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“Don’t eat less than 1200 calories.” “Eating too few calories will slow metabolism.” We’ve heard this diet advice for years, including from Cooking Light’s dietitians. Yet, short-term fasts where calories are cut as low as 500 calories are increasingly popular. New research suggest they may actually speed up metabolism. So, what’s the real story on fasting and metabolism?

Carolyn Williams, PhD, RD
January 09, 2017

Fasting refers to a period of time where little to no food is consumed. It's been done for thousands of years, but a new form of fasting referred to as "intermittent fasting" has gained popularity in the diet world thanks to books like The Fast Diet and the The 8-Hour Diet. Intermittent fasting centers around alternating between short periods of fasting and periods of regular food intake.

Types of Intermittent Fasting

The 5:2 and 16:8 approach are the most common types of intermittent fasting and differ in their daily approach.

  • 5:2 Fasting Method: Two non-consecutive days out of the week are considered "fast days" in which only 25 percent of daily calorie needs are consumed (about 500 for women and 600 calories for men). On the other five days, you eat at your normal calorie level. Some have taken this method one step further by doing a 4:3 approach (also known as alternate day fasting) where a "fast day" is alternated with a day of regular intake.
  • 16:8 Fasting Method: All daily calories are eaten within an 8- to 10-hour window that you choose, but nothing is eaten the other 14 to 16 hours in the day. People often skip breakfast and choose an 8-hour eating window like 11 a.m. to 7 p.m.

Weight Loss and Health Perks of Fasting

Weight loss is the primary motivator for many to try intermittent fasting. Reports of weight loss of 3 to 8 percent of body weight lost in 3 to 24 weeks have been reported. For a 200-pound person, this is approximately 6 to 16 pounds. While weight loss from fasting isn't debated, research studies vary as to whether intermittent fasting is an effective long-term dieting method.

There are other health perks though that appear to come from intermittent fasting. When compared to traditional dieting, research suggests that intermittent fasting:

  • Causes greater fat loss in abdominal area
  • Causes less loss of lean body mass
  • Improves cholesterol numbers and fasting insulin
  • Decreases inflammatory markers.

Metabolism Perks

Surprisingly, research suggests that the effect of intermittent fasting has the same or less negative effects on metabolism compared to traditional dieting. The reason why many think intermittent fasting improves metabolism is due to less loss of lean body mass and greater fat burning. It's impossible to lose weight without losing a little lean body mass, but research suggests that a lower percentage of lean body mass is lost when losing weight with intermittent fasting than with traditional dieting. Preserving more lean body mass means the body's calorie-burning slows less. At the same time, short fasting periods cause the body to tap into fat stores and burn a greater percentage of fat mass for energy.

The Keys to Making It Work

There are two major keys to intermittent fasting success: short fasting periods and "normal" calorie intake.

  • One secret behind this fasting method is that extremely low- or no-calorie intake is only for short periods. If fasting continued without a return to normal calorie intake, then the body's metabolism will slow down. But, short periods like the 5:2 or 16:8 approach appear to maintain (and possibly temporarily increase) calorie burning. The constant flux of fasting and normal intake keeps the body on its toes, rather than allowing it to slow.
  • The other key to success is not equating normal intake to binging or excessive intake. "Normal intake" refers to taking in the body's estimated calorie needs with a healthy variety of foods. It doesn't mean taking in excessive calories or binging on favorite foods.

What Hurts Metabolism

The saying that "if a little is good, then a lot will be better" does not apply to fasting. Though you may be motivated after completing a fasting day to continue with very low calories, the body needs a "normal" calorie day to keep metabolism stoked and to continue burning fat. If extremely low calorie intake is continued day after day without break, then metabolism does appear to slow as the body eventually goes into "starvation" mode.

Overall Verdict

Intermittent fasting appears to be a relatively safe—and possibly more effective—way to diet, as well as improve health. It's not for everyone though, so don't beat yourself up if the thought of 500 calories seems dreadful. Long-term weight loss results from healthy intake that is sustained, so choose an eating plan that works for you.

References:

  1. Intermittent Fasting and Human Metabolic Health
  2. Alternate Day Fasting for Weight Loss - greater weight loss, preserves LBM
  3. Alternate-Day Fasting vs. Daily Energy Restriction - much greater fat loss, while preserving LBM