NEW YORK — 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 personalised 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 — sleek, wearable devices that attach to your arm and 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 in real time which of their favorite foods and snacks can make their blood sugar levels spike and crash, leaving them feeling tired and sluggish after meals. They can reveal how engaging in regular exercise, or simply going for a short walk after a big meal, helps to improve blood sugar control.

And for some people, the devices can provide warning signs that they may be at increased risk of developing Type 2 diabetes and other forms of metabolic disease.

Continuous glucose monitors were developed decades ago to help people with diabetes manage blood sugar. For people with Type 1 diabetes, the devices, 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 growing demand for personalised 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 and serves as its chief medical officer.

“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 like cookies, cake and soda can wreak havoc on their blood sugar levels, studies show that people can have a wide range of responses to many foods.

In one intriguing study from 2015, researchers in Israel followed 800 adults for a week, using continuous monitors to track their glucose levels. They found that even when people ate identical foods — such as bread and butter or chocolate — some people had substantial blood sugar spikes while others did not.

The researchers concluded that a variety of factors unique to every person, such as your weight, genetics, gut microbiome, lifestyle and insulin sensitivity, determine how you respond to different foods.

In general, health authorities consider a healthy fasting blood sugar level — measured after an overnight fast — to be below 100 milligrams per deciliter. It is normal for blood sugar to rise after meals.

But in a 2018 study, researchers at Stanford University found that when they had 57 adults wear continuous glucose monitors for two weeks, many people considered “healthy” by normal standards saw their blood sugar soar to diabetic levels on frequent occasions, a signal that they might be on the road to developing Type 2 diabetes.

Other research shows that such large blood sugar swings are linked to heart disease and chronic inflammation, which is increasingly thought to underlie a wide range of age-related ailments, from heart disease, diabetes and cancer to arthritis, depression and dementia.

The programs, which are not covered by insurance, are not cheap. The starting price for Levels is US$395, which includes a telemedicine consultation and two Abbott FreeStyle Libre glucose monitors that are programmed to run for 14 days each.

Nutrisense offers a variety of packages that range in price from US$175 for a two-week program to US$160 a month for an 18-month commitment.

And January charges US$288 for its “Season of Me” introductory program that includes two glucose monitors, a heart rate monitor, and access to the company’s app for three months.

But are they worth it?

To get a better sense, I signed up to use the Levels program for one month. As a health reporter who writes about nutrition, I try to follow a fairly healthy diet and exercise regimen with plenty of fresh foods and few junk foods or sugary snacks, so I wasn’t expecting to learn much from the program. But I kept an open mind.

To get started, I filled out a brief health questionnaire online. Then Levels shipped me two FreeStyle Libre glucose monitors, which were prescribed by a doctor affiliated with the company.

As instructed, I attached the device — a small patch with a tiny sensor about the size of a human hair — to the back of my arm. The sensor measures “interstitial fluid” beneath the skin, which it uses to estimate blood sugar levels.

The monitor helped me identify foods that I had no idea were spiking my blood sugar, like protein bars and chickpea pasta. But through trial and error, it also helped me find alternatives. One day I ate a salad with grilled salmon and noticed that it caused my blood sugar to soar. I soon realised why: I had drenched my salad in balsamic vinegar, which, it turns out, contains a lot of sugar.

The next day I repeated the meal but with red wine vinegar, which contains no sugar. The result? My continuous glucose monitor showed there was no blood sugar spike or crash.

Dr Means said that people are often surprised to learn just how much sugar is hiding in their foods, especially in things like sauces, condiments and dressings.

But not everyone is the same, and people learn tricks, such as pairing carbs with protein or fats — for example, by adding almond butter to oatmeal or an apple — to blunt the blood sugar response to certain foods.

The monitor also reinforced the value of exercise. I noticed on days when I went for a run, or even a 15-minute walk, that the physical activity helped to keep my blood sugar in a steady range after meals. THE NEW YORK TIMES