What’s The Deal With Intermittent Fasting?

Physio Logic’s Clinical Nutrition Assistant, Rachel Naar, explains a few of the most studied fasting techniques, how they work and the results you can expect out of each based off of both animal and human study findings.

Intermittent Fasting - Alternate Day Fasting, Modified Fasting, and Time Restricted Feeding | https://physiologicnyc.com/clinical-nutrition/


Fasting. We do it when we sleep, that’s at least a given. Unless you’re sleep-eating, which is actually a thing. Night eating syndrome.

But when else throughout your day-to-day is there an absolute food-free zone? What exactly is an intermittent fast and why should you care? Are there actual health benefits?

Below we delve into some of the most studied fasting techniques and separate fact from fiction enabling YOU to be armed with the research to make the most informed decision with regards to fasting.

So, What Is It?

Intermittent fasting is an eating regimen that involves cycling between periods of eating and fasting.

Fasting = Nothing, zilch, nada! (Although some forms allow water, coffee, and zero-calorie beverages).

This eating pattern cares less about which foods to eat, but more so when you eat them. There are a variety of different methods with the common central theme of designated eating times vs. designated fasting time.

Many studies (below) show that these patterns of eating can cause weight loss, improve your metabolism, protect against disease and cancers, and overall improve energy and quality of life.

It is important to note that scientific evidence for health benefits of intermittent fasting in human beings is often derived from animal studies. Your pet rat may not have the same weight loss percentage as you! In addition, most studies are observational in that they overview a religious fasting or are done with a very small sample size of individuals partaking in the experiment.

Now we’ll dive into the main types of fasting identifying each, speak to the animal results, human results, and finally our consensus.

Alternate Day Fasting

  • What

This type of fasting is as exactly as it sounds and involves fasting in which the individual switches off days of eating and drinking with not eating and drinking. There are no restrictions to the days that the individuals eat.

  • Animal Results

Studies with animals concluded that this fasting regiment was as effective as simply consuming less calories through the day in decreasing blood sugars. The results did show a reduction in cholesterol and triglyceride levels.1

  • Human Results

Most sample sizes for testing alternate fasting were only 8 to 30 normal weight adults and were conducted for only a couple of days to a couple of weeks. The studies did find a significant decrease in at least one glucoregulatory marker. However, while there were improvements with HDL cholesterol and triglyceride levels, LDL cholesterol sky-rocketed.2-3

  • Consensus

Can you really imagine eating nothing all day and continuing your workouts? Hangry much? As noted, this method of fasting is just as effective as simple calorie restriction. Work with your doctor and nutritionist to find a sustainable weight loss plan that works for you AND your stomach growls.

Modified Fasting

  • What

This type of regimen generally allows you to eat a small smidgen of your daily calorie allotment needs on fasting days. So in this sense, a fast doesn’t mean ‘nada’ it means a smidgen, limited energy intake. The popular 5:2 diet draws from this regimen. 5:2 means energy restriction for 2 non-consecutive days a week, and usual eating the other 5 days. Similar to alternate day fasting, the individual is not restricted on the days that he or she is not fasting.

  • Animal Results

This type of fasting has shown a significant decrease in visceral fat (fat that is stored deep below the skin that is wrapped around major organs) in mice. In addition, fat cells reduced in size, and cancer risk factors such as cell proliferation decreased .4-6

  • Human Results

The human trials looked primarily at overweight and obese patients who participated in modified fasting for 6 weeks to 8 months. Results included statistically significant weight loss, decreases in fasting insulin (the hormone that regulates levels of blood sugar), but no reduction in fasting blood sugar levels. Some of the studies showed decreases in inflammatory markers, but in general on average 15% of participants reported negative side effects such as feeling cold, low energy, irritable, and depressed.7-10

  • Consensus

Though this may be more tolerable than alternate day fasting, you still may report fatigue, irritation and depression burning through only 20% of your needs on the days you are fasting. Let’s say you require 1500 calories per day, that means you’ll only get 300 calories. That’s only an RX bar and a tablespoon of hummus!

Time-Restricted Feeding

  • What

Allows individuals to consume their calories within specific windows of time regularly. This ultimately induces fasting on a routine basis. Most individuals do this regularly while they sleep. Intermittent may play on the natural sleep-wake cycle or extend the fasting window to slightly longer. Individuals may decide to delay their morning meal, end their last meal at a designated time, or decide to eat only in a designated hour period (ex. 8 hour window). Fasting for 16 hours and eating for 8 is what is known as the 16:8 method.

