Ketosis & Fasting: Benefits, Cortisol, Cancer & Keto-Mojo Tracking

Understanding the Pros and Cons of Ketosis, Fasting, and Metabolic Flexibility

Functional medicine illustration showing ketosis metabolism, cortisol regulation, and ketogenic diet support for cancer health.

What Is Ketosis?

Ketosis is a natural metabolic state in which the body shifts from using glucose (sugar) as its main energy source to burning fat for fuel. When carbohydrate intake is low, the liver converts fatty acids into ketone bodies — primarily beta-hydroxybutyrate (BHB), acetoacetate, and acetone — which become the body’s alternative energy source for the brain, muscles, and organs. (1)

This adaptation evolved to help humans thrive during fasting or food scarcity. Today, ketosis is harnessed therapeutically to improve metabolic health, brain function, inflammation, and longevity. (2)

How Ketosis Works

Under normal conditions, your body relies on glucose from carbohydrates for energy. When carbs are scarce, insulin levels drop, and fat stores are mobilized. The liver converts those fats into ketones, which circulate in the blood and fuel cells. Nutritional ketosis typically occurs when blood ketones rise above 0.5 mmol/L. (3)

Ways to enter ketosis include:

Ketogenic diet: high fat (70–80%), moderate protein (15–20%), low carbohydrate (5–10%)
Intermittent fasting or time-restricted eating
Extended fasting or fasting-mimicking diets
MCT oil or exogenous ketone support

Ketosis, Fasting, and Metabolic Flexibility

Fasting naturally induces ketosis by lowering glycogen and insulin, prompting the body to burn fat for energy. When paired with a clean, nutrient-dense ketogenic diet, fasting trains the body to become metabolically flexible — able to burn both fat and glucose efficiently.

Benefits of Fasting and Ketosis

Cellular Renewal (Autophagy): Fasting triggers cleanup of damaged cells and proteins, enhancing longevity. (4)
Mitochondrial Efficiency: Ketosis boosts energy production and cellular resilience. (5)
Improved Insulin Sensitivity: Lowers fasting insulin and stabilizes blood sugar. (6)
Hormone Regulation: Supports growth hormone and testosterone balance. (7)
Brain Health: Increases BDNF for neuroprotection and sharper cognition. (8)

Potential Downsides

Excess fasting can stress the adrenals and suppress thyroid function. (9)
Women often do best with cyclical fasting that honors hormonal rhythms.
Under-eating or long-term deep ketosis may cause fatigue or hormonal imbalance. (10)

As Dr. Paul Saladino and other functional medicine leaders note, the goal isn’t to stay in ketosis forever — it’s to achieve metabolic flexibility, the ability to switch fuel sources smoothly depending on your energy needs and environment.

The Benefits of Ketosis

Sharper Mental Clarity

Ketones burn cleaner than glucose, providing consistent mental focus and cognitive performance. (11)

Steady Energy

Eliminates sugar highs and crashes for sustained, stable energy throughout the day.

Better Blood Sugar Control

Reduces fasting glucose, A1c, and insulin resistance — powerful tools in reversing metabolic disease. (12)

Anti-Inflammatory and Longevity Support

Ketones act as signaling molecules that activate AMPK and Nrf2, pathways linked to detoxification and anti-aging. (13)

Neurological Protection

Used therapeutically for epilepsy, with emerging data supporting use in Alzheimer’s, Parkinson’s, and multiple sclerosis. (14)

Keto Flu: Why It Happens and What It Really Means

During the first few days or weeks of starting a ketogenic diet, many people experience what’s called the “keto flu.” (15)

Symptoms can include:

Fatigue or low energy
Brain fog, mood swings, or irritability
Headaches or dizziness
Muscle cramps or palpitations
Nausea or sleep disruption

These symptoms occur as your body transitions from burning glucose to using fat and ketones for energy. When carbohydrates are suddenly restricted, insulin levels drop, and the kidneys excrete sodium, potassium, and magnesium — leading to electrolyte imbalances and dehydration. Combined with reduced glycogen stores, this shift can temporarily leave you feeling sluggish and unwell.

