Scientists are learning more about why keeping off lost weight is so hard — and a pair of studies suggest it's more than a question of sheer willpower.

A Dartmouth University study published in Cognitive Neuroscience used brain imaging tests to show that some people have an imbalance between the executive control and reward systems of the brain. This disconnect means you may not have enough natural control over the impulse to reach for food when you're stressed or even when you're happy. It also seems more likely to be the case in people who are chronic dieters.

The other study was done by the University of Michigan and the U.S. National Institute of Diabetes and Digestive and Kidney Diseases, and published in the journal Obesity. It found that the body's internal protection against starvation encourages eating specifically so that you'll regain any lost weight — and at a rate of 100 calories a day for every kilogram or 2.2 pounds dropped. Since that's much more than the average person would naturally burn off, the end result is weight gain.

This doesn't mean your weight loss efforts are doomed, but you will need to make a strong effort to overcome these forces. It will likely take a combination of healthy lifestyle habits and a commitment to practicing mindfulness to squash the emotional urges to eat.

Research published in the American Journal of Preventive Medicine found that maintaining a loss often requires a different approach than you used to lose the weight.

The researchers looked at the habits of 1,165 people and found some intriguing contrasts. People who followed a consistent exercise routine or ate an abundance of low-fat protein foods were better able to maintain a weight loss, but these techniques did not work for losing weight. On the other hand, people who said they did many different kinds of exercise or planned their meals ahead of time found that such steps helped them lose weight, but weren't factors in weight-loss maintenance.