5 December 2017Intermittent Fasting for Weight Loss: Alternate-day fasting and time-restricted feeding

What is the diet?

Intermittent fasting is an umbrella term that covers different types of periodic fasting patterns for the purpose of losing weight and/or improving blood glucose, blood lipids (fats), and other markers of cardiometabolic health.

The two most common types of intermittent fasting diets that have been studied are:

  1. Alternate-day fasting
  2. Time-restricted feeding

Both alternate-day fasting and time-restricted feeding eating patterns include a daily fasting period of at least 12-hours during a 24-hour day.

Alternate-day fasting

Alternate-day fasting is basically what it sounds like - you eat freely on one day, and then the next day, you fast. Most of the research I found on alternate-day fasting defines fasting as:

Intake of noncaloric beverages and water
+
Intake of 25% or less of maintenance calories, typically in the middle of the day

For example, for a person who needs 2000 calories to maintain their weight with their regular level of activity, their fasting day would be 500 calories or less. On their free day, the person eats as desired.

The popular 5:2 Diet is a version of alternate-day fasting. The difference is that there are only 2 fasting days (usually 500 calories/day for women and 600 calories/day for men) instead of an average of 3.5 fasting days per week. The 2 fasting days are non-consecutive, meaning, not back-to-back. You can learn more about this particular diet and another dietitian's review of it in WebMD's The Fast Diet.

Time-restricted feeding

Time-restricted feeding has a number of different patterns, but the common element is to include a minimum 12-hour fast between the last meal of the day and the next meal the following day. In this case, fasting means just consumption of noncaloric beverages and water. An example: if you finish your dinner at 6 PM on Tuesday, then your next meal/snack is no earlier than 6 AM the next day. One common pattern is to restrict calories intake to only 8 hours during the day (e.g. between 10 AM - 6 PM) and then fast for 16-hours.

Both alternate-day fasting and time-restricted feeding eating patterns include a fasting period of at least 12-hours in a 24-hour day.

Safety Issues

Unlike daily fasting for multiple days in a row or longer, it appears that intermittent fasting patterns are safer, are less likely to cause nutritional deficiencies, and do not appear to cause significant loss of lean body mass. Hoddy et al. published a study in 2015 that found alternate-day fasting had either "benign or beneficial effects on eating disorder symptoms" in their adult obese study participants. This is important since you would not want to follow a diet pattern that could increase your risk for developing an eating disorder.

I would like to emphasize that intermittent fasting has mostly been studied in overweight/obese adults who are otherwise healthy.

Intermittent fasting is not appropriate or safe for:

  • People with a history of eating disorder
  • People with a high risk for eating disorder
  • Children and teens
  • Women who are pregnant or breastfeeding
  • People who have had weight loss surgery (bariatric surgery)
  • Anyone with a high risk for hypoglycemia (low blood glucose)

Pros

  • Weight loss equivalent to daily calories restriction
  • Possibly weight loss without loss of lean body mass (and improved fat loss)
  • Calories tracking is not necessary
  • Enjoyment of free eating days
  • No restriction of specific food groups
  • Possible improvement in:
    • blood glucose
    • blood lipids (total cholesterol, LDL, HDL, and triglycerides)
    • IGF1 level (elevated IGF-1 in adults can increase risk for stroke and certain types of cancers)
  • Cons

  • Excessive hunger
  • Bad breath
  • Possible headache, lightheadedness, irritability, dizziness and reduced coordination
  • Fasting can trigger eating disorder behavior in people with a history of eating disorders
  • Fasting can trigger eating disorder behavior in people with a high risk for eating disorders
  • Not a safe choice for children and teens who need to lose weight
  • More safety data is needed for different medical conditions and for the overweight elderly
  • Timing fast days so that it doesn't interfere with work or training
  • Could increase hypoglycemia in people with diabetes who take insulin or a sulfonylurea
  • Cost

    There is the potential for lower food costs since there is periodic fasting.