However, the “keto flu” isn’t a necessary or healthy part of the process. It’s a sign that the approach is too abrupt or too extreme — often lacking electrolyte, caloric, or fat support. In functional medicine, we interpret this as metabolic feedback, not failure.

Common Causes of a Difficult Transition

Overly rapid carb restriction without gradual fat adaptation
Electrolyte depletion from increased urination and sodium loss
Insufficient dietary fat (especially medium-chain fats) to fuel mitochondria
Adrenal or thyroid stress from combining fasting and low-carb extremes
Overtraining, poor sleep, or chronic stress, which raise cortisol and block fat burning

How MCT and Coconut Oil Can Help Ease the Transition

Medium-chain triglycerides (MCTs) — found abundantly in coconut oil — are unique fats that make the transition into ketosis far smoother and more sustainable.

Unlike long-chain fatty acids, which require bile acids, pancreatic enzymes, and the L-carnitine shuttle to enter the mitochondria for oxidation, MCTs completely bypass this transport system.

That difference is crucial: when you’re not yet fat-adapted, your mitochondria are inefficient at transporting long-chain fats across the mitochondrial membrane. This bottleneck — the carnitine shuttle — can lead to fatigue, sluggishness, and “keto flu” symptoms, since the body isn’t yet effective at producing ketones from stored fat. (16)

MCTs, however, skip this step entirely. They’re absorbed directly through the portal vein, delivered straight to the liver, and converted rapidly into ketone bodies — providing immediate energy for both brain and muscles. (17)

Benefits of MCT and Coconut Oil During Keto Adaptation

Immediate Energy Boost: MCTs are rapidly converted into ketones, offering instant cellular fuel even before full fat adaptation occurs.
Enhanced Brain Function: They provide an alternative energy source for neurons, improving focus and mental clarity during early ketosis.
Bypass Metabolic Bottlenecks: Because MCTs don’t need the carnitine shuttle, they deliver energy efficiently even when long-chain fat oxidation is sluggish.
Reduce Keto Flu Symptoms: By improving energy and fueling mitochondria, MCTs help minimize fatigue, brain fog, and electrolyte imbalances.
Train Fat Metabolism: Regular MCT intake helps upregulate mitochondrial enzymes and ease the transition into full fat-burning mode.

Clinical Tip:
Start slowly — 1 teaspoon of MCT oil once or twice daily (often in coffee or tea) — and gradually increase to 1–2 tablespoons as tolerated. Too much too quickly may cause loose stools or nausea. For sensitive individuals, virgin coconut oil offers a gentler start while still containing the key ketogenic fatty acids (C8 and C10).

Functional Insight:

MCTs act as “training wheels” for your mitochondria. By bypassing the carnitine shuttle, they bridge the energy gap between glucose dependence and full fat adaptation — helping you enter ketosis without crashing.

Functional Strategies to Prevent or Ease Keto Flu

Transition gradually: Lower carbs over 2–3 weeks while increasing clean fats (avocado, olive oil, coconut, MCTs).
Replenish minerals: Use quality sea salt or Quinton Isotonic/Hypertonic to maintain sodium and trace minerals.
Hydrate deeply: Aim for 3–4 liters of mineralized, clean water per day to replace losses from increased urination.
Eat enough fat: Avoid going low-carb and low-fat simultaneously; mitochondria need fuel to adapt.
Support adrenals and thyroid: Prioritize rest, circadian rhythm, and adequate calories to prevent hormonal strain.

Monitoring Ketosis: Why Testing Matters

Tracking your ketone levels helps you understand how your body responds to diet and fasting. Not everyone enters or maintains ketosis at the same threshold — stress, sleep, and carb intake can all influence it.

