Expert-reviewed by Ashwini S.Kanade, Registered Dietician and Certified Diabetes Educator with 17 years of experience.
‘Diabetes is a condition brought about by lifestyle. If you change the lifestyle, this condition will leave’, suggested Dick Gregory, the African-American comedian and civil rights activists.
He is partly right, though. Changing your lifestyle will definitely help you fight diabetes, but lifestyle alone does not cause diabetes! This condition is brought on by multiple factors, but genetic factors along with environmental influences are the main culprits.
Digging deeper, the main cause of diabetes is insulin resistance. Diabetes occurs either when enough insulin is not produced by the body or when insulin activity is impaired.
Insulin is a hormone produced by beta cells in the pancreas and this hormone helps the body’s cells to convert sugars (from the carbs and fats we eat) into energy required for the body’s optimal functioning. The extra sugar is converted to glycogen and stored in the liver to be used later when not enough sugar is available for conversion to energy.
One way in which insulin can lead to diabetes is through impaired insulin sensitivity. Insulin sensitivity is a measure to find out how sensitive you are to the effects of insulin. If you have high insulin sensitivity, you will need a lower amount of insulin to remove sugar from circulation than a person who has low insulin sensitivity. Generally speaking, a good sensitivity to insulin is good for health.
Very low insulin sensitivity is called insulin resistance. Insulin resistance means the cells in the body do not respond to the insulin, so sugars cannot enter the cells. Sugar keeps accumulating in the blood raising blood sugar levels. Having high blood sugar levels in your body is a state called hyperglycemia, which in turn leads to diabetes. Although insulin resistance is associated with type 2 diabetes, people with type 1 diabetes too can be insulin resistant.
How does insulin resistance cause diabetes?
Since the cells do not absorb sugar from the blood due to insulin resistance, sugars accumulate in the blood. To meet the increased demand for insulin, the beta cells start producing more insulin. As long as beta cells produce enough insulin, everything is under control. Over time, beta cells cannot keep up with the body’s need for insulin. Sugar builds up in the bloodstream leading to diabetes.
What factors cause insulin resistance?
Genetic factors and environmental factors are responsible for the development of diabetes. Let’s look at type 2 diabetes in particular.
1. Genetic factors
Researchers have identified about 75 gene locations that play a crucial role in increasing the risk for T2DM. In the Indian population, researchers from Karnataka found two gene variants, namely of the TCF7L2 and SLC30A8 genes, that are responsible for the disorder in about 30% of Indians with T2DM. 
However, despite having these risky genes, some individuals do not develop diabetes. The researchers attributed this to other factors such as obesity, diet, environment and lifestyle.
Obesity, especially too much abdominal fat, is probably the most important factor in the development of T2DM.
Fat cells affect metabolism by producing hormones and inflammatory chemicals and releasing non-esterified fatty acids (NEFA). In obese people, secretion of these substances increases manifold. Scientists have found that NEFA is the main culprit that causes insulin resistance and also disrupts the functioning of beta cells, thus causing diabetes. 
Again, how the fat is distributed in your body determines your insulin sensitivity. Those who have fat spread out evenly are more insulin sensitive than those who have much fat in the abdomen and chest area.
3. Gut bacteria
Diabetes is also associated with changes in the intestinal bacterial population. For example, a large amount of the Bifidobacterium species can help improve glucose-tolerance (a marker of type 2 diabetes that shows how well your body can absorb sugars) and reduce low-grade inflammation. On the other hand, high amounts of bacteria such as Bacteroides can cause type 1 diabetes. 
Similarly, the proportion of a bacteria in the gut can also contribute towards the development of diabetes. A study found that the proportion of Firmicutes was significantly lower, while the proportion of Bacteroidetes and Proteobacteria was somewhat higher in people with diabetes compared to those who did not have diabetes. 
A more recent study went a step further and suggested that a type of genetic variation, called single nucleotide polymorphism, in the otherwise harmless gut microbe can cause diabetes. 
4. Psychological / Emotional Stress
Depression, anxiety, and stress are major health issues in India.
When you are stressed, stress hormones such as cortisol are released into the blood, which in turn increase blood sugar levels and inhibit insulin production from the beta cells. 
It not only causes diabetes but may also prevent someone with diabetes to properly manage their disorder. For example, a farmer told some researchers in an interview that he wanted to work but diabetes caused him to faint and fall down. He was scared of wounds becoming infected. This caused him a lot of stress and anxiety, and he could barely work, let alone manage his disease. 
5. Other lifestyle factors
Other risk factors include:
- Sedentary lifestyle
- Low-fibre and high glycemic index diet
- Lack of physical activity
- Alcohol consumption
- Sleep problems
Interestingly, the theory of insulin resistance as the cause of diabetes is now being contested by Dr John Poothullil, a paediatrician and allergist, in his book – Diabetes: The Real Cause and The Right Cure. Instead, he has come out with the fatty acid burn theory, which points to our genetic ability to store fat as the main cause of diabetes. According to him, the muscles instead of using sugar as energy, switch to burning fatty acids for energy. Sugar remains in the bloodstream, eventually leading to high blood sugar, resulting in diabetes. However, this theory is yet to be corroborated. 
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- Sysy Morales. New Book: Diabetes: The Real Cause and the Real Cure. Diabetes Daily. https://www.diabetesdaily.com/blog/new-book-diabetes-the-real-cause-and-the-real-cure-476102/