In some people it's due to genetics, with low-energy-expenditure lifestyle contributing to the condition. This can be termed insulin resistance or IR.  

Some of the most common complaints of people with IR.

Many symptoms occur immediately following a meal of carbohydrates, and others are constant. Keep in mind that these symptoms may also be related to other problems.

1. Fatigue. Whether you call it fatigue or exhaustion, the most common feature of IR is that it wears people out. Some are tired just in the morning or afternoon; others are exhausted all day.

2. Brain fogginess. Sometimes the fatigue of IR is physical, but often it's mental (as opposed to psychological); the inability to concentrate is the most evident symptom. Loss of creativity, poor memory, failing or poor grades in school often accompany IR, as do various forms of "learning disabilities."

3. Low blood sugar. Brief, mild periods of low blood sugar are normal during the day, especially if meals are not eaten on a regular schedule. Feeling jittery agitated and moody is common in IR, with an almost immediate relief once food is eaten.

Dizziness is also common, as is the craving for sweets, pretzels/chips or caffeine.  The old hypoglycemic diet, still in use today, recommends frequent snacks, and individuals with IR usually know to eat often. However, the hypoglycemic diet contains too much carbohydrate for most people.

4. Intestinal bloating. Most intestinal gas is produced from excess carbohydrates and poor chewing habits. Antacids or other remedies for symptomatic relief are not very successful in dealing with the problem. Sometimes the intestinal distress becomes quite severe, resulting in a diagnosis of "colitis" or "ileitis.

5. Sleepiness. Many people with IR get sleepy immediately after meals containing more than 40% or 50% carbohydrates. This is typically a pasta meal, or even a protein meal which includes bread, rolls or potatoes, beer/wine, and a sweet dessert.

6. Increased fat storage and weight. For most people, too much weight is too much fat. In males, a large abdomen is the more evident and earliest sign of IR. In females, it is prominent thighs and buttocks.

7. Increased triglycerides. High triglycerides in the blood are often seen in overweight persons. But even those who are not too fat may have stores of fat in their arteries as a result of IR. These triglycerides are the direct result of carbohydrates from the diet being converted by insulin.

8. Increased blood pressure. It is well known that most people with hypertension have too much insulin stimulation by excess carbs and are IR. It is often possible to show a direct relationship between the level of insulin and the level of blood pressure: as insulin levels elevate, so does blood pressure.

9. Depression. Because carbohydrates are a natural "downer," stimulating serotonin, which produces a depressing or sleepy feeling.

Protein, on the other hand, is a brain stimulant, picking you up mentally.

This is a significant consideration for those trying to learn or perform the duties of their work.

10. IR is also prevalent in persons addicted to alcohol, caffeine, cigarettes or other drugs.

Often, the drug is the secondary problem, with IR being the primary one. When a person with addictions finally lowers carbohydrate intake to tolerable levels, many if not most of the other symptoms may disappear. 

Like many problems, IR is an individual one, affecting different people different ways. You must determine if you are carbohydrate intolerant, and if so, to what degree. Blood tests will only diagnose the problem in the later stages, but the symptoms may have begun years earlier.

As we now know, insulin has many functions. While it can't get glucose into the cells efficiently when they're in a state of insulin resistance, insulin still performs its other tasks, including converting carbohydrates to fat and inhibiting stored fat from being burned.

In a normal person, 40% of the carbohydrates eaten is converted to fat. In the IR person, that number may be much higher.

Many people with IR have a family history of adult-onset diabetes.

Don't think of IR itself as a disease, although left unchecked, it can create problems that lead to disease. It is normal for most humans to be unable to eat large or even moderate amounts of carbohydrates.

As a matter of fact, we evolved for hundreds of thousands of years from the so-called cave man's diet, which consisted solely of meat, fruit, leafy greens, insects. With the onset of modern civilization about 5,000 years ago, our physiology suddenly was asked to digest and metabolize larger amounts of sugar and starch especially grains and refined sugars.