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The Glucose Never Lies — Foundations
7 Fundamentals of The Glucose Never Lies
Seven physiological principles that explain why glucose behaves the way it does in type 1 diabetes — and why understanding these mechanisms makes everything else clearer.
About this page
This page is based on the podcast episode that accompanies the AID series. During that series, co-host Louise and John often moved into technical territory. This episode was created to give everyone a foundation in the concepts that come up repeatedly across the GNL content.
The four topics covered here are:
- What type 1 diabetes is — explained through glucose and insulin
- Why injected insulin behaves differently from natural insulin
- How activity supercharges insulin action
- Why high-fat meals cause prolonged high glucose
Understanding these four mechanisms makes the strategies across the rest of the site considerably easier to understand and apply.
Physiology Fundamentals
Fundamental 1: What is type 1 diabetes?
A useful way to understand what happens in type 1 diabetes is to follow what happens when you eat toast — or any carbohydrate-containing food.
- Toast is chewed, swallowed, and travels to the stomach.
- From the stomach, it moves into the small intestine, where glucose is absorbed into a blood vessel called the portal vein.
- The portal vein connects the intestines to the liver, where glucose is processed.
In people without type 1 diabetes, the pancreas sits above the portal vein, continuously sensing the rise in glucose. When glucose rises, insulin is released directly into the portal vein. This insulin ensures that most of the glucose from the meal is either stored in the liver or transported into cells for energy.
Think of insulin as a transporter — it opens the cell door, allowing glucose to move from the blood into the cell for energy production. Some glucose remains in the bloodstream to be transported to muscle and fat cells, where insulin facilitates entry.
In type 1 diabetes, the beta-cells that produce insulin are destroyed. Insulin must be delivered from outside — by injection or pump infusion into subcutaneous tissue. This changes everything about how insulin reaches the liver and muscles.
Fundamental 2: Why injected insulin behaves differently
A question that comes up constantly: “I counted my carbohydrates correctly, gave the right amount of insulin 15–20 minutes before eating, and my glucose still spiked. Why?”
The key reason is the route of delivery.
When insulin is released naturally, it goes directly into the portal vein. Much of the glucose from the meal is immediately stored in the liver before it can raise blood glucose. When insulin is injected subcutaneously, it enters the outer (peripheral) blood circulation rather than the portal vein. As a result, a much larger proportion of glucose from the meal bypasses the liver and floods the bloodstream — causing the post-meal rises that are so characteristic of type 1 diabetes.
This effect tends to be most noticeable with high-carbohydrate meals in the morning, when insulin resistance is typically higher. This is the structural reason why perfect post-meal glucose control is harder in type 1 diabetes than the mathematics of carbohydrate counting might suggest.
For more detail on the portal vein mechanism and bolus insulin timing, see the bolus insulin guide.
Fundamental 3: How activity supercharges insulin
Physical activity enhances insulin effectiveness through four mechanisms:
- Faster absorption. Activity increases blood flow to the skin, accelerating insulin uptake from the injection or infusion site into the bloodstream.
- Enhanced delivery. Exercise boosts blood flow to muscles, meaning insulin reaches muscle cells more efficiently once it is in the circulation.
- Reduced breakdown. Normally, insulin is partially cleared by the kidneys. During activity, blood is redirected to working muscles, reducing insulin clearance and effectively making the same amount of insulin last longer.
- Non-insulin-mediated glucose uptake. Exercise drives glucose into working muscle cells through a mechanism that does not require insulin at all — particularly relevant during moderate-intensity aerobic activity.
A useful rule of thumb on average: 15 minutes of moderate movement tends to lower glucose by roughly 2 mmol/L (40 mg/dL). Individual responses vary considerably depending on exercise type, intensity, duration, current insulin on board, and residual beta-cell function.
For a deeper exploration of how short bursts of activity can be used strategically, see the activity snacking guide.
Fundamental 4: Why high-fat meals cause prolonged high glucose
High-fat meals slow digestion, which initially delays glucose absorption. This delay can cause glucose to drop if insulin is given too early relative to when the glucose actually arrives.
Later in the digestion process, fat metabolites called diacylglycerols accumulate in liver and muscle cells. These molecules interfere with insulin signalling, preventing glucose from entering cells efficiently. The result is insulin resistance that develops over the hours following a high-fat meal.
A useful way to visualise diacylglycerols: imagine fat molecules as tadpoles. When digested, they break down into diacylglycerols — tadpoles with two tails instead of three. These two-tailed molecules disrupt the insulin signalling pathway at a cellular level, making insulin less effective even though it is present in the bloodstream.
Higher insulin doses or physical activity can help clear these fat metabolites more quickly. CGM data after high-fat meals tends to show a characteristic delayed glucose rise — sometimes appearing four to six hours after eating — that differs from the sharper post-meal rise after a low-fat meal.
For more detail on managing high-fat meals, see the high-fat meals guide.
Key takeaways
- Insulin acts as a transporter, opening cell doors to allow glucose in. In type 1 diabetes, this transporter must be delivered from outside the body.
- Subcutaneous insulin enters peripheral circulation rather than the portal vein, meaning less glucose is captured by the liver after meals — explaining why post-meal spikes happen even with good carbohydrate counting.
- Activity supercharges insulin through faster absorption, better delivery, reduced clearance, and non-insulin-mediated uptake. On average, 15 minutes of moderate movement lowers glucose by around 2 mmol/L (40 mg/dL).
- High-fat meals initially delay glucose rises but later cause insulin resistance via diacylglycerols, leading to prolonged high glucose hours after eating.
- These four mechanisms underpin most of the strategies explored across this site. Understanding them makes it easier to interpret what CGM data is showing and to have more informed conversations with a diabetes care team.
This content is for educational exploration only. It describes average responses and general principles. It is not medical advice and cannot replace individual clinical guidance from your diabetes care team.
