The glycemic index is a ranking of foods based on how they affect blood glucose. It is a standardized tool (based on the average effect of food in a large study population) to understand / predict how food affects blood glucose.
However, there are some limitations because this index does not take into account:
Interpersonal variability: Different people who eat the exact same meal can have highly variable glucose responses, due in part to their body's unique and complex biological environment.
Intrapersonal variability: Different meals with exactly the same amount of carbohydrates can generate highly variable glucose levels in one person. Carbohydrate content alone is often a poor predictor of personal glycemic response.
Lifestyle: Glucose levels can change based on factors such as exercise, stress and amount of sleep. Eg. Chronic sleep deprivation can lead to higher blood glucose levels and slower transport of glucose to the cells.
Variability in eating times: The time of day when we eat can have a major impact on blood glucose levels. Eg. A high glycemic and high-calorie meal can cause a greater glucose and insulin response in the evening than when the same meal is eaten in the afternoon.
Food combinations: The composition of a meal and the order of the foods consumed can cause glucose to change differently than if those foods were eaten separately. Eg. adding (healthy) fats or fiber to a carbohydrate-rich meal can cause the carbohydrates to be broken down more slowly and show a gradual rise in glucose that gives less insulin rise.
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