Weight loss is still incredibly challenging for most people, as evidenced by the growing global obesity epidemic.

According to the latest Centers for Disease Control and Prevention statistics, more than two in three US adults are considered overweight (defined as a body-mass index between 25 and 30), and 36.5% of US adults fall into the obese category (BMI greater than 30).

Many people also want to lose weight. According to a recent study in the Journal of the American Medical Association, approximately half of overweight and obese adults report that they are trying to lose weight. Which is why my patients often ask, "Why aren't I losing weight?"

There is no simple, universal answer to this question, but after nearly two decades of working with patients trying to lose weight, and with the help of two leaders in the field of nutrition and obesity medicine, I've put together this list of some of the most common behavioral and medical reasons you might not be losing weight, even though you think you are doing everything right. I've also tried to include suggestions to get you on the right track to sustained weight loss.

You have calorie 'amnesia'

This is one of the most common and easy-to-fix issues that I see in my practice: Put simply, people often eat more calories than they realize.

Total calories consumed matters when it comes to weight loss, despite what some popular diets claim. People frequently forget about the little things during or between meals that add up calorically and can interfere significantly with weight loss.

The fix: Keep a food journal or track your food intake with an app. Research shows that this simple behavior can double weight loss, and it often helps my patients break through frustrating weight loss plateaus.

You relax your diet too much on weekends

This is another very common issue that is a bit more challenging to fix, as weekends are generally much less structured and more social than weekdays. Three days (Friday, Saturday and Sunday) of more relaxed eating (and in many cases drinking) can easily erase four days of more focused effort.

I don't expect you to be perfect seven days a week, but the majority of successful weight loss maintainers from the National Weight Control Registry (an ongoing study tracking people who have lost at least 30 pounds and kept the weight off for at least a year) maintain a fairly consistent diet throughout the entire week, not just on weekdays.

The fix: Try to limit indulgences if you are trying to lose weight. One night, have a glass of wine or a more indulgent entrée but not both. On another night, split a dessert, skip the wine, and try to choose a healthier entrée. Be careful with having an entire "cheat" meal -- or worse, "cheat" day -- as this can really interfere with weight loss.

Your medication is working against you

Most physicians do not receive adequate training in obesity medicine and nutrition. Many may not be aware that the medications they prescribe may lead to weight gain or make weight loss more challenging.

According to Dr. Caroline Apovian, president of the Obesity Society, frequently prescribed medications that may cause weight gain include Benadryl, Ambien, benzodiazepines, older antidepressant and antipsychotic medications, Paxil, beta-blockers (for high blood pressure), several diabetes medications including insulin, sulfonylureas and thialidazones, and some contraceptive methods, especially Depo-Provera.

The fix: If you are taking any of these medications, talk to your doctor to see whether there is a suitable alternative. (Do not stop any prescribed medication without speaking to your doctor first.) There is a full list of medications that may cause weight gain available online.

You eat too much of a good thing

It is true that recommendations concerning dietary fat have changed in the past decade, and nutrition science has found that unsaturated fat, including nuts, seeds, avocado and olive oil, plays an important role in a healthy diet.

However, you can easily eat too much healthy fat along with other healthy foods, including whole grains.

Fat has more than double the calories of carbohydrates or protein, so fat calories, even if they are healthy, add up more quickly. Whole grains have three times the number of carbohydrates as non-starchy vegetables per serving, so again, portion size matters.

The fix: Practice portion control when it comes to healthy but more calorie-dense foods such as nuts, peanut butter, seeds, avocado, olive oil and whole grains including quinoa, whole-grain cereal and pasta, and brown rice.

Your body is resistant to insulin

This is an issue I often encounter with patients and one that my colleague Dr. Michael Rothkopf, president of the National Board of Physician Nutrition Specialists, confirms is an often overlooked reason for weight loss failure.

It may also require specialized testing to uncover. According to the CDC, approximately 30% of adults and nearly 70% of overweight adults have metabolic syndrome, a constellation of abnormalities strongly associated with insulin resistance and excess insulin production.

Insulin suppresses the release of stored fat and promotes the formation of fat from sugar. The metabolism of a person who has insulin resistance or insulin excess will be shifted away from fat loss and toward fat storage.

The fix: If you are a woman and your waist circumference is more than 35 inches or you're a man with a waist size greater than 40 inches, talk to your doctor about checking for insulin resistance. If you have it, there may be dietary modifications and medications that could help you lose weight.

There are other potential reasons why the scale may not be budging: short sleep duration, excess sedentary behavior, overeating after workouts, stress eating and eating a highly inflammatory diet filled with processed food and inadequate fruits and vegetables. But the above list should get you started figuring out how to get the scale moving in the right direction.