Intermittent Fasting and the Metabolic Switch

Intermittent fasting has recently become a hot topic for those seeking to capitalize on its health benefits. But the practice of fasting — abstaining from food for an extended period of time — dates back thousands of years, with roots in religious practices around the world. This includes Ramadan, the ninth and holiest month of the year in the Islamic calendar, when Muslims worldwide engage in fasting from sunrise to sunset each day to build a closer relationship with Allah and exercise self-discipline, gratitude, and humility.

What is fasting?

Fasting can take different forms. It could include:

1. • Not consuming anything for a short period of time (a matter of hours)

1. • No food for a prolonged period (a day or more)

1. • Avoiding certain foods during a particular stretch of time (like some Christians avoiding meat on Fridays during Lent)

1. • Restrictions on certain foods all the time (as in Jews who follow a Kosher diet)

For Muslims, fasting during Ramadan is a sacred act and one of the five essential pillars of the Islamic faith. People who observe Ramadan voluntarily avoid eating and drinking from sunrise to sunset, and consume food or drink only during specific hours of the day (dusk to dawn). Depending on their geographic location, those who participate in fasting as part of Ramadan could be abstaining from food or drink for anywhere from 12 to 20 hours daily.

Today, the life sciences and health communities refer to fasting for a specific time during a day — and only eating during a set time period — as intermittent fasting. Intermittent fasting can vary in duration and structure, but usually involves fasting for a period of 12 hours or longer. Common schedules include 16/8 or 18/6, when food and drink is only consumed during an 8-hour block or a 6-hour block.

What is the anatomy behind fasting?

So what exactly happens in the body during intermittent fasting? To understand the mechanism of the metabolic switch we must first understand the organs involved, and the type and process of digestion.

Most commonly, food is taken in through the mouth (oral or buccal cavity). The teeth, tongue, and salivary glands work together to grind (or masticate) the food, mix it with saliva, and deliver the resulting softened pulp through the pharynx and down the esophagus via the mechanism of swallowing.

Food continues its journey through the body by next making a stop in the stomach. The stomach acts as a mixer for the food that has been ingested, mechanically deconstructing the food particles through a series of muscle contractions and secreting gastric juices simultaneously that help break down food on a chemical level (chyme). The average stomach can hold up to four liters (about one gallon) of food for about two to four hours before sending it onto its next stop, the small intestine.

The majority of absorbing and digesting water, proteins, carbohydrates, and lipids take place in the small intestine. This chemical breakdown of chyme happens thanks to enzymes and bile taken into the small intestine from the pancreas and liver.

From the small intestine, the chyme passes into the large intestine, where any remaining water is absorbed and any undigested food or bacteria left behind exits the body as waste via the anus.

What is the physiology behind fasting and the metabolic switch?

As we mentioned, there are two types of digestion: mechanical digestion (food is ground into smaller pieces either in the mouth or stomach) and chemical digestion (largely in the small intestine, where enzymes and water break down food into smaller particles) via a process called hydrolysis.

During hydrolysis, nutrients in the body get broken down in the following ways:

Complex carbohydrates (polysaccharides) and disaccharides get broken down and release monosaccharides (glucose, fructose, galactose), disaccharides, or oligosaccharides.

Proteins get broken down into single amino acid molecules to provide the body with essential nutrients for building and repairing cells, and producing enzymes, neurotransmitters, DNA, and RNA.

Fats (lipids) get emulsified by bile salts from the liver and become monoglycerides and fatty acid molecules to be stored and later used as a source of energy to absorb vitamins and produce hormones.

While fasting during Ramadan is an act of faith for Muslims, intermittent fasting as a lifestyle choice is gaining popularity in the secular community, attracting attention for its possible health benefits. During prolonged periods of fasting, “metabolic switching” can occur. During the metabolic switch, stored fat gets converted into fatty acids and glycerol. This is then used as an alternative energy resource after the body uses up all the glucose and glucagon gained from the ingested food. Typically, the metabolic switch occurs 12 to 36 hours after the last intake of food.

The metabolic switch is thought to have several benefits, including:

1. • Preserving muscle mass, by promoting the use of ketones, acids generated when fat is broken down. These ketones then get used for energy instead of the body breaking down the proteins present in muscle.

1. • Decreasing insulin levels, as there is less need for insulin to manage blood glucose levels. This can improve insulin sensitivity, thereby helping prevent insulin resistance.

1. • Weight loss, since the body can burn fat more efficiently when it switches to using ketones. Decreased insulin levels can also help promote breakdown of fat, another weight loss contributor.

1. • Decreased inflammation, thanks to ketones’ anti-inflammatory properties that can help lower the levels of inflammatory markers in the body.

Should I try intermittent fasting?

As you consider the possible benefits of intermittent fasting, be mindful that there are also certain risks associated with it. These include:

1. • Hypoglycemia (low blood sugar) – This is of special concern to those with type 2 diabetes and those that use insulin.

1. • Dizziness – Fluctuations in glucose levels can lead to lightheadedness that can increase the risk of falls, especially for the elderly.

1. • Weakness – Muscles can weaken during fasting if not enough protein is ingested to offset its effects.

Intermittent fasting might also be inappropriate for individuals who are:

1. • Young children

1. • Pregnant or nursing

1. • Dementia patients

1. • Living with an eating disorder

1. • Immuno-compromised

1. • Experiencing a hormonal imbalance

Is intermittent fasting for health reasons appropriate for you? Talk to your doctor to find out.