What Exactly is Happening to Your Body While You're Fasting

How does Fasting Work?

Intermittent fasting is an eating pattern cycle between periods of eating and fasting. The long hours of food restriction during the fasting period give your energy powerhouse, the mitochondria in our cell, a break from working constantly. Fasting also alters the body’s metabolism in a few ways to provide health benefits.

everything you need to know about intermittent fasting 1200

Intermittent Fasting and Cellular Response

Blood glucose, glycogen and insulin

The energy restriction during fasting will reduce blood glucose levels. Meanwhile, the body will start depleting glycogen, the quick access form of energy stored in the liver for energy to maintain blood glucose level ⁽¹⁾.

Insulin plays an important role in the storage and utilization of glucose in our bodies. In conjunction with the depletion of glycogen to maintain blood glucose levels during fasting, the circulating level of insulin is also reduced with improved insulin sensitivity and glucose tolerance^(2,3).

Increases fat oxidation, reduces fat storage

When the body’s glycogen stored in the liver is all used up, our body will seek another source of energy. At this stage, when there is no more “quick access” energy, the body will start breaking down fat tissue to produce ketone bodies as a source of energy inducing a ketogenic state. This change in energy source switches the body’s metabolism from fat storage to fat utilization^(2,4).

Promotes autophagy

Food restriction during fasting causes a drop in insulin and rise in glucagon (to produce more glucose) induced autophagy, which is a state when body cell goes into maintenance and repair mode (5,6). Stimulation of autophagy allows cells cellular “cleansing” to remove damaged cells. This is the key to why intermittent fasting may be protective against stress and promote potentially therapeutic neuronal responses ^(5,7).

Fasting and Weight Loss

Studies have been conducted to identify the association and effectiveness between intermittent fasting, obesity, and weight management. A systematic review conducted in 2020 found intermittent fasting resulted in weight loss, ranging from 0.8% to 12% of baseline body weight in the obese population (8). Studies also showed a reduction in BMI, waist circumference, and most weight loss through intermittent fasting where fat loss (8). Another 2019 systemic review demonstrated time-restricted feeding with hours ranging from 4 to 12 hours achieved greater weight loss than unrestricted eating times ⁽⁹⁾.

Other Health Benefits

Heart health

A large body of evidence has shown intermittent fasting reduced traditional cardiovascular risks factors (4).

Lipid profile

Studies showed reductions in total cholesterol range from 6-21%, LDL level decreased by 7 to 32%, triglyceride level from 16-42% following intermittent fasting interventions ^{(10,11,12,13,14,15,16,17,18)}.

Blood pressure

Studies also showed reductions in systolic pressure and diastolic pressure in people who achieved weight loss following 6 to 24 weeks of intermittent fasting ^{(10,11,15,17)}.

Oxidative stress

Animal trials have found the association between intermittent fasting with energy restriction and improvement in markers of oxidative stress, inflammatory responses to preserve cardiovascular health ⁽¹⁹⁾. A study conducted on overweight adults with asthma, following 8 weeks of alternate-day fasting showed a reduction in markers of oxidative stress, serum inflammation factors, and increased levels of antioxidant uric acid ⁽¹²⁾.

Type 2 diabetes

A recent systemic review and meta-analysis demonstrated time-restricted fasting regimen had significant reductions in fasting glucose concentrations compared to non-time restricted eating ⁽⁹⁾.

Other studies showed intermittent fasting showed promising improvement on glycemic control for people with type 2 diabetes, with a daily fast of at least 16 hours have significantly lower fasting glucose level and increased insulin sensitivity than caloric restriction diet alone ^{(20,21,22,23,24)}.

Brain health & neurodegenerative diseases

Animal studies have found intermittent fasting can enhance brain function, such as learning and memory with increased synaptic plasticity and increased production of new neurons ^(25,26,27).

Excessive energy intake is associated with suboptimal brain function, increased risks of neurodegenerative disorders such as stroke, Alzheimer’s disease, and Parkinson’s disease ^(28,29).

Studies have shown evidence that alternate-day fasting can delay the progression of neurodegenerative diseases by increasing neuronal stress resistance through stimulating autophagy, increasing antioxidant defenses, and reducing levels of pro-inflammatory in animal trials ^{(27,30,31,32)}.

Cancer

The mechanism behind cancer development and prevention is complex. A series of animal studies have shown that intermittent fasting or diet that mimic fasting may delay the progression of cancers ^(33,34). Research in humans also demonstrated evidence that fasting enhances the efficacy and tolerability of chemotherapy in cancer patients ^{(6, 36, 37)}.

Aging & inflammation

Oxidative stress is a key factor for aging, inflammation, and risk factors for chronic disease (38). Clinical and epidemiological data are consistent with the ability to fast to retard the aging process and associated diseases (1). Intermittent fasting regime showed to improve markers of oxidative stress by inhibition of cellular growth pathway, stimulation of autophagy, and ketogenesis ^{(1, 39,40)}.

Reference

Lois Grey

Lois Grey has a BSc in Nutrition and Dietetics and worked as a clinical dietitian to help her clients to manage their nutrition and healthy weight. Lois is part of the Fastyle professional team and helps running the Fastyle blog where she shares nutrition articles on Intermittent Fasting, Healthy Eating, and sustainable lifestyle changes to help Fastyle user achieving their personal goal.

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