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What is Fasting?

The Types of Fasting

So how do you fast? Well, that depends. There are multiple forms of fasting! We will get into the nitty gritty on these in another article, but there is the quick and easy description for each type:

Time restricted feeding (TRF) – Daily pattern of eating during a window of 8hrs or less5 (for example, not eating anything after 8pm until lunch the following day).

Intermittent fasting (IF) – Extended periods of time (16-48 hrs) with very little or no energy intake in tangent to intervals of normal eating5,6 – this is what most people think of as fasting, but the body generally does not reach true biological fasting in this version.

Periodic fasting (PF) – 2 to 21+ days broken up into periods of fasting or fasting-mimicking diets5 – this is true biological fasting.

* Please note that juice cleanses do not fit in any of these categories, and there really is no such thing as a “juice fast”.

Benefits of Fasting

So why should someone consider a fast?  Because it turns out that there are a number of benefits to fasting, and they’re wide-ranging!

In mice models, we see that the benefits of fasting include2:

  • Increased lifespan
  • Decreased diabetes
  • Decreased cancer
  • Increased cognitive functioning (they get smarter vs. their non-fasting peers)

In humans, the benefits are a little different2,7:

  • Maintain healthy levels of metabolic markers including: glucose, blood pressure, insulin, c-reactive protein, insulin-like growth factor 1
  • Weight loss
  • Visceral fat loss
  • Improved cognitive performance

Who Can Fast

But is fasting for everyone? Well, technically the answer is no. Although fasting comes with many benefits, it is not safe for children, elderly or frail individuals, and those that are malnourished. It is also not always safe for people with difficulty regulating their blood sugar, such as diabetics or people with glycogen storage diseases.  Ultimately, it is up to your medical professional to determine if it is appropriate for you to be on a fast.

Is there a way to Fast without Fasting?

Valter Longo, PhD

There actually is a diet where you can get all the benefits of fasting without all the hassle and risk of fasting. It is called a Fasting Mimicking Diet and the incredible researchers at the University of Southern California, under the guidance of Valter Longo PhD, developed this protocol and tested it in a clinical trial. What they ultimately found after the 100 accepted participants went on this fasting mimicking diet was that they lost weight, preserved their muscle mass, and maintained healthy levels of metabolic markers (glucose, triglycerides, cholesterol, blood pressure, c-reactive protein).  This protocol consisted of five days of a scientifically determined ratio of micro- and macronutrients that resulted in the mimicking of the physiology of fasting while eating! This diet was truly food that the body did not sense – it did not activate the nutrient sensing pathways – but it did stimulate fasting.

  1. Longo VD, Mattson MP. Fasting: Molecular Mechanisms and Clinical Applications. Cell metabolism. 2014;19(2):181-192. doi:10.1016/j.cmet.2013.12.008.
  2. Brandhorst et al., A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan, Cell Metabolism (2015),
  3. Ruderman NB, Aoki TT, Cahill GF. 1976. Gluconeogenesis and its disorders in man. In Gluconeogenesis: Its Regulation in Mammalian Species, ed. RW Hanson, MA Mehlman, pp 515–30. New York: Wiley
  4. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22.
  5. Lee C, Longo V. Dietary restriction with and without caloric restriction for healthy aging. F1000Research. 2016;5:F1000 Faculty Rev-117. doi:10.12688/f1000research.7136.1.
  6. Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017 Oct;39:46-58. doi: 10.1016/j.arr.2016.10.005. Epub 2016 Oct 31.
  7. Wei M, Brandhorst S, Shelehchi M, Mirzaei H, Cheng CW, Budniak J, Groshen S, Mack WJ, Guen E, Di Biase S, Cohen P, Morgan TE, Dorff T, Hong K, Michalsen A, Laviano A, Longo VD. Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. Sci Transl Med. 2017 Feb 15;9(377). pii: eaai8700. doi: 10.1126/scitranslmed.aai8700.

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