So how important to healthy nutrition is attention to the glycemic index of food?
High glycemic index meals stimulate high insulin levels. They are followed in some
individuals by an overshoot of the uptake of glucose, low blood sugar (reactive
hypoglycemia), and elevated serum triglycerides. The result may include hunger
from the hypoglycemia, excessive food intake, insulin resistance, the pancreatic
beta cell dysfunction and damage that leads to diabetes, and, over an extended time,
an unhealthy pattern of blood lipids and an increase in the risk for obesity, type 2
diabetes, and heart disease.
The glycemic index (GI) is a measure of the blood glucose-raising potential of the
carbohydrate content of a standard amount of a food compared to a reference food
(generally pure glucose). The glycemic index indicates how rapidly a carbohydrate
is digested and released as glucose (sugar) into the bloodstream. Consumption of
high-GI foods causes a sharp increase in blood glucose concentration (and insulin
release) after a meal that declines rapidly. In contrast, the consumption of low-GI
foods results in a lower blood glucose concentration that decreases more gradually.
Carbohydrate-containing foods can be classified as high- (≥70), moderate- (56-69),
or low-GI (≤55) relative to pure glucose (GI=100). Sugars and most highly refined
starchy foods (e.g., white bread, white potatoes) have a high glycemic index whereas
non-starchy vegetables and legumes tend to have a low glycemic index.
The glycemic load (GL) index takes into account the total amount of the carbohydrate
actually consumed. It multiplies the glycemic index of the food eaten by the
carbohydrate content of the actual serving. The GL of food estimates how much
the food will raise a person’s blood glucose level after eating it. One unit of GL
approximates the effect of consuming one gram of glucose. So, GL may be a better
overall indicator of how a carbohydrate food will affect blood sugar. GL’s of 10 or
below are considered low, and 20 or above are considered high.
Low-glycemic load diets have been shown to provide benefits, including weight
loss, improved insulin metabolism, and better diabetes control. Some meta-analyses
have concluded that meals with high glycemic load and index are associated with
increased risk of cardiovascular disease, especially for women. A meta-analysis
of 28 trials has found that lowering the glycemic index of diets consistently reduced
LDL-C but did not affect HDL-C or triglycerides.
Low-glycemic diets exclude intake of foods with a high glycemic index and/or glycemic
load. These foods include processed foods containing refined starches and/or added sugars, and usually all fruits and certain high-glycemic index vegetables. However, as Katz and Meller have noted, “Evidence that health benefits ensue from
jettisoning fruits, or relatively high-glycemic-index vegetables, from the diet does
not exist.” Ludwig has noted, “…whereas the concept of glycemic index may be
complex from a food science perspective, its public health application can be simple:
increase consumption of fruits, vegetables, and legumes, choose grain products
processed according to traditional rather than modern methods (e.g. stone-ground
breads, old-fashioned oatmeal), and limit intake of potatoes and concentrated sugar.
Indeed, these recommendations would tend to promote diets high in fiber, micronutrients,
and antioxidants and low in energy density.”
In their assessment of the seven dietary patterns that they have identified, Katz
and Meller reinforce this idea. They suggest that low-glycemic eating “…tends to
occur as a by-product of favoring minimally processed, direct-from-nature foods
and avoiding refined starch and added sugars. This basic approach to achieving
reduced glycemic-load is compatible with all or nearly all of the other dietary approaches…”
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