What to Know Before You Try a 10-Day Fasting Diet — and a Safer Way to Fast

Google the "10-Day Fasting Diet" or a "10-day fast" and chances are you won't find one way to follow this so-called diet plan. Fasting, of course, inherently means abstaining from eating and drinking calories — but you can find countless tweaks and adaptations in modern fad diets.

Lately, one modern adaptation called intermittent fasting (which itself has many versions) has grown in popularity. This type of fast is generally safer, less extreme and fairly easy to follow compared to a 10-day fasting diet, which overall, it seems, is typically where followers temporarily eliminate food and stick to drinking clear liquids.

Below, I dig into why this isn't the best route if you're looking to lose weight and keep it off.

You might find people who have tried a modified version of the Daniel Fast — a 21-day detox-esque plan with Biblical roots where followers eat vegan — for just 10 days. You'll also find 10-day fasts that are far more restrictive, including one where followers skip food altogether and subsist only on water — no easy feat, and medically not exactly recommended, but more on that below.

Other 10-day fasters adhere mostly to the principle of water only, but also allow themselves black coffee or maybe even some bone broth. Still other groups drink juice for a week and a half — which sounds like a juice cleanse by another name to me.

Not much! You'll be drinking a lot of water and possibly other liquids like coffee or bone broth. Some followers, however, only drink water.

Wherever you fall on the clear liquid spectrum, you will forgo all other food for 10 days.

In a word, yes. Stick to water — or other clear liquids — for a full 10 days and you'll absolutely lose weight, and quickly. One blogger who fasted for 10 days touted they lost 12 pounds. A Reddit user claims to have lost 24 pounds on a 10-day fasting diet.

In fact, look back at our history of obesity treatment and you'll find that water-only fasting was used in the 1960s and early 1970s. But when research on the topic exposed serious complications, including deaths in some cases, a water-only diet fell out of vogue and research apparently practically halted, as summarized in a 2018 BMC Complementary and Alternative Medicine research article.

Those complications also included fatigue, nausea, insomnia, headaches and more. This isn't all too surprising, but a water-only fast has the potential to be quite dangerous.

People who fasted longer were more likely to report negative side effects, according to the BMC Complementary and Alternative Medicine review. Plus, the fasting studies reviewed were conducted under medical supervision — a must if this kind of fasting is a "diet" you'd like to try.

1. It could help you and your doctor manage certain health conditions: Water-only fasting is thought to be effective for some therapeutic purposes (think: lowering high blood pressure or as an adjunct to some chemotherapy), but there isn't all that much solid scientific data on this form of fasting or the specific 10-day duration.

The science behind fasting is growing a bit, though: An observational January 2019 study in PLOS One found some notable health benefits. Folks who fasted anywhere between four and 21 days lost weight, trimmed down their midsections, improved their blood pressure and even reported better emotional wellbeing.

In this particular study, however, fasting still meant eating up to a total of 250 calories in the form of a mid-day juice and an evening vegetable soup.

1. The side effects are real: In the BMC Complementary and Alternative Medicine article, the most common side effects of fasting were fatigue, nausea, headache, high blood pressure, light headedness, heart burn and back pain. And remember, the researchers in this study found that the longer participants fasted, the more likely they were to experience negative side effects.

2. You'll likely regain the weight: You're more likely to gain back the weight you lose while fasting than you are to keep it off.

In a March 2010 study in Physiology and Behavior, researchers at Cornell University found that women who fasted for a single day weighed less the next day, but then regained the lost weight within four days, even though they didn't up their calorie intake to compensate for that one day of fasting. In other words, they didn't overeat in the days following their fast, but they still regained the lost weight.

This is, of course, just one study, but it exemplifies an unfortunate truth about weight loss: When you lose weight, your metabolism slows to compensate for your smaller size, according to a September 2017 report in Perspectives on Psychological Science.

Then, when you return to your prior eating habits, you will likely regain what you lost — unless you compensate for your slowed metabolism and lower your total calorie intake or are active enough (aka burn enough calories) to make up for the discrepancy.