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Foods can be characterized with a Glycemic Index (GI), which is a scale that represents the rise in blood glucose 2 hours after the ingestion of a food. The Glycemic Index scale is from 0 to 100, where 100 is defined to be the effect of ingesting pure glucose. Any food that does not cause a rise in blood glucose will have a Glycemic Index of zero. A food's Glycemic Response is the food's effect on blood glucose levels over time, starting from the moment the food is consumed.

Sarah Hallberg (Medical Director of the Medically Supervised Weight Loss Program at IU Health Arnett) has the following Rules for Eating, which are consistent with the importance of glycemic response in Blue Zones diets:

  1. If it says "light", "low-fat" or "fat-free", it must stay in the grocery store.
  2. EAT FOOD.
  3. Don't eat anything you don't like.
  4. Eat when you are hungry. Don't eat when you are not.
  5. NO GPS - No grains, potatoes, or sugar.

Ketogenic Diets are by definition low-carb and many people report excellent results with weight loss and improved blood sugar control with it. The only part of the body that absolutely requires glucose are erythrocytes (red blood cells) due to their lack of mitochondria. While weight loss and reduced insulin resistance are great benefits of a ketogenic diet, there can also some minor side effects such as "keto flu" (headaches, muscle cramps, brain fog, etc). Because insulin causes the body to retain water, lower insulin levels result in greater fluid excretion along with some electrolytes (ie, sodium). The loss of electrolytes causes keto flu symptoms and is remedied the consumption of salt. Ketogenic diets reportedly tend to absorb excess body proteins such as excess skin from weight loss and skin tags. The Adkins Diet is one type of low-carb diet and was popular at one of my workplaces several years ago. Although most people who tried it loved the resulting weight loss, no-one was able to stay on it for periods much longer than 6 months. I think the boss really liked it because it gave him the excuse to eat fried chicken all the time.

Dr Paul Mason states that a ketone meter is an excellent tool for finding the optimum level of carbohydrate restriction. His clinic regularly tests for ketone, which is the result of fat metabolism. He reports that higher ketone levels (0.4 mmol/L & higher) act as an appetite suppressant, which is obviously helpful in aiding weight loss by reducing your caloric intake. If you're a diabetic, you're probably already monitoring your blood glucose. Using your glucometer's ketone measuring capability, it is extremely useful to see the effect of various foods on your fat metabolism by keeping a food log correlated with blood ketone. If you don't have a glucometer, you can also measure ketones in your urine although less accurately. You can often get a free glucometer capable of reading ketones (like a FreeStyle Precision Neo and Libre) from your doctor or pharmacy with a coupon from Abbott Laboratories.

Glucose-Ketone Index (GKI) is a means of estimating your level of ketosis and is a unitless number based on mmol/L.
GKI = (serum glucose, mmol/L) / (serum ketone, mmol/L)

If you're using mg/dL for blood sugar, divide your reading by 18.016 to convert to mmol/L. Having any ketone measurement result above zero suggests to me that you have a low IGR, which will also tends to result in lower serum glucose levels. It's not that hard to reach 0.5 mmol/L of ketones without fasting so you would only need a blood sugar measurement of 4.5 mmol/L be in a low ketosis level, which can be maintained through gluconeogenesis on a low-carb diet. Normally, gluconeogenesis will maintain glucose at a level of 70-80 mg/dL (3.9-4.4 mmol/L) in a long-term fasted state using mainly lactate (eg, from muscles). A normal fasting blood glucose level falls within the range of 3.9 to 5.6 mmol/L although the normal lower limit is 3.3 to 3.9 mmol/L.

Symptoms of hypoglycemia don't usually appear until glucose levels have dropped to the range of 2.8 to 3.0 mmol/L (50 to 54 mg/dL). If you are experiencing any hypoglycemic symptoms (eg, nausea, lethargy, impaired mental functioning, irritability, headache, shaking, twitching, weakness in arm and leg muscles, pale complexion, sweating), eat some sugar (and stop fasting). If your blood sugar continues to drop, you risk confusion and unsteadiness and eventually the loss of consciousness when glucose levels fall below 2.2 mmol/L (40 mg/dL). Be aware that persistent exposure to hypoglycemia can potentially result in hypoglycemic unawareness and you may not notice any hypoglycemic symptoms but this appears to be rare for Type 2 Diabetics NOT being treated with insulin (ie, only with diet, exercise, or insulin-sensitizing medicine).

Nutritional Ketosis will typically result in ketone levels in the 0.5-5 mmol/L (with fasting glucose levels) compared to 15-25 mmol/L ketone levels (blood sugar typically over 13.8 mmol/L / 250 mg/dL) and for Diabetic Ketoacidosis.

The following table summarizes various ketosis states:

GKI Ketosis Level Description
>9 Not in Ketosis Not in Fat-Burning Mode
6 - 9 Low Weight loss
3 - 6 Moderate Metabolic Syndrome Treatment
<3 High Epilepsy & Cancer Treatment

More information: