If you are already pre-diabetic, a high-glycemic index (GI) food will have an even greater effect on your blood sugar levels than on those of someone whose insulin works normally. When your insulin does not work properly, your glucose will spike even higher in response to eating a high-GI food. Moreover, when you eat such foods can also have an effect. For example, your glucose levels will rise more following lunch if you ate a high-GI breakfast rather than a low-GI one–despite a greater release of insulin. Thus, the glycemic effect of eating a higher-GI carbohydrate first thing in the morning potentially lasts later into the day as well.
In overweight adults, insulin resistance can also be decreased by the consumption of a low-GI, whole-grain diet (rather than a refined-grain, “white” diet), regardless of your level of excess body fat. Moreover, type 2 diabetic men following a low-GI diet (consuming mostly foods with a GI value of under 40) are also able to improve their BG control, enhance their insulin action, and lower their levels of blood fats after only four weeks on such a meal plan. Such positive results overwhelmingly give credence to the argument that the GI is an appropriate guide to eating more nutritious foods whether you have diabetes, pre-diabetes, or insulin resistance.
There are other factors to consider as well. Although a higher fat content lowers the immediate effect of most foods on your blood sugars (for example, the GI value of a baked potato is higher than that of potato chips), many types of fat are actually detrimental to your metabolism and can cause insulin resistance on their own. In particular, saturated fats (solid at room temperature), such as those found in abundance in red meat, butter, dairy fat, and tropical oils (palm and coconut), decrease insulin action and result in higher BG levels.
Trans fats, which are created during manufacturing and are commonly found in margarines, crackers, baked goods, and other processed foods, have a similarly negative effect on insulin action; luckily for consumers, manufacturers are now required by law to list trans fat content on food labels and, accordingly, many foods are now being made with fewer of these heart-unhealthy fats.
Combination foods with significant carbohydrate and fat content, such as pizza, have a lower GI value because the fat slows down the absorption of the carbohydrates, so that it takes longer than two hours for the pizza to be metabolized and fully exert its effects on your blood sugars. While eating this fat load may initially slow down your carbohydrate absorption, its later glycemic effect should not be discounted. The saturated fat in pizza, for example, will make you more insulin resistant four to five hours later, when it finally hits your circulation and reduces your uptake of glucose. The heightened state of insulin resistance that results raises your blood glucose level and worsens your glycemic response to high-GI foods the next time you eat. Thus, despite its medium-GI value, pizza–with its high content of refined carbohydrates and saturated dairy fat–is best eaten only in very small quantities (no more than one to two slices), and you should eat plenty of salad or another low-GI, low-fat, high-fiber food along with it. Thankfully, healthy fats such as those found in fish, nuts, and olive oil don’t appear to similarly increase levels of insulin resistance.
Next week, I’ll discuss more about healthy eating practices.