All food contains energy – that’s why we eat them, after all. This energy makes its way into our blood in the form of glucose, a simple sugar that our organs use to power their metabolism and support our own. When we look at different foods, one way to compare how our body will handle their energy is by calculating their glycemic index (GI).
Foods with lower GI values are likely going to be helpful in the maintenance of a balanced lifestyle. The reason for this is simple – rapid changes in blood glucose levels are harmful, so minimizing them is an essential component of a holistic health optimization plan. If you replace foods high in starch and have relatively high GI values with foods digested more slowly or have less starch to digest, the result will be a more even level of blood glucose over the day.
Typically, foods will change over time – hence why they have sell-by and eat-by dates. For fruits and vegetables, they are usually born with a lot of starch, but this starch will break down into sugars over time. In fact, as some of this sugar ferments, it forms alcohol. There are some hypotheses that we like alcohol because it’s associated with a more readily available source of sugar in nature. For our purposes, though, it’s just important to remember that an apple eaten at an orchard will be very different in terms of sugar than an apple left to ripen on a kitchen counter fully. If you’re looking at oven fries, most probably, the GI will be as advertised. But fruits and vegetables will do some digestion for us the longer they are left to ripen, and you can find a chat of different options and their GIs here.
Although a food’s GI conveys valuable information for planning your diet, we need to know more to understand the physiologic effects of nutrition on our blood glucose levels. At first, the GI is calculated as an average value over many individual people. For any individual, though, their unique responses to a given food are what matters for their health, not how the food generally affects other people on average. The GI is, of course, sound and valuable as a guide, but it would be even better to know how a food will affect a specific person.
There is also a wide range of nutritional information that can make food good or bad for us, or more specifically, for a person. Generally, foods that provoke smaller responses in blood glucose levels are also better for weight control, but not always. For example, vegetable oils can have a very low glycemic index but will also be very high in calories. Trade-offs like this are abundant in nutrition, and no one figure, however useful on its own, will be the definitive factor for every person.
The principal method for this calculation is by measuring the blood glucose levels in someone’s body for two hours after they eat a specific food. The total rise in blood glucose levels over those two hours is then compared to the effect of pure glucose on that same person’s blood glucose levels over two hours. The resulting ratio is the GI for that food, 100 for pure glucose, as the two levels would be the same. For something like a vegetable low in starch, it could be much lower. The sample size is at least ten people and the amount of food tested is typically fifty grams. Remember, the lower the GI of a specific food, the slower or lower the rise in blood glucose levels, so foods with a high GI will have a harsher effect on blood glucose levels.
A food is considered to have a low glycemic index if it has a GI of below 55, which would signify that it will release around half as much glucose into the blood over the first two hours. Likewise, a food is considered to have a high glycemic index if the GI is over 70, while values between 55 and 70 are considered moderate. For example, if you follow a keto diet, then using the GI can be a valuable point of departure for choosing foods that will work well for you. As we said above, the GI of a specific food is altered by many factors, including how it’s prepared, processed, and eaten in combination with other foods. Since the GI is calculated from a 50-gram sample, how much food will be eaten at a single meal is also relevant.
To try and address this last problem – that most people will not eat precisely 50 grams of a specific food at once – we can use the glycemic load (GL) to paint a more realistic picture of a food’s effect on blood glucose levels. To get this value, we start with the GI, which is accurate while remaining agnostic to the amount of food eaten. We then multiply the GI by the number of carbohydrates in the serving size and divide the result by 100, accounting for how starchy a particular food type is compared to others.
If a food has a low GI but a high proportion of carbohydrates, the GL will “correct” the value upwards to account for the high overall load of sugar in the food. Most foods will have similar GI and GL, at least relative to other foods, as starchy foods tend to be digested quickly as well. Some people prefer the GL for this reason, as it has the extra step to try and make sure nothing is missed. After all, you can always choose to be informed by both values. GLs of over 20 are considered high, those 10 or under are considered low, and those from 11 to 19 are considered moderate.
Since it’s exceedingly rare to find a person who eats 50 grams of pure glucose, we’ll take the common step of setting white bread as the baseline value. So, for this chart, when you see a GI of 25, for example, it means there will be a quarter as much blood glucose level rise over the first two hours of digestion compared to white bread. Along with the other limitations of the GI mentioned above, this is one reason we consider it a good starting point but by no means the end of the journey towards discovering which foods work best for you.
Some people use a continuous glucose monitor (CGM) to provide a more precise readout of their blood glucose levels. This device measures the interstitial fluid in your arm to estimate the glucose level in the blood over a set period, usually ten days or two weeks. You can even see how a specific food works for you relative to the sample population from which the GI is derived by comparing your glucose results of the first two hours after eating the food to the reported GI. You can also alter how you prepare food, how quickly you eat it, the portion size, and similar techniques to calibrate your blood glucose levels within a healthy range.
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