Nighttime Fasting: A New Way to Lose Weight?

If you couldn’t let anything cross your lips from 5:00 p.m. to 9:00 a.m., but you were allowed to eat anything you wanted for eight hours a day and still lose weight, would you try it? That’s the apparent bottom line of a rat study published in the journal Cell Metabolism, which recently stirred up the weight loss pot. 

Scientists put groups of mice on different diet regimens for 100 days. One group of rodents ate healthy food while animals in two of the groups chowed down on high-fat, high-calorie feed. Half of the junk food eaters were allowed to munch whenever they wanted to while the others only had access to feed for the eight hours they were most active. The conclusion: even though they ate a fatty diet, the mice who were forced to fast for 16 hours were almost as lean as those who ate the healthy fare. Interestingly, the round the clock junk food eaters became obese and developed health problems, even though they consumed about the same amount of fat and calories as the time-restricted junk food fed mice. 

The researchers who conducted the study say that this single strategy: simply extending the nighttime fast is a cheap and easy weight loss approach free from side effects, but I’m not sure I agree. As a health professional my primary goal is always optimal health, so when I hear about studies that essentially send the message that you can eat poor quality food and still lose weight, I feel like it does a real disservice to consumers. Any time you lose weight, no matter how you do it, even the most unhealthy way possible, you'll see some positive health indicators, perhaps a reduction in cholesterol, blood sugar, blood pressure, etc. But long-term, to optimize energy, wellness, and even looks (hair, skin, etc.), the nutrients found in healthy foods need to show up for work day after day. 

Over the years I’ve met numerous clients who have lost weight eating restricted amounts of unhealthy food, but they struggled with side effects from dry skin and dull hair to bad breath, constipation, fatigue, crankiness, and a run down immune system. And if it was an approach they couldn’t maintain, they gained all the weight back.

Also, my private practice clients who eat meals at consistent times (breakfast within an hour of waking up and the remaining meals three to five hours apart) do far better long term than those who try to eat a larger breakfast, taper the size of the meals as the day goes on, and stop eating earlier in the evening. In my experience the latter just isn't sustainable or practical for most people. But eating a healthy dinner at say 6:00 p.m. and a healthy snack at 9:30 p.m., then going to bed at 11:00 p.m., keeps hunger from getting out of control, curbs cravings, fits better with most people’s social life, and can be sustained, which is the real key to losing weight and keeping it off. 

Many of my clients are long-term or even when we're not actively working together we're in touch regularly so I "follow" them for a long time, sometimes years. Seeing what really works for people after months or years, and what fizzles out, what makes people feel good, and what robs them of their energy, gives me a bird’s eye perspective that makes me skeptical of oversimplified approaches but I’d love to hear from you. What do you think? Would limiting your eating time to your most active eight hours of the day work for you? And do you think the quality of your diet is important? Please tweet your thoughts to @cynthiasass and @Shape_Magazine.

Nighttime Fasting: A New Way to Lose Weight? -2

Cynthia Sass is a registered dietitian with master's degrees in both nutrition science and public health. Frequently seen on national TV, she's a SHAPE contributing editor and nutrition consultant to the New York Rangers and Tampa Bay Rays. Her latest New York Times best seller is S.A.S.S. Yourself Slim: Conquer Cravings, Drop Pounds and Lose Inches.

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