Whey Protein's Impact on Insulin Resistance & Blood Glucose
If you've been in the lifting game for a semi-significant period of time, you've heard the note consuming whey protein and simple carbohydrates like dextrose immediately post-workout is crucial to "spike" insulin levels and maximize muscle protein synthesis (MPS). Since the early days of weight training, lifters have been employing this practice to increase muscle mass during a bulking phase and preserve lean mass during a cutting phase.
A few years ago there was a shift in recommendations within the fitness community. Simple carbohydrates are no longer "required" to spike insulin levels because whey protein appeared to sufficiently spike insulin levels by itself.
Related: Finding the Perfect Post-Workout Carb to Protein Ratio
A recent study published in the scientific journal, Diabetes, suggested that protein, specifically prolonged consumption of whey protein, may spike insulin to dangerous levels. This implied that whey protein could lead to insulin resistance, a common predecessor of type 2 diabetes. 
Insulin Resistance Battle: Whey Protein vs. Leucine
A recent study entitled "Protein Ingestion Induces Muscle Insulin Resistance Independent of Leucine-Mediated mTOR Activation" by Gordon Smith, et al. was published in the May 2015 edition of Diabetes, a journal of the American Diabetes Association.
In this article, the authors administered either whey protein dosed at 0.6g/kg of fat-free mass or the amount of free-form L-leucine found in the equivalent dose of whey protein.  Each group consisted of 11 women, classified as sedentary, weight-stable, and between the ages of 50 and 65. 
Researchers found that both leucine and whey protein increased the mammalian target of rapamycin (mTOR) by 30% over baseline; mTOR is responsible for muscle protein synthesis.  Furthermore, Smith, et al. found that whey protein decreased the rate of glucose uptake from the blood by peripheral tissues, such as skeletal muscle by ~25%, whereas free-form L-leucine did not induce muscle insulin resistance.  
In short, the authors believe that protein, particularly whey protein, causes insulin resistance, a trait that typically leads to the development of Type 2 Diabetes.
Does Whey Protein REALLY Cause Insulin Resistance?
Before you switch to free-form L-Leucine diet, let's examine why these findings are ludicrous and in no way applicable to the fitness community.
The "sample" used in this study is abysmal at best; only 22 people, all of which sedentary post-menopausal women within a 15-year age range. If that's not enough information to make you stop reading or take the study with a grain of salt, let's delve further.
Our readers at Tiger Fitness are NOT sedentary individuals. We enjoy high intensity and frequency weight and cardiovascular training. Even a single instance of moderate intensity exercise can increase glucose uptake by 40+%. 
Let's say whey protein does decrease glucose uptake. Even after a bout of moderate intensity exercise, glucose uptake increases by a net of 15+%. That's in sedentary, untrained individuals.
Improved glucose uptake is an important component of building muscle and minimizing fat gain while eating a hypercaloric (re: mass gain) diet. If a similar study were to be performed in the future, researchers should vary the gender, age, and training experience. I'm willing to bet under these circumstances the results would be exceptionally different in non-sedentary individuals.
The Insulin Resistance and Diabetes ConnectionInsulin resistance does not always lead to type 2 diabetes. A study of 331 individuals, conducted by researchers at the Joslin Diabetes Center, Harvard Medical School, and Beth Israel Medical Center, found that insulin resistance doesn't accurately predict your likelihood of developing type 2 diabetes if you don't have a family history of Diabetes. 
So before we jump to the conclusion that whey protein causes diabetes, let's give the researchers credit; they didn't outright say whey protein causes diabetes. Rather, they claimed protein, specifically whey protein, causes insulin resistance which can lead to type 2 diabetes if not managed properly and if you have a family history of Diabetes. However, other factors such as bodyweight, age, gender, race, and activity level can also influence insulin resistance.
This study contained only two groups - whey protein and free-form L-leucine. Including a non-dairy, whole food protein source like chicken, beef, or egg, whether it be in unprocessed or powder form, would have been beneficial.
More specifically, using egg protein,with a biological value of 100 and net protein utilization of 94, would provide the best comparison to whey protein. Eggs have a biological value of 104 and net protein utilization of 92. 
Any time you process and filter a food to increase protein content and remove undesired ingredients like carbohydrates and fats and in the case of L-leucine, other amino acids, your body is going to respond and process the protein differently. It would also be interesting to compare L-leucine to other free-form essential amino acids like isoleucine and valine.
Multiple studies indicate that protein, specifically whey protein, can actually improve blood glucose levels in healthy, obese, and individuals with type 2 diabetes.
Yes, Whey Protein is Insulinogenic
It's well-established that whey protein is insulinogenic, which means it stimulates the production of insulin. For all the lifting bros, whey protein contributes to the "insulin spike". The effects and magnitude of this insulin spike will be debated until the end of time.
One study found that consuming whey protein versus white wheat bread led to 87% and 139% higher insulin levels 15 minutes and 30 minutes post consumption, respectively.  Let me reiterate, spiking insulin isn't always a bad thing; it's beneficial for replenishing glycogen stores within the muscles, particularly after an intense training session.