Methods of Ketone Testing

Urine Strips: Inexpensive but unreliable once fat-adapted (measure acetoacetate, not BHB).
Breath Meters: Convenient but less precise.
Blood Monitors: The gold standard — measure beta-hydroxybutyrate (BHB), the main circulating ketone.

Keto-Mojo: The Functional Medicine Favorite

At Denver Sports & Holistic Medicine, we recommend Keto-Mojo as the most accurate and affordable at-home ketone and glucose monitor.

Why Keto-Mojo Stands Out

Dual Measurement: Tracks blood glucose and BHB, allowing calculation of the Glucose Ketone Index (GKI) — a powerful marker of metabolic health.
Clinical Accuracy: FDA-approved and validated in research.
Smart Integration: Syncs with the Keto-Mojo app, Apple Health, or Cronometer to trend your data.
Functional Use: Perfect for fasting, longevity, or metabolic protocols.
Affordable Strips: More cost-effective for long-term tracking.

Target Range:

Nutritional ketosis: 0.5–3.0 mmol/L BHB
Therapeutic ketosis (for specific medical protocols): 3–5 mmol/L, under supervision

Why the Keto Diet Can Backfire: Cortisol, Gluconeogenesis, and Blood Sugar Spikes

1. Low Carbohydrate Intake Triggers a Stress Response

When carbohydrate intake drops very low, the body perceives this as an energy deficit or starvation signal. In response, the hypothalamic-pituitary-adrenal (HPA) axis activates — releasing cortisol and, to a lesser degree, adrenaline (epinephrine).
These hormones are designed to mobilize stored energy to protect the brain and maintain blood sugar when food is scarce.

Cortisol maintains glucose levels during fasting or stress.
It does this by:

Stimulating gluconeogenesis— the production of new glucose in the liver from amino acids, glycerol (from fat breakdown), and lactate when glycogen stores run low.
Increasing lipolysis (fat breakdown)
Reducing glucose uptake in muscle to spare glucose for the brain

So even though you’re eating almost no carbs, cortisol can raise fasting blood sugar by pushing your liver to produce more glucose.

2. Gluconeogenesis Keeps Blood Sugar Elevated

In nutritional ketosis, moderate gluconeogenesis is normal and necessary. But if cortisol remains high due to chronic stress, fasting, or calorie restriction, this pathway becomes excessive.

That means:

Blood glucose stays elevated (often 90–110 mg/dL or higher)
Ketone production may stall despite low-carb intake
You feel tired, anxious, wired, or crave sugar — even when “in ketosis”

This is why some keto dieters see paradoxically higher fasting glucose despite perfect adherence. (18)

3. Chronic Cortisol Elevation Blocks Fat Adaptation

High cortisol also directly impairs mitochondrial efficiency. When the stress response dominates:

The body prioritizes glucose over fat as its emergency fuel
Insulin sensitivity worsens
Muscle protein is broken down for glucose production
Thyroid conversion (T4 → T3) slows, reducing metabolic rate

In essence, your physiology shifts from “fat-burning and repair” to “fight-or-flight survival.” (19,20)

4. Other Triggers That Exacerbate Cortisol on Keto

Overtraining while in deep ketosis (no glycogen = stress response)
Under-eating calories or protein (starvation signal)
Sleep deprivation or circadian disruption
Extended fasting without proper refeed days
Adrenal fatigue or pre-existing HPA axis dysfunction

These factors compound to elevate cortisol, which then drives morning blood sugar spikes and even insulin resistance over time — the opposite of what keto is meant to achieve.

