When You Eat, Not Just What, May Impact Health

Reviewed by Michael W. Smith, MD on May 07, 2015
From the WebMD Archives

May 7, 2015 -- Watching when you eat, without necessarily changing what or how much, may yield big health benefits, including weight loss.

Although more research is needed, intriguing new findings in people and mice suggest that eating within a strict 8- to 12-hour time frame each day changes metabolism at the genetic level, lowering blood sugar and body weight, even without cutting calories.

Scientists think the changes may be powerful enough to lower the risks for cancer, heart disease, dementia, and diabetes.

The latest study on this phenomenon, called time-restricted eating, looked at the link between meal timing and blood sugar control in more than 2,200 women. The average age of women in the study was 47, and the average body mass index (BMI) was 28, making them overweight.

Poor blood sugar control is a risk factor for diabetes and cancer, among other things. Blood sugar that swings wildly before and after eating indicates that the body isn’t very sensitive to insulin, the hormone that signals cells to take in calories from food. That means more insulin has to be released from the pancreas to get the blood sugar into cells. The trouble is that extra insulin doesn’t just impact blood sugar. It also promotes the growth of cells -- including cancer cells. And over time, the body can’t keep up with the demand for more and more insulin. When that happens, blood sugar levels climb dangerously high, leading to diabetes.

The women in the study reported what and when they ate and gave blood samples. Researchers could see how high their blood sugar climbed after meals and how steady their blood sugar had stayed over the previous 2 to 3 months. About half the women reported not eating or drinking anything for at least 12 hours, from 6 p.m. to 6 a.m., for example. The other half fasted for less than 12 hours, eating both early and late.

“What we found, in general, [is] that women who fasted for longer nightly intervals had better blood sugar control than those who didn’t fast as long, and that was independent of other eating behaviors such as how many calories women were eating,” says study researcher Catherine Marinac, a doctoral candidate in public health at the University of California at San Diego.

“That’s similar to what was seen in animal models,” she says.

Interesting Animal Studies

Satchin Panda, PhD, an associate professor at the Salk Institute in La Jolla, CA, studies time restriction in mice and flies.

Panda took mice that share the same genes -- the mouse version of identical twins. He fed some of them normal mouse chow while others got high-fat food. In each group, some of the mice were allowed to eat whenever they wanted, while others only had access to food for 8 hours each day.

The mice that were fed normal chow, which was low in fat, ate at night, which is when mice are naturally awake. But the mice on the high-fat diet started eating all the time and seemed to lose their natural ability to feel satisfied. They quickly got fat and seemed to be on the road to diabetes.

But the mice that were time restricted didn’t become obese. That was remarkable, Panda says, because they were still eating the same number of calories as the others and getting those from a high-fat diet. He has since repeated those results with different kinds of high-fat and high-sugar diets. Each time, it turns out the same way. No matter how poor the diets of the time-restricted mice are, they don’t gain weight.

What’s more, Panda says, time restriction seems to act like a switch that improves blood sugar control and turns down inflammation, two major drivers of chronic disease. They also have better endurance and better motor coordination.

That makes sense to Mark Mattson, PhD, chief of the laboratory of neurosciences at the National Institute on Aging. He’s found evidence that going for long periods of time without eating benefits the brain. In mice bred to have diseases like Alzheimer’s and Parkinson’s, fasting seems to protect their brain cells. And fasting animals perform better when he’s tested their learning and memory in mazes.

Mattson says time restriction mimics the way our ancestors had to eat. Back in the hunter-gatherer days, food was scarce. Early humans had to hunt for hours before scoring their next meal. He says it wouldn’t make sense for their brains and bodies to become more and more sluggish the hungrier they got.

“If your brain isn’t working well when you haven’t eaten for a long time, then you’re going to be at a competitive disadvantage in being able to acquire food,” he says.

Mattson is getting ready to test another kind of time-restriction, one that alternates 2 days of fasting with 5 days of normal eating, in people with early signs of Alzheimer’s disease.

He’s so convinced that time restriction works, he’s doing it himself.

“I go sometimes 18 hours without eating,” he says.

Panda says most of the technicians who work in his lab are trying time restriction, too.

“Everybody that looks at these mice realizes what a profound impact this has on metabolism, body weight, etc.,” he says, “and that spurs them to want to do something. So I think almost everybody in the lab is affected by this.”

More Research Needed

Sadly, mice are not humans. And it’s harder than you might think to tease out the potential benefits of time restriction in people.

For one thing, the mice are eating the same food at every meal. That’s not the way people eat.

“In humans, the diet is very different at different times of the day,” Panda says.

In 2012, researchers at Brigham Young University in Utah asked 29 healthy, college-aged men not to change their diets, but to time restrict their eating. They stopped eating between 7 p.m. and 6 a.m. every day for 2 weeks, then ate as usual for 2 more.

During the time-restricted period, the men lost about a pound of body weight, but they also ate about 200 fewer calories than they did on the days when they could eat whenever they wanted. So it’s not clear whether fewer calories or less late-night snacking whittled their waistlines.

Another flaw of studying time restriction in people has to do with memory. The most practical way to study dietary habits in large groups of people is just to ask them to write down everything they ate and when they ate it over the last day or so. The trouble is that people can be bad at remembering that stuff, especially little things like coffee creamer and mayonnaise on a sandwich.

Even though it’s not perfect, sometimes it is the only way to test a question to see if it merits bigger, more expensive studies.

That was the case for Marinac’s study. She’s quick to point out that her study can only show links. She can’t yet say whether an extended overnight fast will lead to better blood sugar control.

But she’d like to find out. She says she’s writing a grant now in the hopes of funding a larger study to test time restriction in people. Researchers at Northwestern University in Chicago are also testing time restriction in people to see if it changes body weight and certain indicators of diabetes and cancer risk.

Experts are hopeful these future studies will connect a few more dots. They’d like to know, among other things, if it matters when someone starts their fast each day, and they hope to learn exactly how long a person should go without eating to see benefits.

“We’re kind of using 12 hours as a sweet spot. We think a 12- to 14-hour fast would be attainable, feasible," and would improve measures of metabolism, she says.

Show Sources


Satchin Panda, PhD, associate professor, Regulatory Biology Laboratory, Salk Institute, La Jolla, CA.

Mark Mattson, PhD, Chief, Laboratory of Neurosciences; Chief, Cellular and Molecular Neurosciences Section, National Institute on Aging, Bethesda, MD.

Catherine Marinac, doctoral candidate in public health, The University of California at San Diego, San Diego.

Cell Metabolism, June 6, 2012.

Cell Metabolism, Dec. 2, 2014.

Proceedings of the National Academy of Sciences, Nov. 25, 2014.

British Journal of Nutrition, December 2013.

Cancer Epidemiology, Biomarkers & Prevention, April 20, 2015.

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