Smoking tobacco acutely and chronically reduces insulin sensitivity (1, 2, 3), possibly via:
- Increased inflammation
- Increased circulating free fatty acids (4)
Glucocorticoids and stress
Glucocorticoids are "stress hormones" secreted by the adrenal gland, under the direction of the brain, that influence glucose metabolism among other things (hence the name). The major glucocorticoid in humans is cortisol, while in rodents it is corticosterone. A disorder called Cushing syndrome is caused by the oversecretion of cortisol in humans. Cushing patients exhibit insulin resistance and an elevated risk of diabetes (6).
Cortisol is part of the body's way of dealing constructively with stressful situations, so it is increased by both physical and mental stressors. As with inflammation, it's helpful acutely, but may be damaging if overused. It may be one link between excessive stress and insulin resistance.
This is not a can of worms I've opened on WHS yet, but I think cooking temperature is a significant factor in health. Gentle cooking methods preserve nutrients, form fewer potentially damaging compounds such as advanced glycation end-products, lipid peroxides and heterocyclic amines, and do not increase the energy density of foods. In diabetics, a diet composed of gently cooked foods increases estimated insulin sensitivity, improves glucose control, and substantially decreases markers of inflammation, compared to one composed of similar foods cooked by higher-heat methods (7, 8).
One study showed that a gently cooked diet increases estimated insulin sensitivity in non-diabetics, relative to a high heat cooked diet, when eaten for one month (9). Unfortunately, the result was confounded by a lower energy intake in the gently cooked food group. Maillard reaction products (browning) are formed during high-heat cooking, and contribute substantially to the palatability of food. Think grilled steak vs. steamed steak. In addition, high heat (as opposed to wet heat) cooking increases the energy density of food by evaporating water. Both of these factors promote increased food intake, which is what the study observed. We can't know whether the difference in estimated insulin sensitivity was due to differences in energy intake, a direct effect of the high-heat cooked food, or both. They didn't report changes in body weight. Regardless, my opinion is that gentle cooking methods such as steaming, boiling and poaching are generally superior to frying, roasting and grilling for health. Deep-fried food is poison.
Age is a dominant risk factor for many health conditions, including obesity, cardiovascular disease and the most common metabolic disorders. Insulin sensitivity generally declines with age, although not greatly (10). However, most people in affluent countries become more sedentary and gain fat with age, so the change may not have anything to do with age per se. Indeed, when investigators adjust for body composition, the effect of age on insulin sensitivity disappears (11). Fitness, not age itself, seems to be the important factor.
As with essentially all biological traits, insulin sensitivity is linked to your genetic inheritance (12), and the genetic contribution is likely substantial.
Low birth weight
People who were born small and/or thin are at a higher risk of insulin resistance and diabetes later in life, and this is true even if researchers factor out differences in adult BMI (13, 14, 15). This suggests that the prenatal environment influences metabolic health throughout life.
OK, I'm done
I'm sure there are plenty of other factors that influence insulin sensitivity, but in this series I've covered the important ones I'm aware of. In the next and final post in the series, I'll review the diet and lifestyle implications of all this.