5. How to Fix It: Functional Medicine Strategies

To avoid the cortisol–glucose rebound, balance is key:

Add Strategic Carbohydrate Refeeds
Include 1–2 carb-containing meals per week (root vegetables, fruit, squash) to reset leptin, thyroid, and glycogen stores.
Prioritize Sleep & Stress Reduction
Poor sleep can raise cortisol by 30–40% and negate the metabolic benefits of ketosis.
Use breathwork, meditation, acupuncture, or adaptogens (ashwagandha, rhodiola, holy basil).
Support Adrenals and Electrolytes
Use mineral-rich sea salt, magnesium, and potassium (Quinton or trace minerals) to stabilize blood sugar and stress response.
Track Your Pattern
Use Keto-Mojo or continuous glucose monitoring (CGM) to see how stress, sleep, or fasting affect your blood sugar. You’ll often see morning glucose spikes that mirror cortisol rhythms (the “dawn phenomenon”).
Personalize Your Approach
For patients with adrenal fatigue, thyroid issues, or chronic stress, a cyclical keto or low-carb Mediterranean approach is safer and more sustainable.

Key Clinical Insight

“If your blood sugar rises on keto, it’s not always the food — it’s often the stress chemistry.”

The solution isn’t stricter carb restriction; it’s restoring metabolic balance and resilience to stress.

Keto and Cancer: Supporting the Metabolic Terrain

One of the most compelling therapeutic applications of ketosis is in the realm of cancer metabolism.
Emerging research supports what functional medicine and metabolic oncology have long suggested — that cancer is not just a genetic disease, but also a metabolic one, driven by impaired mitochondrial function and altered glucose metabolism.

The Metabolic Theory of Cancer

Healthy cells can flexibly burn both glucose and fat for fuel.
Cancer cells, however, are often locked into aerobic glycolysis (the “Warburg effect”) — meaning they rely almost exclusively on glucose and glutamine to generate energy, even when oxygen is available.

By reducing glucose availability through ketosis, we can target a metabolic vulnerability in cancer cells while nourishing healthy cells with ketones, which they can use efficiently for ATP production and repair. (21,22,23)

In simple terms: Ketosis may help starve cancer cells of their preferred fuel while strengthening the mitochondria of healthy cells.

Potential Benefits of a Therapeutic Ketogenic Diet

Reduces circulating glucose and insulin, which can lower growth signals like IGF-1 that promote tumor proliferation. (24)
Enhances mitochondrial resilience and reduces oxidative stress in healthy cells. (25)
May increase the effectiveness of conventional therapies, such as chemotherapy and radiation, by sensitizing cancer cells to oxidative damage. (26)
Improves energy, mood, and weight stability in patients undergoing treatment.

Clinical Insights: Not a One-Size-Fits-All Approach

While ketogenic therapy shows promise as an adjunct in cancer care, it must be personalized and medically supervised.
Certain patients — particularly those with adrenal fatigue, thyroid dysfunction, or cachexia (weight loss due to cancer) — may not tolerate deep ketosis well.
A modified or cyclical ketogenic diet, with strategic carbohydrate and protein adjustments, often achieves better long-term balance.

At Denver Sports & Holistic Medicine, we use functional lab testing, nutritional genomics, and metabolic assessments to tailor each plan — ensuring mitochondrial support, detoxification, and hormone stability alongside therapeutic ketosis.

Cautions and Considerations

Avoid extreme calorie restriction or fasting without medical guidance during active cancer treatment.
Ensure adequate micronutrient and antioxidant support (magnesium, selenium, zinc, B-vitamins, CoQ10).
Pair ketosis with gentle detoxification and mitochondrial support (NAD precursors, PQQ, sulforaphane, or Quicksilver AMPK+ Charge).
Monitor progress through labs, Keto-Mojo tracking, and clinician oversight.

Key Takeaway

Ketosis isn’t a cancer cure — but it’s a powerful metabolic tool that can shift the body’s terrain toward healing and resilience.
When guided by a practitioner trained in metabolic medicine, the ketogenic diet can complement conventional care, reduce side effects, and improve quality of life.

Paul Saladino’s Animal-Based Perspective

Dr. Paul Saladino, once a strict carnivore advocate, now emphasizes metabolic cycling — alternating between fat- and carb-based metabolism to support hormone and thyroid balance. His philosophy:

“Ketosis is a tool, not a prison. Flexibility is health.”

