In my nutrition practice specializing in eating disorders and co-occurring conditions, the topic of blood sugar regulation comes up frequently. Many clients at some point may be told by their health care provider (!!) to fast longer or cut carbs to “fix” their high blood sugar or high A1c, but these strategies often backfire, or contribute to symptoms worsening. The healthcare providers recommending this to patients often may not fully understand, or may unintentionally oversimplify how metabolism really works. To understand blood sugar, and especially in the context of complex health conditions, we need to first talk about insulin and glucagon and what they do for and in our bodies.

What Is Insulin?

Insulin is a hormone released by the pancreas in response to rising blood sugar after eating, particularly carbohydrate-rich foods. It helps glucose enter your cells to be used for energy or stored in the liver and muscles.

• Main function: Lowers blood sugar by storing or using glucose

• When it rises: After meals (especially carbs)

• Other roles: Suppresses the release of stored energy
  
  

What Is Glucagon?

Glucagon is also produced by the pancreas, but it has the opposite effect of insulin. It helps raise blood sugar between meals or during fasting by telling the liver to release stored glucose or produce it from non-carb sources.

• Main function: Raises blood sugar when it’s too low

• When it rises: During fasting, overnight, or low-carb diets

• Other roles: Triggers fat breakdown when energy is low
  
  

The Fed vs. Fasted State

• In a fed state, insulin rises and glucagon drops

• In a fasted state, glucagon rises and insulin drops

This hormonal rhythm is natural and necessary. But when it becomes dysregulated due to illness, stress, or prolonged restriction, blood sugar control and overall energy balance are affected.

Why This Matters for Brain Health

The brain depends on glucose for nearly all its fuel. It doesn’t store much energy, so it relies on a steady supply through meals and blood sugar regulation.

If insulin and glucagon are out of sync, the brain may experience:

• Brain fog

• Irritability

• Trouble concentrating

• Fatigue or dizziness

The Interplay: Why Balance Matters

Insulin and glucagon work together to maintain stable blood glucose levels throughout the day, regardless of when or what you eat. When you consume carbohydrates, insulin increases and glucagon decreases. During fasting or low-carb intake, insulin decreases and glucagon rises.

This is a normal and healthy response. However, problems arise when one or both of these hormones become dysregulated, such as in insulin resistance, diabetes, prolonged undernourishment, or chronic stress.

Fasting, Low-Carb Diets, and A1C: What You Should Know

Reducing carbohydrate intake or increasing fasting windows can sometimes lead to improved short-term glucose levels, but that does not mean it will automatically improve A1C or long-term metabolic health.

Here’s why:

• A low-carb diet may reduce post-meal glucose spikes, but if insulin sensitivity remains poor, the underlying issue is not resolved.

• During extended fasting or carbohydrate restriction, the body may rely more heavily on gluconeogenesis (the liver creating glucose from non-carbohydrate sources), which still raises blood sugar—just through a different mechanism.

• Some individuals may experience higher fasting glucose levels due to increased glucagon activity, especially in the early morning (often referred to as the “dawn phenomenon”).

• Prolonged restriction, especially without adequate overall nutrition, can trigger a stress response in the body that affects glucose regulation, hormone balance, and nervous system function.

How This Applies to Autoimmunity, POTS, MCAS, EDS, and Dysautonomia

For individuals living with chronic conditions such as autoimmune disease, POTS (Postural Orthostatic Tachycardia Syndrome), MCAS (Mast Cell Activation Syndrome), EDS (Ehlers-Danlos Syndrome), or other forms of dysautonomia, this hormonal balance becomes even more critical.

These conditions often involve:

• Instability in blood sugar regulation

• Greater sensitivity to stress hormones like cortisol and adrenaline

• Disrupted autonomic nervous system activity

• Delayed gastric emptying and digestive challenges

• Increased energy demands or poor energy production

In this context, fasting or carbohydrate restriction can backfire. Skipping meals or avoiding carbohydrates may lead to more frequent glucose crashes, which can trigger or worsen symptoms like:

• Dizziness

• Brain fog

• Heart palpitations

• Nausea

• Temperature dysregulation

• Fatigue

These are not just “blood sugar symptoms.” They are tied into the autonomic nervous system and can impact quality of life, especially in individuals who are already managing complex, overlapping diagnoses.

What This Means for the Brain and for Eating Disorders

This blood sugar dynamic also has serious implications for the brain, especially in those with anorexia, ARFID, or other eating disorders.

The brain is one of the most glucose-dependent organs in the body. It cannot store glucose the way muscles and the liver can, and it relies on a continuous supply of energy from food or gluconeogenesis. In the context of fasting, malnutrition, or chronic energy deficit, several things happen:

1. Glucose availability drops, impairing cognitive function and concentration.

2. The brain receives less fuel, which can increase symptoms like brain fog, irritability, obsessionality, depression, and anxiety.

3. Insulin and leptin signaling change, affecting appetite regulation and body awareness.

4. The nervous system becomes increasingly sensitized, making individuals more vulnerable to overstimulation and stress.

In anorexia nervosa and other restrictive eating disorders, glucagon remains elevated for extended periods, while insulin remains suppressed. Over time, this places enormous strain on the liver, increases catabolism (breaking down of tissue), and alters brain chemistry in ways that reinforce the illness itself.

Ultimately, it's important to know that fasting states are not neutral. They have physiological consequences that affect both body and brain. For individuals with eating disorders, these effects can perpetuate the cycle of restriction, anxiety, and poor cognitive functioning. For individuals with chronic illness, they can lead to increased symptom burden and reduced resilience.

*Note: This post and all contents of this website are provided for informational purposes only and are not medical advice or intended to be used as such. The explanations provided above describe the typical biochemical programming of insulin and glucagon in individuals without insulin-dependent diabetes. In people with Type 1 diabetes or advanced insulin-dependent Type 2 diabetes, the body either does not produce insulin or does not respond to it effectively. This alters the hormonal balance and glucose regulation described here. In these cases, external insulin is necessary to support blood sugar control, and glucagon responses may also be impaired due to pancreatic dysfunction. 

Check back in soon for Part 2!