Eating and Fasting

Fasting is the abstinence from consuming food and/or beverages for different periods of time. There are many different types of fasting: during holy month of Ramadan (absence of water and food intake from sunrise until sunset), intermittent fasting (short term fasting- less than 24 hours; long term fasting- longer than 24 hours), alternate day fasting, prolonged fasting (for more than 3 days) (1).

What happens when we eat?

Every time we consume calories, an insulin response in our body is triggered and blood glucose levels rise, staying up for around 90 minutes to a couple hours. Insulin is tasked with carrying glucose to the cells and storing it in the liver and muscles, like glycogen. If blood glucose level is still high even after glycogen is replenished in the muscles and liver, the remaining glucose will be stored as fat. Therefore, when insulin levels increase, the body goes into fat-storing mode.

All foods raise insulin to some degree. In particular, refined carbohydrates (white flour, white bread, white rice, pastries, sodas, snacks, pasta, sweets, breakfast cereals and added sugars) tend to significantly raise insulin compared to other nutrients, such as healthier fats (avocado, cheese, dark chocolate, whole eggs, fatty fish, nuts, olive oil, coconut oil, full fat yogurt) but insulin still goes up in both cases.

Fasting is one of the mechanisms developed to lower blood glucose and decrease fat stores. Eating triggers the liver’s natural response to break up and digest food (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14).

What happens when we fast?

1. Postabsorptive Phase: Occurs 6 to 24 hours after fasting. Blood sugar and insulin levels fall. The liver breaks down glycogen for energy, stores last for approx. 24 to 36 hours (1).

2. Gluconeogenesis: Occurs 24 hours to two days after fasting. Glycogen stores have run out. The liver produces new glucose from amino acids in a process called gluconeogenesis. In non-diabetics, glucose levels fall but stay within the normal range (1).

3. Ketosis: Occurs 2 to 3 days after fasting. Low insulin levels stimulate lipolysis, the breakdown of fat for energy. Triglycerides, the main constituents of natural fats and oils., are broken into the glycerol backbone and three fatty acid chains. The glycerol is used for gluconeogenesis, so amino acids can be reserves for protein synthesis. The fatty acids are used directly for energy by most tissues of the body, excluding the brain. The body uses fatty acids to produce ketone bodies, which are capable of crossing the blood-brain barrier and are used by the brain for energy. After four days of fasting, approximately 75% of the energy used by the brain is provided by ketones. The two major types of ketones produced are beta-hydroxybutyrate and acetoacetate, which can increase up to time 70 times during fasting (1).

4. Protein Conservation Phase: Occurs 5 days after fasting. High levels of growth hormone maintain muscle mass and lean tissues. The energy for basic metabolism is almost entirely supplied by fatty acids and ketones. Blood glucose is maintained by gluconeogenesis. Increased norepinephrine levels prevent any decrease in metabolic rate. There is a normal amount of protein turnover, but it is not being used for energy (1).

References

1. Fung, Moore (2017) The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting

2. Rhonda Patrick (2017) https://www.foundmyfitness.com/topics/fasting

3. Basu et al (2006) Dietary Factors That Promote or Retard Inflammation

4. Menendez et al (2005) Oleic acid, the main monounsaturated fatty acid of olive oil, suppresses Her-2/neu (erbB-2) expression and synergistically enhances the growth inhibitory effects of trastuzumab (Herceptin) in breast cancer cells with Her-2/neu oncogene amplification.

5. Fulgoni (2013) Avocado consumption is associated with better diet quality and nutrient intake, and lower metabolic syndrome risk in US adults: results from the National Health and Nutrition Examination Survey (NHANES) 2001-2008.

6. Mozaffarian et al. (2010) Trans-palmitoleic acid, metabolic risk factors, and new-onset diabetes in U.S. adults: a cohort study.

7. Taubert (2007) Effects of Low Habitual Cocoa Intake on Blood Pressure and Bioactive Nitric Oxide

8. Vander et al. (2005) Short-term effect of eggs on satiety in overweight and obese subjects.

9. Wang et al. (2006) n-3 Fatty acids from fish or fish-oil supplements, but not alpha-linolenic acid, benefit cardiovascular disease outcomes in primary- and secondary-prevention studies: a systematic review.

10. Jackson, Hu (2014) Long-term associations of nut consumption with body weight and obesity

11. Bauchamp et al. (2005) Phytochemistry: ibuprofen-like activity in extra-virgin olive oil.

12. Liu (2008) Medium-chain triglyceride (MCT) ketogenic therapy.

13. Morelli (2014) Yogurt, living cultures, and gut health.

14. Arnason et al. (2017) Effects of intermittent fasting on health markers in those with type 2 diabetes: A pilot study