    Social Support

    If you follow an intermittent fasting program on your own, then you will have to find and develop your own support network. Online support is an option. MyNetDiary has a Community Forum that helps people connect with each other.

    Summary

    Intermittent fasting could be an effective method to lose weight in overweight adults while also improving fasting blood glucose, blood lipids, and IGF-1 levels. These changes could help reduce risk for Type 2 diabetes, stroke, and certain types of cancers.

    My main concern about this diet is that I still worry that it is not a safe option for a number of population groups. I would not recommend this diet for overweight children or teens. Also, fasting is a known trigger for eating disorder behaviors in people with a history of eating disorders and in people with a high risk for eating disorders - so these folks should never engage in fasting for weight loss.

    I would also hesitate to recommend this type of diet to an athlete who has to perform at their physical peak, or anyone who works in a dangerous occupation that requires a high level of physical coordination.

    And lastly, although smart use of intermittent fasting could help those with insulin resistance (e.g. prediabetes and Type 2 diabetes), I would recommend that the person work with their physician and diabetes educator before experimenting. This is especially important for people who take medication with a high risk for hypoglycemia such as insulin or a sulfonylurea.

    Does it work?

    Yes, studies show that you can lose weight with this diet. Although results will vary by individual, one review study reported an average weight loss of 3-8% body weight for periods of 8 weeks or longer (see Patterson et al. reference at the end of this post). For a 200 lb person, that would be 6 - 16 lbs. Alternate-day fasting typically results in more weight loss than time-restricted feeding.

    Who would most benefit from this diet?

    For adults who are overweight, who are otherwise healthy, who seem to tolerate hunger well, and who have not been successful with daily calories restriction (e.g. food logging/calories counting).

    Is it viable long term?

    Theoretically, it is possible barring safety issues that might emerge. I could not find a study with long-term follow-up past 1-year.

    Do I like this diet?

    I think intermittent fasting could work well for adults who hate dieting, tolerate hunger both emotionally and physically, and enjoy the concept of free days where food intake is not restricted.

    Personally, I wouldn't follow alternate-day fasting. When my calories intake is too low, I get very hungry, lightheaded, irritable, and my productivity plummets. I am physically active most days of the week. I am one of those people who have successfully lost weight by creating a small daily calories deficit from eating a little less and exercising a little more. I learned how to create the needed calories deficit by tracking food and exercise. And I have maintained my weight loss for over 7 years with the assistance of tracking.

    On the other hand, having worked in the area of weight control and diabetes for so many years, I know how difficult it is for people to lose weight and keep it off. And a lot of folks really dislike careful tracking of their food and exercise, especially on a daily basis for months and years. So it is important to explore and test alternate ways that people can lose weight without causing themselves harm or taking a lot of time. I think intermittent fasting could be an effective and safe method for weight loss, for the right person. If you are interested in trying intermittent fasting, I recommend that you have a discussion with your healthcare provider about it first. That person knows your medical history and can best advise you.

    Resources

    For an excellent current review of the literature on intermittent fasting (and more detail on how and why intermittent fasting works), I highly recommend this somewhat technical but thorough article written by Anton SD et al. (2017): Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Open access at the time of this post publication is available at Wiley Online Library: http://onlinelibrary.wiley.com/doi/10.1002/oby.22065/pdf

    Patterson, RE et al. Intermittent Fasting and Human Metabolic Health. J Acad Nutr Diet. 2015 Aug; 115(8): 1203-1212. HHS Public Access available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4516560/

    For basic information about other diets, I recommend WebMD's Weight Loss & Diet Plans A - Z and U.S. News & World Report's Best Weight Loss Diets.

    Katherine Isacks, MPS, RD, CDE
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    Disclaimer: Please note that we cannot provide personalized advice and that the information provided does not constitute medical advice. If you are seeking medical advice, please visit a medical professional.

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