We all know the simple truth that when you consume more calories than you use, you gain weight. We might accept that the opposite is true, what we might call the basic rule of weight loss: when you eat fewer calories than you burn, you will lose weight.

While this is true, there are other reasons for weight gain or for lack of weight loss despite a rigorous weight loss regimen. There are a variety of nutritional, health and lifestyle components that cause weight gain. Here are 10 of those ways we gain weight, apart from the calories in/calories out theory.

Deficient fat in our diet. Essential fatty acids, also known as omega-3 and omega-6 good fats, which we find in flaxseeds, fish, nuts and seeds, really are essential. In addition to providing nourishment, fats maintain the body’s metabolic rate. The higher our metabolism, the more calories we expend doing routine daily activities.

Indigestion. When your body doesn’t digest the food you feed it, that food will transform into fat. Indigestion is a basic culprit in weight gain, especially for those who eat a reasonable diet.

Food allergies. Allergies produce internal side effects that may directly and indirectly promote the appearance of weight gain. Bloating and swelling in our intestines does not affect fat production, but it will fatten your waistline.

Spikes in insulin. A little known health issue that might affect 25% of the population, insulin resistance makes the body produce too much insulin in order for the insulin to be effective. Since the presence of insulin tells the body to metabolize glucose instead of fat, excess of insulin will trigger the body to hold on to fat, even when the fat might otherwise be used for energy.

Hypothyroidism. The thyroid is highly involved in regulating our metabolism, which is the pace at which our body uses calories. If you have a thyroid imbalance, your metabolism could be adversely affected.

Prescription drugs. Some prescription drugs tend to promote weight gain: certain antidepressants (particularly Prozac, Paxil and Zoloft), steroid anti-inflammatories (prednisone), drugs prescribed for bipolar disorder, and medications prescribed for hyperthyroidism.

Underdeveloped muscles. Muscles are our primary engine for metabolizing calories. Any physical, whether walking around your block or running up a mountain, will use more calories if there are more muscles doing the physical activity. Better muscle volume also increases our calorie burn when the body is at rest. Simply said, the bigger the muscles you have, higher the metabolism.

Dehydration. When your body doesn’t have sufficient water to function, it holds on to the fluid it does have. This causes fluid retention in the cells, which adds inches to our body.

Sugar substitutes. Be leery of the alternative sweeteners that brag zero calories. There are 2 reasons why they may promote weight gain to the unsuspecting dieter. A 2008 research study by Duke University discovered that Splenda contributes to weight gain and destroys good bacteria in the digestive track. Other research suggests that artificial sweeteners raise insulin levels, which causes the body to store fat.

Bad food combinations. There is medical research that shows that the ratio of carbohydrates, protein and fat directly influences our insulin production, which in turn, impacts how much fat our body burns or stores. This relates to every meal, including snacks. If you consume a snack of a rice cake with sugar free jelly, you are only eating carbs — no protein and no fat. Even though your calorie intake is merely 150 calories, your insulin levels will spike as though you had just eaten several donuts. The key to keeping insulin levels low is to always include a little protein and fat with every carb you eat.