Metabolic Flexibility: The Goal of Human Biology

TL;DR: Most clinics focus on the scale. We focus on making you metabolically flexible. Metabolic dysfunction, especially insulin resistance, is the root cause of much of what goes wrong in middle age. At Fishtown Medicine, we fix the engine using fasting insulin, continuous glucose monitors (CGMs), and DEXA scans.

What Is Metabolic Flexibility?

Metabolic flexibility is your body's ability to switch between fuel sources. A flexible system burns glucose when food is available and burns fat when it is not. A stuck system feels foggy and tired the moment a meal is delayed. If you have stubborn belly fat, brain fog after lunch, or a 3 PM energy crash, you are likely metabolically inflexible. Your body has forgotten how to burn fat for fuel.

What Is Insulin Resistance?

Insulin is the hormone that tells your cells to "open the door" for glucose, your blood sugar. When you eat processed carbs and sugar for years, the cells get tired of the constant knocking and lock the doors. That is insulin resistance.

• The result: Your pancreas panics and pumps out more insulin (often 5 to 10 times normal levels) to force the doors open.
• The damage: High insulin blocks fat burning, fuels chronic inflammation, and accelerates aging.

Why "Normal" Blood Sugar Misses the Problem

Standard medicine waits for your fasting glucose or A1c (a 3-month average blood sugar marker) to rise before diagnosing pre-diabetes. That is too late. Your glucose can stay normal for 10 to 15 years while your insulin levels climb to dangerous heights compensating. By the time A1c crosses into pre-diabetes territory, the metabolic damage is already underway. We test fasting insulin to catch this process a decade earlier.

How Do We Test for Metabolic Dysfunction?

We do not guess. We verify with the right tools.

Test	What It Tells Us
Fasting insulin	Insulin resistance often shows up here long before glucose changes
HOMA-IR	A calculated score using fasting glucose and insulin
A1c	A 3-month blood sugar average
Lipid panel with ApoB	ApoB measures the number of bad-cholesterol particles, a stronger predictor than LDL alone
Fasting triglycerides and HDL	The triglyceride to HDL ratio is a quick metabolic health proxy
DEXA scan	Body composition: visceral fat, lean mass, bone
Continuous glucose monitor	Real-world glucose patterns over 2 weeks

What Is the Optimization Toolkit?

We build the plan in layers. Foundation first, then refinement, then advanced tools.

1. Foundation (Protein, Fiber, Sleep)

Before fancy data, we stabilize the inputs.

• Protein: We set specific gram targets based on body weight to protect muscle and drive satiety. Most adults benefit from 0.8 to 1 gram per pound of ideal body weight.
• Fiber: 30 to 40 grams a day acts like a brake pedal for blood sugar.
• Sleep: You cannot out-diet poor sleep. We address circadian rhythm first.

2. Refinement (CGMs)

Once the foundation is set, we use continuous glucose monitors (CGMs) to fine-tune.

• The reality: CGMs add awareness, not magic.
• The use case: When standard labs do not match your symptoms, or when we want to test how your body reacts to specific foods like oatmeal or rice.

3. Body Composition (DEXA)

The scale lies. It does not tell us if you lost fat or muscle. DEXA scans (a low-dose X-ray that measures fat, muscle, and bone) tell us:

• Visceral fat: The dangerous deep belly fat around organs
• Lean (muscle) mass: The metabolic engine that disposes of glucose

4. Advanced Tools (Medications and Zone 2)

• Pharmacology: We use GLP-1 agonists (a class that includes Wegovy and Zepbound) when appropriate, never without a plan to protect muscle. We use metformin (an inexpensive insulin sensitizer) earlier in many cases.
• Zone 2 cardio: Slow, steady aerobic work builds mitochondria (the energy-producing parts of cells), which raises your engine's efficiency.

Guidance from the Clinic

"Insulin is the canary in the coal mine. By the time A1c is high, the canary has been dead for years. I would rather catch the change early and do something light, than wait and need something heavy."

A common question I hear: "My A1c is normal. Am I fine?" My honest answer: not necessarily. Many of my patients have a normal A1c and a fasting insulin of 15 or 20 (we want under 7). They feel tired, carry stubborn belly fat, and have rising blood pressure. Standard medicine tells them they are healthy. They are not, they are early.

Scientific References

1. Reaven GM. Banting lecture 1988. Role of insulin resistance in human disease. Diabetes. 1988;37(12):1595-1607. The foundational paper.
2. Kraft JR. Detection of Diabetes Mellitus In Situ (Occult Diabetes). Lab Med. 1975;6(2):10-22. Early data on fasting insulin testing.
3. DeFronzo RA, Tripathy D. Skeletal muscle insulin resistance is the primary defect in type 2 diabetes. Diabetes Care. 2009;32(Suppl 2):S157-S163.
4. Lin SX, Carnethon M, et al. Cross-classification of fasting insulin and the metabolic syndrome. Atherosclerosis. 2011;218(2):494-499.
5. Lincoff AM, et al. Semaglutide and Cardiovascular Outcomes in Obesity Without Diabetes. N Engl J Med. 2023;389(24):2221-2232. The SELECT trial.

Medical Disclaimer: This resource provides clinical context for educational purposes. In the world of Precision Medicine, there is no "one size fits all", the right metabolic plan must be matched to your unique lab work, physiology, and goals. Consult Dr. Ash to determine if this approach is right for you, especially if you have chronic health conditions or are taking prescription medications.