I'm going to share with you one of my favourite topics — and if you're a patient, you've likely heard me passionately advocate for this on more than one occasion. It's time to shine a spotlight on a simple, yet profoundly significant lab test: fasting insulin.
Despite the fact that we're currently grappling with an escalating epidemic of chronic metabolic diseases — diabetes, obesity, heart disease, cancer — an essential piece of the puzzle continues to be overlooked.
That piece is insulin regulation, specifically, the checking of fasting insulin levels.
Fasting insulin can effectively screen for blood sugar and metabolic dysfunction, often years before more routine tests like fasting glucose or A1C.
This simple lab test is central to understanding metabolic disease, and yet, is continually omitted on routine lab panels. I'm on a mission to shed light on this primary care blindspot.
What insulin does
Insulin, a hormone produced by your pancreas, is central to regulating your blood sugar levels. It allows your cells to take in sugar (glucose) from your bloodstream for energy. When functioning optimally, insulin maintains a delicate balance, ensuring your blood sugar levels stay within a healthy range.
However, things can go wrong. High insulin levels (hyperinsulinemia) often precede the development of type 2 diabetes — sometimes by many years. Chronically high insulin levels can cause your cells to become resistant to insulin's effects, a condition known as insulin resistance. This leads to more insulin production, creating a vicious cycle that can ultimately result in metabolic disorders.
A study published in The Lancet demonstrated that high fasting insulin levels were associated with an increased risk of ischemic heart disease. Another study in the International Journal of Cancer established an association between elevated fasting insulin levels and an increased risk of breast cancer.
Why fasting insulin is critical
Simply put, checking fasting insulin levels provides a valuable early warning sign. By the time blood glucose levels rise enough to be diagnosed as prediabetes or diabetes, your body has likely been struggling with hyperinsulinemia and insulin resistance for years.
In primary care, we typically check fasting blood glucose and HbA1c levels — both of which are important. However, they don't paint the complete picture. A fasting insulin test can detect underlying insulin resistance even when blood glucose levels are normal. By adding this test, we're not just identifying disease sooner — we're getting a chance to prevent it.
Insulin and cancer
Not only does insulin regulation play a vital role in metabolic diseases and heart health, but an emerging body of evidence also suggests a connection to cancer initiation and progression.
Chronic hyperinsulinemia is becoming recognized as a potential risk factor for several types of cancers. Researchers believe this could be due to insulin's ability to promote cell growth and division — a characteristic that can become dangerous if cells begin to multiply uncontrollably, as they do in cancer.
A study in the International Journal of Cancer revealed a direct correlation between elevated fasting insulin levels and an increased risk of breast cancer in postmenopausal women. Another noteworthy study in Cancer Epidemiology, Biomarkers & Prevention identified a strong association between high insulin levels and colorectal cancer. Research also implicates hyperinsulinemia in prostate, pancreatic, and endometrial cancers.
In supportive cancer care, the role of monitoring fasting insulin levels becomes even more crucial. As we work to support our patients' overall health, understanding their insulin regulation guides us in developing personalized care strategies. In fact, downregulating high insulin levels is one of the primary targets of supportive cancer care.
The IGF-1 connection
Elevated insulin levels can increase IGF-1 activity. IGF-1 is one of our body's primary growth hormones. So while high IGF-1 might be great for a young bodybuilder, it's not so great in diseases like cancer, where we are metabolically trying to starve growth. Insulin inhibits the production of insulin-like growth factor-binding proteins (IGFBPs), which bind to IGF-1 in circulation and reduce its bioavailability. Thus, high insulin levels lead to increased bioavailability of IGF-1 — amplifying its growth-promoting effects.
The simple take-home message, in terms of cancer at least, is that high fasting insulin drives tumour growth.
What's a good target?
The "normal" reference range for fasting insulin is between 20 and 180 pmol/L (LifeLabs). These reference ranges are derived from a population with poor metabolic health to begin with, so interpreting the test requires some nuance.
While there is no clear consensus on optimal levels, the metabolic threshold I like my patients to establish is less than 50 pmol/L. Fasting insulin levels exceeding 50–60 pmol/L may suggest insulin resistance, a precursor to type 2 diabetes and a risk factor for other metabolic and cardiovascular diseases.
There are many intricacies to understanding chronic disease, but underlying most of them is inflammation and insulin. It's high time we start treating insulin levels with the respect they deserve in the pantheon of metabolic health markers.
I encourage you to speak to your healthcare provider about getting your fasting insulin levels checked. More importantly, I encourage each of you to embark on positive lifestyle changes and/or metabolic therapies that result in improved fasting insulin levels. Your metabolic health will thank you.
