(CNN) Have you made your New Year's resolution yet? If not, there's still time.

If last year is any indication, the majority of Americans will set goals pertaining to their health for 2016. The No. 1 resolution for 2015 was "stay fit and healthy" (37%), according to a January Nielsen survey -- 32% said they wanted to "lose weight."

"When we look at how consumers are planning to meet their health and fitness goals, what is surprising is how this year's resolutions stack up against last year's behavior," the Nielsen researchers wrote. "For example, 43% of Americans say they plan to lose weight by making healthier food choices, but 76% said they did not follow a weight loss or diet program in 2014."

Perhaps that's not at all surprising. It seems as though every few weeks, a different diet is touted as the best option, only to be bumped out of the top spot by another plan, sometimes with contradictory science and reasoning.

"Don't eat meat." "Don't eat dairy." "Low-fat is the answer." "Sugar is toxic."

It's no wonder Americans looking to lose weight don't know where to begin. Many aren't sticking with their diets because they're not achieving the results they want.

Now, a first-of-its-kind study is showing us something our experience told us all along: one size (or diet) rarely fits all.

'Healthy' foods differ by individual

An Israeli study, published last month in the medical journal " Cell ," concludes that even after eating identical meals, the way those meals are then metabolized differs -- sometimes dramatically -- from one person to another.

You've probably heard the term "glycemic index" (GI) touted on more than a few diet commercials. Simply put, GI measures how much a particular food is expected to raise your blood sugar. High blood sugar levels are closely linked to health issues such as diabetes and obesity.

The problem with the GI system is that it's based on studies that average how small groups of people respond to various foods. This recent study, out of the Weizmann Institute of Science, found that the GI of any given food is not a set value, but instead depends in large part on the individual who is eating it.

"Most dietary recommendations that one can think of are based on one of these grading systems," said Eran Segal, of Weizmann's Department of Computer Science and Applied Math. "However, what people didn't highlight, or maybe they didn't fully appreciate, is that there are profound differences between individuals. In some cases, individuals have opposite responses to one another, and this is really a big hole in the literature."

The study's authors gave the example of a middle-aged woman with obesity and pre-diabetes, who had tried and failed to achieve positive results with a rage of diets over the course of many years. She learned that her "healthy" eating habits may have actually been contributing to her weight problem. Her blood sugar levels spiked whenever she ate tomatoes, which she did many times over the course of the study, mistakenly thinking she was making a healthy food choice.

"For this person, an individualized tailored diet would not have included tomatoes, but may have included other ingredients that many of us would not consider healthy, but are in fact healthy for her," said Eran Elinav, of Weizmann's Department of Immunology. "Before this study was conducted, there is no way that anyone could have provided her with such personalized recommendations, which may substantially impact the progression of her pre-diabetes."

Why the drastic difference?

The researchers began by asking their 800 volunteers to complete health and lifestyle questionnaires, as well as undergo a handful of body measurements and blood tests.

Over the course of the week-long study, the participants were asked to honestly record their sleep, exercise and the food they ate using a mobile app food diary. They also had their blood glucose measured every five minutes, for the entire week, using a subcutaneous (under-the-skin) glucose monitor. Participant meal reporting closely matched the biometric data obtained from their glucose monitors.

Researchers then provided analysis of the participants' personalized responses to food. "Measuring such a large cohort without any prejudice really enlightened us on how inaccurate we all were about one of the most basic concepts of our existence, which is what we eat and how we integrate nutrition into our daily life," said Elinav. "In contrast to our current practices, tailoring diets to the individual may allow us to utilize nutrition as means of controlling elevated blood sugar levels and its associated medical conditions."

As it turns out, the key to understanding the vastly different ways our bodies process food lies deep inside our gut.

We each have trillions of bacteria in our digestive tract. In fact, scientists have calculated we have 100 times more bacteria in our gut microbiome than we have genes in our own genome.

In order to put this gigantic variable to the test, the Weizmann researchers conducted microbiome analyses on stool samples from each of the study's participants.

"Growing evidence suggests bacteria are linked to obesity, glucose and diabetes," Elinav and Segal said, "and the study demonstrates that specific microbes indeed correlate with how much blood sugar rises post meal."

That explains why tomatoes might be a perfectly healthy option for one person, but may sabotage even the best of intentions when it comes to the diet of another individual.

The power of personalized nutrition

"I think a lot of this is really about setting expectations, which is critical for success," said Dr. Melina Jampolis , a physician-nutrition specialist. "You can't just expect to lose weight by eating the exact same thing as your neighbor, husband, best friend or co-worker."

Her new book, " The Doctor on Demand Diet ," provides a customized eating, exercise and behavioral plan she says will optimize your chances of success, without forcing you to eliminate any major food groups.

"I can tell you from 15 years' experience that it is absolutely much harder for some people to lose weight than others," said Jampolis. "We are constantly uncovering more and more reasons why, including things like blood sugar response to foods, as discussed in this study."

"I think it also highlights the importance of taking the time to keep a journal, which can help you identify how you respond to different foods and really develop an optimal eating plan to satisfy hunger, maintain good energy levels and lose weight if you need to," she said.

To get started, Jampolis recommends moving toward a more plant-based diet, rich in fiber and less highly processed food, refined sugar, refined grains and saturated fat.

Emerging science surrounding the gut microbiome suggests it is a modifiable risk factor that can be genetically addressed through dietary intervention in as little as a few days.

"In addition, we know that regular physical activity, stress management and adequate sleep improves blood sugar control," Jampolis said.

And just like that, it all comes together, just in time for 2016.

Without all three health and wellness pillars of diet, exercise and sleep, you're never going to get healthy -- and stay healthy.

JUST WATCHED How to break bad habits -- and form new ones Replay More Videos ... MUST WATCH How to break bad habits -- and form new ones 02:17

"The beginning of the year is maybe one of the best times to try and make some new changes in your life, but it's not always easy, no matter when you start," said CNN's Chief Medical Correspondent, Dr. Sanjay Gupta

His first piece of advice: Try to disrupt your environment, even just a little bit. Whether than means letting go of people who are empowering bad habits or avoiding places where those bad habits occur, disrupt your environment and you'll be far more likely to claim victory with your New Year's resolution.