For many, a low-carb diet is a powerful tool for controlling blood sugar levels and improving metabolic health. The fundamental principle is that by reducing dietary carbohydrates, you reduce the primary fuel source that raises blood glucose. However, the body is a complex system, and a simple reduction in carbs doesn't always lead to perfectly stable readings. For some, especially in the morning or after specific meals, blood sugar can remain stubbornly high.
The Role of Gluconeogenesis
When you restrict your carbohydrate intake, your body must find another way to produce glucose for the few organs that still require it, most notably the brain. This metabolic process is called gluconeogenesis, which literally means "new glucose formation".
- Your liver uses non-carbohydrate sources like amino acids (from protein) and glycerol (from fat) to create glucose.
- In a truly well-adapted state, this process provides just enough glucose to meet your needs, with the rest of your body happily running on ketones produced from fat.
- However, if you're eating a high-protein diet, your liver's glucose production can be stimulated, potentially leading to higher blood sugar than you expect.
The Dawn Phenomenon
The dawn phenomenon is a natural, hormonal event that causes a rise in blood sugar in the early hours of the morning, typically between 3 and 8 a.m.. It affects both people with and without diabetes, but those with impaired insulin responses will notice it more significantly.
- During the night, your body releases hormones such as cortisol, adrenaline, and growth hormone to prepare you for waking up.
- These hormones prompt your liver to release stored glucose (via glycogenolysis) and produce new glucose (via gluconeogenesis).
- In people with normal insulin sensitivity, a release of insulin keeps this morning blood sugar spike in check. But for those on a low-carb diet who may be metabolically compromised, this doesn't happen as efficiently, resulting in a higher morning reading.
Adaptive Glucose Sparing (Physiologic Insulin Resistance)
This is a normal and beneficial metabolic adaptation for those on long-term very low-carb or ketogenic diets. As your body becomes adept at using fat for fuel, your muscles become resistant to taking up glucose. This is not the same as the pathological insulin resistance seen in type 2 diabetes.
- Your muscles, which are typically large consumers of glucose, switch to using fatty acids as their primary fuel.
- This change, often described as muscles being in "glucose refusal mode," ensures that the small amount of glucose produced by the liver is spared for the brain and other glucose-dependent organs.
- The effect is a slightly higher but steady fasting blood glucose reading, as the glucose circulates longer in your bloodstream. Your fasting insulin levels, however, should be low.
Chronic Stress and Poor Sleep
Stress and a lack of quality sleep are often overlooked but powerful drivers of high blood sugar. The link is the hormone cortisol.
- Chronic stress causes your body to release cortisol, a hormone that raises blood sugar by prompting the liver to produce more glucose.
- Poor sleep has a similar effect, increasing cortisol levels and weakening insulin sensitivity.
- This makes blood sugar control harder, especially for those with existing insulin resistance, and can defeat the purpose of a carefully managed diet.
Dehydration
If you are not drinking enough water, your blood glucose levels can rise. This is particularly relevant on a low-carb diet, where the initial loss of water from glycogen depletion increases your risk of dehydration.
- When dehydrated, your blood becomes more concentrated, increasing the concentration of glucose within it.
- Your kidneys work harder to excrete the excess glucose, but if fluid intake is insufficient, your blood sugar can remain elevated.
Hidden Carbohydrates
Not all low-carb-labeled foods are created equal. Many processed low-carb or keto-friendly products contain hidden ingredients that can cause a blood sugar spike.
- Artificial Sweeteners and Sugar Alcohols: Some, like maltitol, can significantly impact blood sugar despite having lower net carbs.
- Processed Ingredients: Sauces, dressings, and pre-packaged foods often contain added sugars or starches. Always read ingredient labels carefully.
Comparison: Physiological vs. Pathological Blood Sugar Elevation
To differentiate between a benign metabolic adaptation and a concerning health issue, it's helpful to consider the context of the elevated blood sugar.
| Feature | Physiologic Insulin Resistance (Adaptive Glucose Sparing) | Pathologic Insulin Resistance (Type 2 Diabetes) |
|---|---|---|
| Cause | Normal metabolic adaptation to low glucose availability on a low-carb/keto diet. | Excessive insulin levels leading to cellular resistance and dysfunction. |
| Fasting Glucose | Mildly elevated but stable, often highest in the morning due to dawn phenomenon. | Consistently and progressively high. |
| Fasting Insulin | Low and stable, showing good insulin sensitivity overall. | High, indicating the body is working overtime to manage blood glucose. |
| Response to Carbs | Higher glucose response to a small carb load, as muscles prefer fat. | Impaired glucose metabolism after carb consumption. |
| Associated Symptoms | Typically none, or feeling of increased energy and mental clarity. | Fatigue, thirst, increased urination, weight gain, and other diabetes symptoms. |
Conclusion
High blood sugar on a low-carb diet, while counterintuitive, can be caused by a range of factors, from the body's natural adaptive processes to lifestyle influences like stress and dehydration. For many, a slightly elevated morning reading is a normal sign of metabolic adjustment rather than a health problem. For others, it may point to hidden dietary pitfalls or the impact of stress. It is crucial to look at the full picture, including fasting insulin levels and overall health metrics, to determine the root cause. Consulting with a healthcare professional is always recommended to interpret your specific readings and tailor your diet and lifestyle effectively.
An excellent resource for learning more about the intricacies of metabolic health is The Diet Doctor's guide on fasting blood glucose on low-carb or keto, which explains these phenomena in greater detail.
