“The Most Important Lab Test Your Doctor Has Probably Never Checked”
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.
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. Fasting insulin can effectively screen blood sugar and metabolic dysfunction, often years before more routine tests such as fasting blood glucose or hemoglobin A1C.
Before diving in, let's understand what insulin is. 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 (1).
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 (2).
A study published in "The Lancet" demonstrated that high fasting insulin levels were associated with an increased risk of ischemic heart disease (3). Another study in the "International Journal of Cancer" established an association between elevated fasting insulin levels and an increased risk of breast cancer (4). It's clear that imbalances in insulin regulation can cascade into more severe health problems.
So why is fasting insulin so 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 (5).
In primary care, we typically check fasting blood glucose and HbA1c levels (as aforementioned), 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 (6).
This is why I'm standing on my metaphorical soapbox today. As a primary care provider, I believe it's our responsibility to advocate for preventative care that puts patients first. Checking fasting insulin levels should become a routine part of blood work, just like cholesterol or glucose tests.
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 (7).
The aforementioned study published 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 (8). This is of significant concern as breast cancer remains one of the most common cancer diagnoses in women. It’s also a primary reason why many postmenopausal women have difficulty achieving optimal metabolic health and target body weight.
Another noteworthy study in "Cancer Epidemiology, Biomarkers & Prevention" identified a strong association between high insulin levels and colorectal cancer (9). And it doesn't stop there - research also implicates hyperinsulinemia in prostate, pancreatic, and endometrial cancers (10).
There's no denying the association between elevated insulin levels and cancer risk, and it's becoming increasingly clear that maintaining proper insulin regulation could be a key strategy in cancer prevention.
In supportive cancer care, the role of monitoring fasting insulin levels becomes even more crucial. As we work to support our patients' overall health and improve their outcomes, understanding their insulin regulation can guide us in developing personalized care strategies. In fact, downregulating high insulin levels is one of the primary targets of supportive cancer care. It’s why nutrients such as berberine or medications such as metformin are so often prescribed in supportive cancer care. (11)
While research continues, the takeaway is clear: addressing insulin regulation offers another potential avenue for cancer prevention and supportive care. This yet again highlights the importance of regularly checking fasting insulin levels.
Interpreting the Fasting Insulin Test
The “normal” reference range for fasting insulin is between 20 and 180 pmol/L. This range will vary depending on which lab is utilized (LifeLabs). These reference ranges are derived from a population with poor metabolic health to begin with, so interpreting the test requires some nuance. It’s also why falling within “normal limits” doesn’t imply good general health. In diseases like cancer, which are highly dependent on insulin for growth, improving insulin is very important.
The Interplay between Fasting Insulin and IGF-1 in Cancer
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), particularly IGFBP-1 and IGFBP-2, which bind to IGF-1 in the circulation and reduce its bioavailability. Thus, high insulin levels can lead to increased bioavailability of IGF-1, amplifying its growth-promoting effects. I think the simple take-home message, in terms of cancer at least, is that high fasting insulin drives tumour growth.
So what’s a good general target for fasting insulin?
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 (12).
“The target we like to establish is <50 pmol/L”
There are many intricacies to understanding chronic disease, but underlying most of them is inflammation and insulin. Consequently, it's high time we start treating insulin levels with the respect they deserve in the pantheon of metabolic health markers. Recognizing the critical role of insulin regulation could transform how we approach and treat chronic metabolic diseases. And that is a transformation worth fighting for.
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:)
American Diabetes Association. (2018). Insulin, Medicines, & Other Diabetes Treatments.
Reaven GM. (2005). Insulin resistance, cardiovascular disease, and the metabolic syndrome. Diabetes Care.
Jeppesen J, et al. (1995). Insulin resistance, the metabolic syndrome, and risk of incident cardiovascular disease. The Lancet.
Pisani P. (2008). Hyper-insulinaemia and cancer, meta-analyses of epidemiological studies. Archives of Physiology and Biochemistry.
Tabák AG, et al. (2012). Trajectories of glycaemia, insulin sensitivity, and insulin secretion before diagnosis of type 2 diabetes: an analysis from the Whitehall II study. Lancet.
Bonora E, et al. (2006). Homeostasis model assessment closely mirrors the glucose clamp technique in the assessment of insulin sensitivity. Diabetes Care.
Arcidiacono B, et al. (2012). Insulin resistance and cancer risk: an overview of the pathogenetic mechanisms. Experimental Diabetes Research.
Gunter MJ, et al. (2009). Insulin, Insulin-Like Growth Factor-I, and Risk of Breast Cancer in Postmenopausal Women. Journal of the National Cancer Institute.
Yamaji T, et al. (2009). Hyperglycemia, Insulin Resistance, Impaired Pancreatic β-Cell Function, and Risk of Pancreatic Cancer. Journal of the National Cancer Institute.
Pollak M. (2008). Insulin and insulin-like growth factor signalling in neoplasia. Nature Reviews Cancer, 8(12), 915-928.
Podhorecka M, Ibanez B, Dmoszyńska A (2017). Metformin - its potential anti-cancer and anti-aging effects.
Wallace TM, Levy JC, Matthews DR. Use and abuse of HOMA modeling. Diabetes Care. 2004;27(6):1487-1495.