There’s no best diet: Bodies respond differently to the same foods

There are many different ways to eat, and plenty of people who will tell you that their plan is the best way to stay fit.

But a new study is calling their bluff.

Scientists had 1,100 adults in the US and UK eat the same common foods (like muffins for breakfast and sandwiches for lunch), and tracked participants’ glucose levels before and after meals. The results showed that no two individuals’ reactions were the same — more evidence that there’s no such thing as a perfect, one-size-fits-all diet.

“Even we were surprised by the results,” Tim Spector, an epidemiologist and professor at King’s College in London who led the study, told Business Insider. “Just because some diet or recommendation is out there doesn’t mean that you fit it.”

The findings back up other recent work that undermines the conventional “eat this, not that” wisdom about dieting.

“On the personal level, we now know there is no diet or dietary intervention that is right for everyone, or even for an individual throughout their lifespan,” a different group of nutrition researchers wrote recently in The Lancet.

Study participants ate a lot of muffins

Spector presented some of his research— which was done in conjunction with researchers at Harvard and Massachusetts General Hospital — at the American Society of Nutrition conference this month. For the study, participants were given a “big pack of standard meals” to eat. Sensors called continuous glucose monitors were embedded under their skin to track their blood-sugar levels, and they were also given tool to measure glucose via blood samples collected by pricking their fingers. ( Continuous glucose monitors have only been on the market for about two-and-a-half years.)

The subjects — 60% of whom were identical twins — then tracked their reactions to what they ate for 14 days. They also wore wristbands to monitor their exercise and sleep habits, and took photos of all the food they ate during the study.

Nearly every morning for the two-week study period, the participants ate a muffin for breakfast.

“I know Americans love muffins for breakfast, but some of the Brits didn’t quite find it as good as their English breakfast,” Spector said.

Three kinds of muffins were on the menu: a high-fat, low-sugar muffin; a standard muffin with an average amount of fat and carbohydrates; and a third, so-called “light” muffin that had more sugar but less fat and was “easier to digest,” Spector said. The researchers wanted to know: Would all the participants’ insulin levels spike after they ate the muffins, or would the glucose monitors register only a tiny blip, reflecting a fairly stable insulin response? Would one of the muffins perform best across the board?

It turned out that it was hard to generalize. Some participants responded fine to foods like muffins, bread, or bananas, while others did not.

The researchers found that it was easier to predict how a person might react to a certain food based on their own previous glucose monitor readings, rather than any dietary guidelines meant to apply across a population. Fitness level was not a predictor of glucose response, either.

“There wasn’t a difference between sporty and non-sporty people that we could see,” Spector said. “There were many people who ran every day that had a bad fat response.”

It’s more evidence that everyone — even identical twins — is different when it comes to nutrition.

“We should be personalizing diets and not just trying to squeeze everyone into the same shoe size,” Spector said. “For most people, we can make basic recommendations about how they respond to carbs in general, or fatty foods.”

To follow up on these findings, Specter is launching another nutrition study in the US soon in conjunction with scientists at Stanford, Harvard, and Tufts. (The team is enrolling participants now.)

Spector said he lost 20 pounds in 3 years

The researchers hope the data they gathered about participants’ responses to different meals could help each participant determine which specific foods their body responds best to. It could even give them clues about which times of day to eat and exercise, based on the real-time measurements from their glucose monitors.

“The glucose monitors were recording every few minutes, automatically throughout the two weeks,” Spector said. “You couldn’t have done this study five years ago, the technology just wasn’t there.”

Specter also tried this glucose-monitoring strategy on his own body. He said his own blood-sugar readings seemed to spike every time he ate his go-to lunch of a tuna sandwich with grapes.

“All these tests showed that I responded really badly to bread and grapes,” he said.

Begrudgingly (Spector said he loves grapes and bread), he started incorporating more apples into his diet instead of grapes, since his body appears to respond better to those. Specter said he also now opts for carbs like rice or pasta in place of bread, and tries to include more nuts, seeds, and fermented foods like cheese and kefir into his meals.

So far, the changes appear to be working: Spector said he lost 20 pounds in three years.

He has also co-founded a personalized nutrition company based on this research called Zoe. The Zoe team has raised $27 million from investors, and wants to eventually use the results of this and future studies to develop an at-home test and app that could help people measure their bodies’ reactions to foods and tweak their diet and exercise patterns accordingly. (Zoe was involved in the new study.)

What you can do for your gut: Eat a wide variety of plants

For now, Specter’s findings suggest it’s hard to know how your body’s response to certain foods compares to that of others. But he said there are still a few things everyone can do to eat well.

One of the best ways to improve your health is to foster your microbiome— the microorganisms in your gut that help the body absorb nutrients from the food you eat and stave off disease. Scientists like Spector are increasingly finding that eating a variety of plants is important for a healthy microbiome.

“Most people in the US have non-diverse microbes and they could definitely improve their gut health,” Spector said. “We think that the more microbes you’ve got, the better your metabolism is.”

To boost your microbiome, it helps to incorporate more fiber into your diet (which is easy to get from fruit, vegetables, nuts, seeds, and whole grains) and consume an array of fermented foods.

“The more junk food you eat, the more [your microbiome] shrinks,” Spector said.

A 2018 study tracked the microbiomes of 10,000 people across the US, UK, and Australia, then compared people who ate 10 or fewer different plants each week (including soup ingredients and grains in bread) to those who ate 30 or more different plants.

Unsurprisingly, the plant-lovers were shown to have far healthier microbiomes.