Intermittent Fasting: Can you have your cake now and still lose weight?

Maintaining a healthy weight is not always easy in our world of fast food and super-sized portions. Dieting strategies come in all shapes and sizes and often come down to personal preference. Intermittent fasting -- a structured eating period that alternates between periods of eating and fasting -- is one that can allow you to enjoy the pleasures of eating all those French fries and servings of pasta and still lose or maintain your weight. While this might seem like a win-win situation, critics are concerned about potential adverse effects.

What is intermittent fasting?
Kerri Link Heckert, MS, RD, registered dietitian at Children's Hospital of Philadelphia,  says that intermittent fasting is more of a schedule than a diet.

"It could take the form of alternate days taking a full day off from eating or modified fasting like the 5:2 diet (5 days of regular eating, 2 days of only 500 calories) or time-restricted eating (set hours during the day for eating/fasting)," said Heckert, who also writes for the Goal Getter blog.

According to Heckert, while intermittent fasting does offer some benefits -- weight loss, improved HDL ("good") cholesterol, triglycerides and inflammatory markers, and reduced fat mass, reduction in meal frequency can also lead to increased blood pressure, LDL ( "bad") cholesterol, fasting glucose and delayed insulin response compared to eating more frequently

Research growing, but not conclusive
Researchers have been studying fasting for many years. Valter Longo, PhD, director of the Longevity Institute at the University of Southern California, has studied fasting in mice and found that two to five days of fasting each month reduced biomarkers for diabetes, cancer and heart disease. Later studies in humans demonstrated similar results.

Longo's work focuses more on prolonged and periodic fasting than what intermittent fasting entails, but it highlights the impact that fasting has on the body.

"We usually apply a 4 to 5-day fasting mimicking diet done every month to every 3-4 months, depending on need. We have evidence that periodic fasting is more effective, easier to do, and with less potential side effects," Longo said.

"The 4-5 days of fasting causes the body to switch to a mode that uses mostly visceral/abdominal fat for fuel. It also turns on cell's repair systems and kills bad cells to replace them with new ones by activating stem cells.

"We have done one clinical study on a 5-day fasting mimicking diet in which we showed that using it once a month for 3 months lowers risk factors for diabetes, cardiovascular disease and cancer in humans. In mice, starting the [fasting mimicking diet] at middle age reduced cancer by nearly 50 percent, reduced inflammation including dermatitis, increased bone density and relative lean body mass while reducing abdominal fat. It also increased cognitive performance in old mice and extended the lifespan by 11 percent."

Researchers at the Intermountain Heart Institute in Murray, Utah, also found that, during periodic fasting, the body converts LDL in fat cells to energy, reducing diabetes risk factors.

Other research has focused on intermittent fasting. A 2011 study that compared the 5.2 program to a regular low-calorie diet found that those in the intermittent fasting group lost slightly more weight and belly fat, and retained more muscle.

In 2013, there was a study on intermittent fasting published in the British Journal of Vascular Disease. The researchers found that not only does intermediate fasting promote weight loss in older individuals but it may reverse diabetes and offer cardio protection.

Still, critics who are concerned about possible dangers of intermittent fasting call for more research to be conducted.

Intermittent fasting is not for everyone
According to Cynthia Sass, MPH, RD, author of Slim Down Now: Shed Pounds and Inches with Pulses -- The New Superfood, how intermittent fasting works depends on who is doing the fasting.

"Some research shows it can help blood pressure, insulin sensitivity, and inflammation, but this is generally in overweight adults with conditions like metabolic syndrome. Some of my non-athlete clients have had good results, but for others, intermittent fasting has triggered food cravings, crankiness, obsessive thoughts about food, and rebound overeating, as well as poor sleep, and the loss of muscle," Sass said. "Also, it may lead to dehydration, low blood sugar, fatigue, and acid reflux."

Sass does not believe that intermittent fasting is ideal for athletes and that it may prevent recovery from exercise, which can increase the chance of injury risk, as well as limit adequate fuel for exercise, which can reduce energy, strength, and endurance.

"I think it's highly individual.  I think it's important to know your personality. For example, if you're a person who tends to focus more on food when it's restricted, it may backfire for you. I've had clients tell me they got out of bed in the middle of the night and binged after attempting an overnight fast, or binged the day after a fast. Others don't have that issue though."

In a report for ABC News, Sass highlighted four possible unwanted effects from intermittent fasting: rebound overeating, poor sleep -- which in turn can increase cravings for sweet and fatty foods -- fewer nutrients, and muscle loss. Restricted calories mean less of the good stuff like veggies, fruit, lean protein, and healthy fats and carbohydrates.

Growing in popularity
Unlike Sass, Evan Shaulson, Unite Fitness group coach and personal trainer, is an advocate of intermittent fasting including for athletes and dedicated exercisers.

"Intermittent fasting lets you eat big, satisfying meals even while at a calorie deficit.  If you're only eating for 4-8 hours of the day, you get to eat your entire day's allotment of calories in that window," Shaulson said. "That means big meals, delicious foods, and keeping your favorites in the dietary lineup."

According to Shaulson, "professional athletes, celebrities, and even endurance competitors swear by intermittent fasting. The one major exception is for people who exercise first thing in the morning.  Early morning training sparks up a pretty big appetite and the last thing you'll want to do after an intense workout is to fast for six hours."

"That being said, it's perfectly safe to exercise fasted, and some people [like me] actually prefer to exercise fasted.  There's even evidence to suggest that cardiovascular exercise while fasted may promote fat loss beyond what is possible with food in the system."

Loss of muscle is not an issue either, Shaulson said. "Fasting has actually been shown to boost growth hormone levels which acts to preserve muscle tissue at the expense of fat."

Shaulson acknowledges that people may face certain pitfalls when trying intermittent fasting for the first time: "I want to emphasize how important the adjustment period is.  In order to make the biological adjustment possible and tolerable, you have to ease into it. "

He recommends starting by skipping breakfast and having your first meal later in the day. Then gradually reduce the size of your earlier meals until it feels like you don't need them.

For Shaulson, the biggest risk with intermittent fasting is not eating enough calories to stay healthy. So it is crucial to keep track of what you are consuming during your eating periods. He also advises that it is not suitable for children, people with diabetes or who are prone to hypoglycemia, females with low body fat percentages who are prone to irregularity of their period, and anybody with eating disorders, especially binge eating and bulimia.

"Personally, intermittent fasting has been amazing for me," Shaulson said. The main reason I do it is to make my lifestyle more enjoyable.  Since I'm a trainer, I like to keep my body lean and muscular.  Most people think that it must be easier for me to maintain a low body fat percentage all year round, but it's actually not.  I'm very prone to overeating and gaining weight."

By saving a majority of his calories for later in the day, Shaulson is able to eat his favorite foods, and a lot of them, every day.

"The approach I've crafted for myself isn't exactly "The Warrior Diet" or "The Eating Window", but it's the combination that's right for me. Finding the most enjoyable way to do this becomes something of an art form, and once you've found your rhythm, every day becomes incredibly satisfying."

Here is Shaulson's typical routine:

Shaulson explained that this strategy leaves him feeling full and completely satisfied after eating less than 2,500 calories.

"The key thing to remember is that you're trying to find the most enjoyable way to maintain an appropriate calorie intake so you can reach your fitness goals," Shaulson said.

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