A new crop of digital health companies is offering consumers an unusual way to transform the way they eat, with the promise of improving metabolic health, boosting energy levels and achieving a personalized road map to better health. Their pitch: Find the foods that are best for you by seeing how they affect your blood sugar levels.

The companies, which include Levels, Nutrisense and January, provide their customers continuous glucose monitors — wearable devices that measure your body's glucose levels 24 hours a day, no skin pricks required. The devices transmit that data to your smartphone, allowing you to see in real time how your glucose levels are affected by your diet, sleep, exercise and stress levels.

The devices can show users which of their favorite foods can make their blood sugar levels spike and crash. They can reveal how engaging in regular exercise, or simply going for a short walk after a meal, helps to improve blood sugar control. For some people, the devices can provide warning signs of a risk of metabolic disease.

For people with Type 1 diabetes, continuous glucose monitors, which require a doctor's prescription, are considered the standard of care, freeing them from the burden of having to prick their fingers multiple times a day to check their blood sugar. But now digital health companies are using the devices to market programs that tap into the demand for personalized nutrition, a multibillion-dollar industry.

"We've had trackers for many other things like sleep, stress and fitness," said Dr. Casey Means, a surgeon who co-founded Levels. "But a continuous glucose monitor measures an internal biomarker like a tiny lab on our arms. This is the first time it's been used for a mainstream population for the specific purpose of making lifestyle decisions."

While most people know that eating sugary junk foods can wreak havoc on blood sugar levels, studies show that people can have a wide range of responses to foods. A 2015 Israeli study of 800 adults found that even when people ate identical foods, some people had blood sugar spikes while others did not.

And a 2018 study, researchers at Stanford University found that when they had 57 adults wear continuous monitors for two weeks, many people considered "healthy" saw their blood sugar soar to diabetic levels on frequent occasions.

"The nice thing about using a CGM is that it's an early way of catching what's going on, and it gives you a chance to change your behavior," said Michael Snyder, a professor in genetics at Stanford.

The programs are not cheap. The starting price for Levels is $395, which includes a telemedicine consultation and two glucose monitors that are programmed to run for 14 days each. Nutrisense offers packages that range from $175 for a two-week program to $160 a month. And January charges $288 for its introductory program.

Dr. Aaron Neinstein, an endocrinologist at the University of California, San Francisco, prescribes the devices to most of his patients with diabetes. And by wearing a CGM himself, he found, for example, that a type of soup that he regularly ate was causing a "surprisingly sustained elevation" in his blood sugar levels, leading him to cut back on it.

"There is so much unhealthy food all around us, and we're in an epidemic of metabolic disease," he said. "If people can use these devices to test different foods and get a little feedback on what are the behaviors that are making them less healthy, then that seems like a valuable thing to me."