a low glycemic index increase blood sugar levels slowly (making it easier for your body to regulate blood sugar level). One way to check the glycemic index of your food sources is to use databases like this, and try to avoid foods in the high range.
If you don't want to look up all your foods, here are some rules of thumb:
- Foods low in carbohydrates (like meat) tend to have a lower glycemic index (since blood sugar comes from carbs, this makes sense). That said, it's hard to get adequate vitamins and minerals without consuming some carbs, and high carb doesn't necessarily mean high glycemic index.
- Carbs in foods can come from sugar, starch, and fiber (on the nutrition label, only sugar and fiber are listed, but [Total carbs] - [sugar] - [fiber] = [starch]). Sugar has the highest glycemic index since it doesn't take much to break it down; the body can absorb it almost immediately, spiking blood sugar. Starch takes longer to break down, so it enters the bloodstream more slowly. Fiber goes mostly undigested, so it doesn't contribute much to blood sugar, and has a very low glycemic index. It also makes food harder to digest, prolonging the digestion process. So, in general, high-fiber, slow-sugar foods have a lower glycemic index. This is one reason whole grain bread is so much healthier than white bread.
- If you have a sugar craving, fructose ("fruit sugar") has a much lower glycemic index than glucose. So to satisfy your sweet tooth eat fruit or veggies (these foods are also high in fiber). Be careful though, consuming large amount of fructose (such as soda sweetened with high-fructose corn syrup) will still cause weight gain.
- Eating carbs along with fat and protein lowers the glycemic index of a food. It's kinda counterintuitive, but chocolate has a lower glycemic index than, say, jelly beans (pretty much pure sugar), because it contains protein and fat. Protein and fat also prolong digestion, just like fiber. Don't overdo it on the fat though. Fat yields over twice as many Calories/gram as carbs or protein, so eating too much fat can rack up Calories quickly. Certain types of fats can increase your health risks, so I would recommend trying to get unsaturated fat rather than trans or saturated fat (however, there's been some debate around here about that lately).
- There are a few foods that slow the emptying of your stomach into your small intestine. Since most food is absorbed into your body in the small intestine, this slows down the entry of carbs into the bloodstream, lowering glycemic index. Not only that, having food in your stomach for longer also increases feelings of fullness. Protein, fat, and fiber all have this effect (See my answer here for mechanisms). There is also evidence that other substances, such as vinegar and cinnamon do the same (Addressed in another question here). Incorporating these into your cooking could help lower glycemic index.
As you can see, the way you combine foods has a huge effect on glycemic index, so instead of just looking for low glycemic index foods, you want to try to create low glycemic index meals.