Jennifer Aniston and Reese Witherspoon are intermittent fasting fans - but is it safe?

Jennifer Aniston and Reese Witherspoon credit fasting for their youthful looks. [Photo: Getty]

Jennifer Aniston and Reese Witherspoon are almost as famous for their youthful looks as their successful acting careers.

Now, the actresses have revealed how they stay looking so young without resorting to expensive skincare, Botox or surgery.

But their gruelling routines are not for the faint hearted.

Jennifer, 50, who is most famous for her role on ‘Friends’, fasts for 16 hours a day by opting not to eat anything between dinner and lunch the following day. Even on her “days off”, the actress still starts with a celery juice and no breakfast.

Mother-of-three Reese, 43, who became a household name after playing Elle Woods in ‘Legally Blonde’, also goes without food first thing, getting by on just a coffee and green juice.

Reese Witherspoon and Jennifer Aniston speak during an event launching Apple tv+ at Apple headquarters on March 25, 2019 [Photo: Getty]

Speaking of her daily regimen, Jennifer told Radio Times: “I do intermittent fasting, so no food in the morning. I noticed a big difference in going without solid food for 16 hours.”

Jennifer and Reese met after the Academy Award winning actress starred as her younger sister Jill in series six of ‘Friends’.

The pair quickly became close, with Reese even calling on Jennifer to keep her looking and feeling her best.

“Jen knows so much about health and fitness that I always defer to her,” she said.

READ MORE: Everything you need to know about the 16:8 diet

After her green juice and coffee, Reese reportedly has a 7.30am work out, which she does six times a week.

Less of an early bird, Jennifer rises at around 9am, before exercising five times a week. Even on her non-fasting days, the actress still “enjoys” a celery juice, work out and morning meditation.

Speaking to Radio Times, the famous pair had never even heard of a fry up.

Reese Witherspoon and Jennifer Aniston at ELLE's 18th Annual Women in Hollywood Tribute in 2011 [Photo: Getty]

How does intermittent fasting work?

Fasting has garnered a lot of media attention over the past few years for its effect on weight loss.

While past diets focused on cutting calories and avoiding sugar, fasting places more emphasis on when, rather than what, you eat.

“Essentially, fasting in its various forms is about calorie restriction which will inevitably lead to weight loss in most cases,” Rob Hobson, registered nutritionist at Healthspan, told Yahoo UK.

Perhaps the most famous is the 5:2 diet, created by TV medic Dr Michael Moseley.

This involves fasting for two days a week, with people being advised to cut their calorie intake to just a quarter of what they would normally eat, around 500 a day for women and 600 for men. Some even go all day with no food at all.

On the remaining five days, the dieter is free to eat as normal.

By the end of the week, fasters should have consumed less, or burnt more, calories than they took in, resulting in weight loss.

Similar to the 5:2 diet, alternate day fasting has people eating normally one day and seriously cutting calories the next.

READ MORE: Naomi Campbell can go more than a day without eating. Is this healthy?

“The 16-hour fast is about limiting the time available to eat so by proxy reducing your calorie intake during the day,” Mr Hobson said.

“This is the easiest of fasting techniques to follow as you can eat your evening meal early (6pm -7pm) and then your first meal the following day at 10am-11am.

“You can drink tea, coffee, herbal teas and water up until your first meal to stave off hunger. This method may be difficult for night owls or people with a busy social life that involves eating late into the evening.”

While it may sound like a lot of willpower is required, advocates claim knowing a tasty, “normal” meal is just a few hours away encourages them to stick to their fast.

But, critics argue simply cutting the number of calories consumed over the day, but otherwise eating normally, has the same weight-loss effect.

A study by the University of California, Berkeley, found obese rats lost just as much weight with alternate fasting as simple calorie reduction. The two approaches were also equally as effective at lowering the animals’ insulin and blood-sugar levels.

However, some maintain going “cold turkey” and abstaining from food all together, for a set period, is easier than counting calories.

Scientists from the Pennington Biomedical Research Center in Baton Rouge, Louisiana, had 22 non-obese adults fast every day for 22 days. At the end, the participants lost up to 2.5% of their body weight.

When it comes to diets like the 5:2, a team from the University of California, San Diego, looked at nine studies that had participants fast for several days a week. They found in seven of the trials, fasters lost up to 8% more weight than the non-fasting “controls”.

The diet was also linked to lower levels of “bad” cholesterol, as well as reduced blood sugar, insulin and inflammation.

READ MORE: Intermittent fasting: How to follow the scientifically-proven weight loss method

Studies into time restricted fasting, à la Jennifer and Reese, have been equally as positive. A study by Brigham Young University in Utah found a group of 29 men lost around 2.1% of their body weight after going without food for 11 hours a day over two weeks.

Similarly, a team from the US Department of Agriculture found dieters who consumed just one meal in the afternoon for eight weeks lost 4.1% more weight than those who ate as normal three times a day.

Is intermittent fasting safe?

It’s important to note that fasting isn’t suitable for all. Experts recommend pregnant women and those with pre-existing conditions, like diabetes, talk to their GP first.

While studies suggest fasting is effective, followers are advised to stay hydrated throughout the regimen. When they do eat, fasters should ensure they consume a range of foods to prevent nutritional deficiencies. They should also not overindulge to avoid undoing all their hard work.

When starting a fast, dieters should expect to feel hungry, as well as possibly battling a host of other unpleasant symptoms.

“Fasting days can cause tiredness and affect your ability to focus on daily tasks,” Mr Hobson said. “Training on fasting days may also be difficult as glucose stores become depleted.

“However, if planned with a little thought, you can prepare meals and snacks that are low calorie whilst high in fibre and protein to maintain fullness between meals.

“High-water foods such as soups and stews are useful dishes to prepare on fasting days. Be aware this approach does not mean you can eat what you want on the other five days. The remaining days should be made up of a healthy balanced diet.”