He encourages seasonal inclusion of clean carbohydrates (like raw honey and fruit) to mirror ancestral rhythms — promoting thyroid function, glycogen balance, and circadian health. (18)

Who Should Consider Ketosis or Fasting?

Beneficial for:

Insulin resistance, prediabetes, or type 2 diabetes
Neurological or cognitive decline
Inflammation, autoimmune, or mitochondrial dysfunction
Weight loss or longevity protocols

Use caution with:

Adrenal fatigue or thyroid disorders
Underweight individuals or athletes
Pregnancy and breastfeeding

Functional Medicine Perspective

At Denver Sports & Holistic Medicine, we view ketosis, fasting, and ketone monitoring as part of a precision metabolic strategy — tailored to each person’s unique physiology, hormones, and stress response.

Through advanced lab testing (glucose, insulin, thyroid, cortisol, and mitochondrial markers) and real-time tracking, we help patients achieve fat adaptation, metabolic flexibility, and sustainable energy — without deprivation or burnout.

Key Takeaway

Ketosis and fasting are powerful metabolic therapies when personalized and cycled intelligently. They can enhance mental clarity, energy, and longevity — but the key is balance and support.

Use ketosis to heal — and metabolic flexibility to thrive.

Ready to Personalize Your Ketosis Journey?

At Denver Sports & Holistic Medicine, we tailor ketosis, fasting, and metabolic nutrition plans to your unique biochemistry. Through advanced lab testing and real-time ketone tracking, we help you enter ketosis safely, avoid burnout, and restore true metabolic flexibility.

Request a Free 15-minute consultation with me, Dr. Martina Sturm.

Frequently Asked Questions (FAQs)

If you’re considering a ketogenic or fasting approach, it’s natural to have questions about safety, duration, and long-term impact. Below, we’ve answered some of the most common questions we hear at Denver Sports & Holistic Medicine from patients exploring ketosis for energy, weight loss, or metabolic health.

These FAQs combine functional medicine insight with evidence-based research to help you apply ketosis safely, avoid common pitfalls like the “keto flu,” and understand when it’s time to transition back toward metabolic flexibility — the ability to burn both fat and carbohydrates efficiently.

Q1: What is ketosis, and how does it work?

Ketosis is a natural metabolic state in which your body burns fat instead of glucose for energy. When carbohydrates are restricted, the liver produces ketones (BHB, acetoacetate, and acetone), which fuel the brain and muscles efficiently while reducing inflammation and stabilizing blood sugar.

Q2: What causes the “keto flu”?

The keto flu occurs when the body transitions too quickly into ketosis, leading to electrolyte loss, dehydration, and low energy. Common symptoms include fatigue, brain fog, and muscle cramps. It’s usually a sign that carb restriction was too abrupt or fat intake and minerals are inadequate.

Q3: How can I prevent or fix the keto flu?

Transition gradually over 2–3 weeks, increase electrolyte intake (sodium, magnesium, potassium), and include MCT or coconut oil to provide fast fuel while your body adapts. MCTs bypass the L-carnitine shuttle, providing instant ketone energy and easing the adjustment phase.

Q4: What makes MCT oil different from other fats?

Unlike long-chain fats, MCTs don’t need bile or the carnitine shuttle to enter mitochondria. They’re absorbed directly through the portal vein and converted into ketones by the liver — providing a quick, clean energy source that supports brain and metabolic function.

Q5: How do I know if I’m in ketosis?

You can test ketone levels with blood, breath, or urine tests. The most accurate method is blood testing for beta-hydroxybutyrate (BHB). At Denver Sports & Holistic Medicine, we recommend the Keto-Mojo meter for precise, affordable tracking of both glucose and ketones.

Q6: How long should I stay in ketosis?

Ketosis isn’t meant to be permanent for everyone. Short cycles (2–8 weeks) can reset metabolism, but staying in deep ketosis long term may affect thyroid and hormone balance. Dr. Paul Saladino’s “animal-based” approach recommends cycling between fat- and carb-based metabolism for optimal flexibility.