  • Animal Results

In animal studies, time restricted feeding was associated with reductions in body weight, cholesterol, triglycerides, blood sugar, and improvements to insulin sensitivity (how well your body takes up blood sugar).11

  • Human Results

In a human study, a prescribed nighttime fasting interval of >11 hours resulted in a significant decrease in weight. No biomarkers were assessed in this study.12 In another crossover study, consuming one meal per day was associated with reductions in weight greater than consuming 3 meals per day, a decrease in blood sugar, and improvements with LDL (decrease) and HDL (increase) cholesterol. This type of fasting ritual did not generate a mean change in tension, depression, anger, vigor, fatigue, or confusion.13

  • Consensus

The data highlights the potential importance of synchronizing intermittent fasting regimens with daily circadian rhythms (sleep-wake cycle) to maintain optimal metabolic function. While it may be difficult to consume only 1 meal per day, having a time of night when then “kitchen is closed” may stop additional munching and excess calories that are not being utilized for energy. The majority of excess calories are generally consumed after dinner.

Final Thoughts…

  • If you have had or have an eating disorder, disordered eating or obsessive thoughts about food and weight loss, intermittent fasting may be triggering.
  • If you have diabetes and experience frequent low blood sugars, intermittent fasting may be dangerous to your health.
  • Fasting and intense exercise may lead to low blood sugar which can cause dizziness and lightheadedness.
  • Your body is at its peak for blood flow and gastric emptying (digestion) when you’re awake. It may be advantageous to make use of your body’s ability to break down food at its most efficient state, versus waiting to eat most calories later in the day.
  • Eating in conjunction with your circadian rhythm may improve gastrointestinal function and metabolism.
  • If you’re approaching a new nutrition regimen, best to seek out advice from a nutritionist and a doctor before implementation.


  1. Varady KA, Hellerstein MK. Alternate-day fasting and chronic disease prevention: A review of human and animal trials. Am J Clin Nutr. 2007;86:7-13.
  2. Halberg N, Henriksen M, Soderhamn N, et al. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol. 2005;99:2128-2136.
  3. Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E. Alternate-day fasting in nonobese subjects: Effects on body weight, body composition, and energy metabolism. Am J Clin Nutr. 2005;81: 69-73.
  4. Varady KA, Roohk DJ, McEvoy-Hein BK, Gaylinn BD, Thorner MO, Hellerstein MK.Modified alternate-day fasting regimens reduce cell proliferation rates to a similar extent as daily calorie restriction in mice. FASEB J. 2008;22:2090-2096.
  5. Varady KA, Roohk DJ, Hellerstein MK. Dose effects of modified alternate-day fasting regimens on in vivo cell proliferation and plasma insulin-like growth factor-1 in mice. J Appl Physiol. 2007;103:547-551.
  6. Varady KA, Roohk DJ, Loe YC, McEvoy-Hein BK, Hellerstein MK. Effects of modified alternate-day fasting regimens on adipocyte size, triglyceride metabolism, and plasma adiponectin levels in mice. J Lipid Res. 2007;48:2212-2219.
  7. Williams KV, Mullen ML, Kelley DE, Wing RR. The effect of short periods of caloric restriction on weight loss and glycemic control in type 2 diabetes. Diabetes Care. 1998;21:2-8.
  8. Johnson JB, Summer W, Cutler RG, et al. Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma. Free Radic Biol Med. 2007;42:665-674.
  9. Varady KA, Bhutani S, Church EC, KlempelMC.Short-term modified alternate day fasting: A novel dietary strategy for weight loss and cardioprotection in obese adults. Am J Clin Nutr. 2009;90:1138-1143.
  10. Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: A randomized trial in young overweight women. Int J Obes Suppl. 2011;35: 714-727.
  11. Rothschild J, Hoddy KK, Jambazian P, Varady KA. Time-restricted feeding and risk of metabolic disease: A review of human and animal studies. Nutr Rev. 2014;72:308-318.
  12. LeCheminant JD, Christenson E, Bailey BW, Tucker LA. Restricting nighttime eating reduces daily energy intake in healthy young men: A short-term cross-over study. Br J Nutr. 2013;110: 2108-2113.
  13. Carlson O, Martin B, Stote KS, et al. Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women. Metab Clin Exp. 2007;56: 1729-1734.
  14. Patterson RE, Laughlin GA, Sears DD, et al. Intermittent Fasting and Human Metabolic Health. Journal of the Academy of Nutrition and Dietetics. 2015;115(8):1203-1212. doi:10.1016/j.jand.2015.02.018.

Looking for nutritional guidance? Schedule an appointment with our Clinical Nutritionist today. Fill out the form below to get started.

Contact Us