Fasting: how does it impact your health and disease risk?

Most of us would agree that losing body weight and maintaining it is a priority. Most of us also would agree that this is easier said than done. Let me talk to you about fasting.

Research reveals fasting has many health benefits, including improved cardiovascular health, reduced cancer and neurodegenerative disease risk, gene repair, and potentially increase longevity. Here I would like to present an overview of fasting to help you make an informed decision on whether it may be right for you.

What happens to your body when you fast

In your body’s normal state the body depletes its glucose stores in the liver and muscles, as your body's main source of energy. When you fast, this store of glucose is used up. Fat tissue becomes the next source of energy for the body and gets burnt instead of glucose. Another mechanism that makes fasting beneficial for health is its effect on insulin sensitivity.

Types of fasting

Intermittent fasting is an eating pattern where you cycle between periods of eating and fasting. Intermittent fasting involves restricting calorie intake on some days and eating freely on others. The 5:2 approach is the most common, where you eat 500-600 calories on your 2 fasting days and eat regularly on the remaining days of the week. The basis of intermittent fasting is often linked to our ancestors who did not have access to food around the clock and instead had periods of feast and famine.


An alternative intermittent fasting plan is only eating within an eight to ten-hour window (i.e. only consuming food between 10am-7pm).

Caloric restriction diets are generally harder to adhere to as they involve reducing energy intake every day by 20%–50%.

What does the research say?

There is a large body of research to support the health benefits of fasting, however most of it has been conducted in laboratory settings on animals from yeast and worms, to mice and monkeys; research data on humans are sparse.

Caloric restriction

Calorie restriction (eating less every day) promotes both weight loss, longevity and other health benefits in animal studies, however this kind of diet is not attainable for most people.

Calorie restriction was tested in humans in the CALORIE study. In the CALORIE study, researchers asked participants who were normal-weight and moderately overweight to decrease their daily calories by 25%. 191 participants completed the 2-year study. On average, the group of non-obese adults were only able to cut down by 12% over the 2-year study. Significant changes in the study’s main aims: resting metabolic rate and core body temperature didn’t occur. However, researchers found calorie restriction improved blood pressure, cholesterol, and insulin resistance. The calorie restriction group lost an average of 10% of their body weight in year one, and maintained this weight during year two, demonstrating it is feasible and safe to adhere to a daily diet long-term. This study also assessed calorie restriction’s effects on quality of life such as mood and hunger-related symptoms and found no adverse effects.

Intermittent fasting

Intermittent fasting was tested in a study of 71 U.S adults, conducted over 3 months. Researchers found that fasting for five days a month (750 and 1,100 calories per fasting day) reduced their waistlines and total body fat, but not muscle mass, which is important.  The diet also reduced cardiovascular risk factors including blood pressure, signs of inflammation (measured by C-reactive protein levels), as well as fasting glucose and reduced levels of IGF-1, a hormone that affects metabolism. The intermittent fasting group lost an average of 2.6 kilograms and the control group, who didn’t change their eating habits remained at the same weight.

Fasting or caloric restriction diets? Which method works best?

It depends on the individual which method is best for you. Research shows that you can get the same benefits of calorie restriction through intermittent fasting. Researchers explored the effects of intermittent and caloric restriction on weight loss and various biomarkers among 107 overweight or obese woman. The researchers found that intermittent restriction was as effective as daily restriction for improving weight loss, insulin sensitivity, blood pressure, and reduction in leptin, total and LDL cholesterol, triglycerides, lipids and other health biomarkers.  

Fasting and the brain?

Intermittent fasting twice a week could help decrease your risk of neurodegenerative diseases like Alzheimer’s and Parkinson’s while also improving memory and mood. If you don’t eat for 10–16 hours, your body will go to its fat stores for energy, and ketones will be released into the bloodstream. Ketones help positive changes in the structure of synapses that are central for learning, memory, and overall brain health. While research on fasting diets and their impact on preventing neurological diseases still has a long way to go, the early evidence is promising.

Will fasting help you to live longer?

Across all types of intermittent energy restrictions diets, we don’t know the longer-term benefits or harms on humans.  Whether caloric restrictions or fasting can extend life span in humans is still an open-ended question and is difficult to measure in randomised controlled trials.

Some tips to start?

  1. Make sure you have adequate and healthy nutrition between fasting times.
  2. Avoid sugary foods because they make you hungry.
  3. Be sure to stay hydrated.
  4. Exercise - Physical activity will prevent loss of muscle mass, which would be unhealthy.

Some groups of people who should not fast include:

  • Women who are pregnant or breastfeeding
  • Children and teenagers
  • Type 1 diabetics
  • People taking medications that require food to be consumed at the same time
  • People recovering from surgery
  • Those with chronic stress (adrenal fatigue)
  • People with eating disorders.
  • People over 70 years of age

If you are unsure if fasting may be for you, consult your GP before you begin.

If you wish to receive regular information, tips, resources, reassurance and inspiration for up-to-date care, that is safe and sound and in line with latest research please subscribe here to receive my blog, or like Dr Andreas Obermair on Facebook. Should you find this article interesting, please feel free to share it. 




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