More than two-thirds of adults in the United States are overweight or obese.

But most of those who are trying to lose weight are probably failing, given the bleak data on people who manage to keep off excess weight.

One of the many aids developed to help are weight loss apps for smartphones. Their usefulness is mixed, but the hunt is on to find more effective tools.

Drexel University psychology professor Evan M. Forman, co-director of the Laboratory for Innovations in Health-Related Behavior Change, and his team are working on two new apps that they hope will make the weight loss quest easier. One helps users figure out the specific triggers that lead to diet disaster; the other is aimed at retraining how the brain responds to specific foods. Both are being tested, and local volunteers are needed for the effort.

Why is losing weight so difficult?

If you're trying to lose weight, you need to burn more energy than you're taking in. We've pretty much worked out formulas to help people achieve that balance. We know it as a science, how to do it. And we can explain that to people. We can help them choose the right foods and the right kinds of physical activity.

One thing we don't know is how to help people adhere to the plan, to stick with it. This is very hard for people. One reason is that, biologically, we're hardwired to seek out food. As early humans, if any food was available, we needed to eat it right away. You never knew where the next calories would come from. But in the modern world, calories are only a few feet or inches away. Our biology, plus that kind of environment, means we're really in trouble.

Enter weight loss apps.

There's no shortage of them. It is estimated that there may be as many as 10,000. And we know from many, many studies that careful tracking is virtually required. If you're not tracking your physical activity and your intake consistently and accurately, you're very unlikely to lose weight and keep it off. The best apps are good at helping you track activity and food intake, and helping you set appropriate goals - how much food energy you should take in versus how much you expend.

But even the very best apps don't offer the tools necessary to meet goals.

Can you elaborate?

The apps are limited in their ability to give you the motivation and the commitment - and especially the accountability - that you need to be successful. Some try to do that. They give you visual tools so you can see your progress. Some give you reinforcing messages. And some have certain psychological or cognitive strategies for helping you make changes or deal with challenges.

However, most people end up slipping maybe once a day, maybe every other day. They either eat something that wasn't on the plan, or instead of having one portion of an entrée, they have double. Or at 10 p.m., they're eating ice cream. We know now that those slips add up. They explain why people aren't losing weight.

How is the new app you're working on different?

We are trying to figure out how to help people prevent those slips. Through some of our research and other people's research, we realized that when you slip isn't completely random; it follows a pattern. All sorts of things contribute to those patterns. Your emotional experience in any given moment - whether you feel anxious or bored or even happy - is one. It seems to matter whether you have skipped a meal, what time of day it is, how you slept the night before. And it varies somewhat from person to person.

So, with funding from the Obesity Society, the country's main scientific organization that's combating obesity, we created an app that took in all these factors - about 20 or so - part automatically, and part based on information the individual would provide by answering questions from time to time. It's called DietAlert. It synthesizes all this information mathematically and determines through a special kind of statistics whether you are about to slip from the plan. The first iteration is to make the app a companion to Weight Watchers, so people sign up for the plan and use the app to enhance it.

We also created a suite of interventions that we know from other studies can help people to prevent slips. At the point where the person is in danger of slipping from the plan, the app activates something we call a risk alert, telling the person, "You are at high risk right now of lapsing. Here's why. And here's what you should do about it." It's almost like someone tapping you on the shoulder right when you need it.

As scientists, we're interested in whether you can really make those predictions, and if you can, whether it helps people. So far, we're getting great results, and we're very excited about it. But we're still in a testing phase.

What's next?

Another app we are developing, DietDash, works through a completely different mechanism. It trains something called inhibitory control - basically, the brakes. It's been shown that that's a basic brain capacity. And if you practice very basic skills around stopping an impulse, you can get better at it.

If, for instance, someone eats ice cream and candy bars, we customize the program to show pictures of ice cream and candy bars. They then have to use "clicks" to respond as quickly to the pictures in various ways that train new brain responses to those foods. It's all more complicated than that, but in essence, what we're saying is that the training will change the response of the brain to that ice cream and candy bars at a deep level.

There's evidence it works for short periods of time. But can it for long periods? We're running our first study. Anecdotally, people have reported an amazing amount of success. But we need a lot more rigorous testing.

The idea that you could do a real quick brain exercise every day, and that it will make an important difference when it comes to what you eat, is obviously an exciting idea. If we can achieve that, it will be pretty remarkable.

The lab is seeking participants for both studies. Participants should be between 18 and 85 years old, looking to lose weight, and have an iPhone or home computer. For more information, to find out if you are eligible, or to sign up, email myhealth@Drexel.edu, call 215-553-7100, or visit Drexel.edu/weightloss.