Fasting vs. Calorie Restriction
People are always on the lookout for quick and easy ways to lose weight, which is why there are approximately 5 billion new diet books written every year — that’s our unscientific, but probably not inaccurate, estimate. Speaking of unscientific, the problem with the plans in these books is their so-called “new” approaches to weight loss haven’t been tested, well, scientifically. And when they get scientists to pay attention, the plans often fail to produce the stellar results they claim.
One weight-loss method that’s gotten a lot of attention lately is fasting, where people go without eating for several hours (usually 16 or more) or even a full day. Search the internet and Reddit boards, and you’ll find passionate devotees talking about the advantages of fasting for weight loss, muscle gain and other benefits, like improved mental clarity. And while there isresearch to back up some of these claims, much of it is preliminary or was conducted on animals.
Lately, researchers have been looking into fasting more deeply and are starting to fill this knowledge void. A study published this May in the Journal of the American Medical Association: Internal Medicine sought to find out whether fasting was any better or worse at helping people lose weight than a more traditional diet. Here’s what they discovered:
WHAT DID THE STUDY EXAMINE?
The study looked at the effectiveness of two diets:
One was alternate-day fasting, where subjects ate a very small number of calories (25% of their daily energy needs) on “fasting” days, and an excessive number of calories (125% of their daily need) on “feasting” days.
The other program was more like what most of us imagine when we think of dieting: a calorie-restriction model where people aimed to eat slightly less than their energy need (75%) every day.
WHO TOOK PART IN THE STUDY?
One hundred people total: 86 women and 14 men, aged 18–64 (the mean age was 44). They were obese, but did not have metabolic diseases (like Type 2 diabetes or metabolic syndrome). Their cholesterol counts and blood pressure all fell within the normal range. The subjects were divided into three groups — those who were put on an alternate-day fasting plan, those who were put on a calorie-restriction plan and those who received no intervention. Changes in their bodies were rigorously tracked with researchers using a digital scale (DEXA machine) and MRI scanner to record subjects’ weight and body fat throughout.
HOW LONG DID THE STUDY LAST?
A full year. The first six months were a weight-loss phase, and the second six-months were a weight-maintenance phase.
Both the alternate-day fasting and caloric restriction groups lost about the same amount of weight — the fasters dropped and kept off 6% of their body weight on average, while the calorie-restrictors came in at a 5.3% loss. Those in the fasting group struggled with adherence, however. There were slightly more dropouts in the group (13 of 34 fasters quit, compared to 10 of 35 bowing out of the caloric-restriction group). The fasters often ate more than their target goal on fasting days, while consuming less than the allowed mark on feasting days.
The researchers also monitored the subjects’ cardiovascular health through markers, like their cholesterol counts. That’s where one curious result occurred, which researchers cannot explain: By the end of the second six-month interval, those in the fasting group had experienced an increase in their LDL cholesterol. (LDL is what we think of as the “bad” cholesterol.) The caloric restriction group did not.
WHAT ARE THE TAKEAWAYS?
If your goal is to lose weight, fasting is an option you can try. It appears to work about as well as daily caloric restriction. Alternate-day fasting, which this study examined, is one option. Other popular ones include a 12-hour daily fast, a 16-hour daily fast (often called the “Leangains” diet), a 20-hour daily fast (known by many as the “Warrior” diet), or a 24-hour fast once per week.
Keep in mind that, prior to this study, a lot of the most compelling research showing body composition benefits of fasting was performed on “hard gainer” young males (i.e. guys who have a hard time building muscle). The results may not translate so well to other populations.
One group of people who should be especially careful with fasting is women. The approach can cause a cascade of hormonal issues in women, including thyroid imbalances and menstrual cycle issues.
If you try a fasting diet, do so in consultation with your doctor, especially if you have any cardiovascular risk factors like high cholesterol or blood pressure. Monitor both of them closely. And, perhaps most important, pay attention to how you feel throughout. The body often sends signals when something is going right or wrong. It’s our job to tune in to them.
Remember that no diet approach is magical. What matters is what works for you!