Disclaimer: I am not a medical professional. The body of text below follows from my understanding of the studies I have read during my extensive research. This post should not be use as substitution for medical advice from a doctor.
These days it seems like everyone has a different and contradictory perception of what causes type 2 diabetes. While the underlying mechanisms have not been nailed down by medical science with absolute certainty yet, there is a lot of available data. I will do my best to share what I have learned while avoiding the act of simply re-posting conclusions from mainstream media articles. This is all based on my own understanding after reading medical journals.
What Is type 2 diabetes?
Type 2 diabetes is a condition in which your body becomes resistant to the effects of insulin, resulting in buildup of sugars in the bloodstream (also known as glucose). For most people, glucose is the most readily available source of energy. It is used for energy first and foremost, before the body begins to metabolize fat. Insulin acts as a gateway for glucose to enter your muscle cells to be used for energy. When those cells become resistant to insulin, there is nowhere for that glucose to go, so it continues to circulate and buildup in your bloodstream.
Separating symptoms from root-cause
We all know what that extreme levels of sugar in the bloodstream results in the negative symptoms associated with type 2 diabetes. However, does that mean that sugar causes type 2 diabetes? The evidence appears to suggest that sugar is not the sole cause for type 2 diabetes. So what is the cause? This is where the gaps in modern medical science begin to show. However, there is much data regarding the physical properties of the muscle cells of patients with type 2 diabetes compared to healthy patients. So what is the difference? The single factor which seems to correlate most strongly to insulin resistance is the level of intramyocellular lipids (or IMCL for short). In other words, fat stored in your muscle cells. Some IMCL is considered to be healthy and can be used as a source of energy when glucose is not available. However, too much strongly correlates to insulin resistance. The logical next question for any recursive thinker is "what causes buildup of IMCL?". So far, the evidence suggests that buildup of IMCL occurs when there is more fat in your blood than what can be immediately used for energy. Some fats (such as saturated fat) seem to correlate more strongly with buildup of IMCL, although it is believed that all fats contribute. This includes both dietary fats as well as fats resulting from lipogenesis (conversion of excess calories into fat, including sugars and protein). So while carefully regulating the amount of sugar that you consume may be the way to prevent negative symptoms resulting from type 2 diabetes, it does nothing to reduce or eliminate the underlying problem that causes type 2 diabetes.
Can type 2 diabetes be cured/reversed
The official answer is no. Patients who have had Type 2 Diabetes for long periods of time may not be able to produce a normal amount of insulin anymore due to the way the pancreas reacts to this condition. However, there may be steps a person can take to reduce their levels of IMCL or otherwise increase insulin sensitivity. As mentioned above, IMCL is utilized as a source of energy when glucose is not available. This is essentially an energy reserve. To force your body to use that energy reserve you will likely have to consume a caloric deficit. This means you will have to consume less calories than what your body needs for energy. While drastically reducing the amount of calories you consume can trigger your body to lower it's basal metabolic rate (the amount of calories your body expends in it's resting state), combining calorie restriction with enough exercise will inevitably result in your body using it's energy reserves. Not only will daily exercise greatly help in reducing your levels of IMCL, it has also been shown as an effective way to increase insulin sensitivity independently of your IMCL levels. In fact, endurance athletes who have been training for many years have been shown to be quite sensitive to the effects of insulin even despite having very high levels of IMCL. This leads us to our next point.
Are intramyocellular lipids the only relevant factor?
There is a lot of emerging data that suggests that some people can have very high levels of IMCL without showing any signs of insulin resistance. Specifically, endurance athletes who have been training for a number of years. So as it turns out, simply measuring a person's levels of IMCL is not sufficient to determine their risk of developing type 2 diabetes. Another factor believed to be related is "muscle fatty acid oxidative capacity", which is the body's capacity to rapidly break down lipids into their smaller components. muscle fat oxidative capacity seems to correlate to level of exercise. More on this subject can be found here: https://www.cambridge.org/core/journals/proceedings-of-the-nutrition-society/article/highfat-diet-muscular-lipotoxicity-and-insulin-resistance/DE65595E73123E2362D1431775714A2A/core-reader. Do to the length of that article, I do not feel comfortable trying to condense it all down until a short paragraph. Please read it for yourself. I promise it is very informative.
A note on many so-called "experts"
While there are many researchers who have a deep and thorough understanding of the current data on this subject, most typical general practice doctors have not studied the subject thoroughly enough to be able to understand the underlying problems presented by type 2 diabetes. Most doctors are trained to recognize symptoms, make a diagnosis, and then treat the symptoms (usually by writing prescriptions and/or recommending that you diet & exercise). If you choose to question a doctor about what causes type 2 diabetes, please be careful and understand that many doctors have conflicting ideas on this subject. Even many doctors who have studied this topic extensively are guilty of "confirmation bias". This means that they went into the subject with a pet theory and were simply looking to be correct. Rather than being open to all the evidence, they may have been guilty of selectively taking in some evidence, while ignoring evidence that does not support their pet theory (ie. Dr. John Mercola who was simply looking for evidence that carbs cause diabetes and that high fat diets are the cure. That is NOT true!). I am not telling you to distrust the opinions of all doctors. I am simply warning you to be careful and remember that just because a doctor believes something, that does not automatically make it true. Do your best to understand all the relevant pieces of information rather than blindly accepting the conclusions of others.
Final Thoughts
While there is still much debate within the medical community regarding the root cause of type 2 diabetes and what particular diets are best for preventing it. My belief from the studies I have read is that type 2 diabetes is caused by excess intramyocellular lipids in combination with a lack of an appropriate level of muscle fat oxidative capacity. In other words, too many calories (especially from fats) and not enough exercise. Ways to reduce calorie consumption may include eating more fibrous foods (ie. plants) as well as reducing or eliminating consumption of meat, dairy, eggs, pasta, breads, cooking oils, and processed sugars.