Before we wrap up this article, let's examine studies that challenge the notion that whey protein induces muscle insulin resistance.
Researchers who performed a comprehensive analysis of literature related to dietary whey protein found that it reduces blood glucose levels in healthy individuals, obese individuals, and those with Type 2 Diabetes.  When human subjects consumed just 9 grams of whey protein before a ham sandwich meal, they experienced significantly lower post-meal changes in blood sugar compared to the control group. 
Another study of 12 healthy human subjects found that once again, consuming just 9g of whey protein before a meal with 25g of carbohydrates decreased post-meal fluctuations in blood sugar.  In fact, it appears that whey protein before a meal actually improves insulin response.
Now let's examine a few studies on rodents, our genetic cousins. One study on rats found that whey protein didn't affect fasting blood glucose level and actually improved insulin resistance.  To avoid confusion here, let me clarify - improved insulin resistance is the same as decreased insulin resistance.
Another study on rats found that consuming whey protein hydrolysate increased GLUT-4 concentrations in the plasma membrane without affecting insulin levels.  GLUT-4 is the mechanism that brings glucose into muscle cells and those with type 2 diabetes experience decreased GLUT-4 concentrations.
A study on rats found that consuming whey protein isolate (100g per liter of drinking water) alongside a high-fat diet for 11 weeks actually improved glucose tolerance and insulin sensitivity.  So there you have it folks, multiple studies contrasting the recent finding that whey protein induces muscle insulin resistance.
Whey Protein Can Improve Blood Glucose LevelsDon't let this one-off study scare you into ditching whey protein. Whey protein is considered a "supplement", which means it's not required for gaining size, strength, or power. However, it's a tasty, convenient, and cost-effective way to increase your protein intake and improve recovery.
Multiple studies indicate that protein, specifically whey protein, can actually improve blood glucose levels in healthy, obese, and individuals with type 2 diabetes. Instead of blaming whey protein for insulin resistance, let's focus on decreasing portion sizes and processed junk while increasing physical activity in the general population. Those are the kinds of steps that will decrease insulin resistance in both the short and long term.
References1) "Protein Ingestion Induces Muscle Insulin Resistance Independent of Leucine-Mediated MTOR Activation. - PubMed - NCBI." National Center for Biotechnology Information. N.p., n.d. Web. 23 July 2015.
2) "Too Much Whey Today, Type II Diabetes Tomorrow - Human Study: Whey Reduces Muscular Glucose Uptake by 25%! - SuppVersity: Nutrition and Exercise Science for Everyone."SuppVersity - Nutrition and Exercise Science for Everyone. N.p., n.d. Web. 23 July 2015.
3) "Glucose Disposal Rate." Diabetes Education for Healthcare Professionals. National Diabetes Education Initiative, 2015.
4) "Does Exercise Without Weight Loss Improve Insulin Sensitivity?" Diabetes Care. N.p., n.d. Web. 23 July 2015.
5) "Insulin Resistance Is a Poor Predictor of Type 2 Diabetes in Individuals With No Family History of Disease." Diabetes Education for Healthcare Professionals. National Diabetes Education Initiative, 2015. Web.
6) Hoffman, Jay R., and Michael J. Falvo. "Protein - Which is Best?" Journal of Sports Science and Medicine. N.p., Sept. 2004. Web.
7) "The Insulinogenic Effect of Whey Protein is Partially Mediated by a Direct Effect of Amino Acids and GIP on ?-cells. - PubMed - NCBI." National Center for Biotechnology Information. N.p., n.d. Web. 23 July 2015.
8) Sousa, Gabriela TD et al. ?Dietary Whey Protein Lessens Several Risk Factors for Metabolic Diseases: A Review.? Lipids in Health and Disease 11 (2012): 67. PMC.
9) Gunnerud, Ulrika J. et al. ?Effects of Pre-Meal Drinks with Protein and Amino Acids on Glycemic and Metabolic Responses at a Subsequent Composite Meal.? Ed. Jean-Marc A. Lobaccaro. PLoS ONE 7.9 (2012): e44731. PMC.
10) "Effects of Whey Proteins on Glycaemia and Insulinaemia to an Oral Glucose Load in Healthy Adults; a Dose-response Study. - PubMed - NCBI." National Center for Biotechnology Information. N.p., n.d. Web. 23 July 2015.
11) "[Whey Protein Improves Insulin Resistance Via the Increase of Antioxidant Capacity in Model Rats]. - PubMed - NCBI." National Center for Biotechnology Information. N.p., n.d. Web. 23 July 2015.
12) "Whey Protein Hydrolysate Increases Translocation of GLUT-4 to the Plasma Membrane Independent of Insulin in Wistar Rats. - PubMed - NCBI." National Center for Biotechnology Information. N.p., n.d. Web. 23 July 2015.
13) Shertzer, Howard G. et al. ?Dietary Whey Protein Lowers the Risk for Metabolic Disease in Mice Fed a High-Fat Diet.? The Journal of Nutrition 141.4 (2011): 582?587. PMC.