Q7: Who should avoid or modify the ketogenic diet?

People with adrenal fatigue, hypothyroidism, pregnancy, or chronic stress should use a more moderate or cyclical approach. Functional medicine practitioners personalize ketosis based on labs, hormone balance, and metabolic resilience.

Q8: What’s the ideal ketone range for health benefits?

For most, nutritional ketosis begins at 0.5 mmol/L and peaks around 2–3 mmol/L of BHB. Therapeutic ketosis (e.g., neurological or cancer support) may range from 3–5 mmol/L, under clinical supervision.

Q9: What foods support healthy ketosis?

Focus on whole-food fats (avocados, olive oil, coconut, pasture-raised eggs, grass-fed meats), low-starch vegetables, and clean proteins. Avoid processed “keto” snacks with seed oils or sugar alcohols, which cause inflammation and disrupt gut balance.

Q10: How long should I do a ketogenic diet safely?

The ideal length of time to stay in ketosis depends on your goals, health status, and hormone balance.

For most people, short to moderate cycles (4–12 weeks) are safe and effective for improving insulin sensitivity, fat loss, and inflammation. After that, many benefit from transitioning to metabolic flexibility — reintroducing healthy carbohydrates (like root vegetables or fruit) while maintaining a foundation of stable blood sugar and fat metabolism.

Long-term, strict ketosis can lower thyroid hormone (T3), reduce leptin, and increase cortisol, especially in women, athletes, or those under stress. This is why cyclical or targeted keto — alternating between fat- and carb-based metabolism — tends to be safer and more sustainable.

At Denver Sports & Holistic Medicine, we personalize each plan using lab testing (thyroid, insulin, cortisol, and nutrient panels) to ensure ketosis supports your unique physiology rather than depleting it.

Q11: What are the signs it’s time to come out of ketosis?

If ketosis begins to feel depleting rather than energizing, it may be time to reintroduce clean carbohydrates. Common signs include fatigue, poor sleep, low thyroid function, menstrual irregularities, irritability, or stalled weight loss.

These signals often mean that hormone or adrenal balance needs carbohydrate support. Strategic refeeding with nutrient-dense carbs like fruit, root vegetables, or squash can restore leptin, thyroid hormone (T3), and glycogen levels — improving metabolism and mood.

At Denver Sports & Holistic Medicine, we teach patients to listen to their physiology and use lab data and metabolic markers to know when to cycle out of ketosis safely.

Q12: Why does my blood sugar go up on keto if I’m eating almost no carbs?

This is a common frustration—and it’s often caused by stress chemistry, not carbohydrates. When carbs drop too low or fasting becomes excessive, the body perceives a survival threat and releases cortisol, your main stress hormone.

Cortisol tells the liver to make glucose through gluconeogenesis, raising blood sugar to ensure your brain has fuel. Over time, this can stall fat loss, increase morning glucose, and make you feel wired but tired.

In functional medicine, we correct this by restoring metabolic balance, not by restricting further. Strategic carb refeeds, MCT oil, improved sleep, and adrenal support help lower cortisol so blood sugar stabilizes naturally.

FAQ Summary

A ketogenic diet can be a powerful therapeutic tool — when it’s applied intentionally and personalized to your body’s needs. Whether you’re using ketosis for insulin resistance, inflammation, brain health, or performance, the key is balance, monitoring, and strategic cycling.

At Denver Sports & Holistic Medicine, we help you determine:

How long ketosis is appropriate for your goals
When to reintroduce healthy carbohydrates
How to use tools like Keto-Mojo to track real progress
And how to support your thyroid, adrenals, and mitochondria throughout the process

If you’re ready to explore personalized nutrition and metabolic testing, schedule a consultation to design your ideal path toward sustainable energy and metabolic resilience.

👉 Request a Free 15-minute consultation

Resources

Ketosis: What It Is, the Science Behind It, and When It Works Best