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    Intermittent Fasting Explained

    June 7, 2023

    The benefits of Intermittent Fasting have been studied since the late 1990s, when an article was published in the New England Journal of Medicine exploring the benefits of calorie restriction on the life span of animals [1].

    Since then, further studies have suggested that caloric restriction over the course of your lifetime can improve life expectancy and reduce the risk of chronic diseases such as obesity, type 2 diabetes, cardiovascular disease, cancers and neurodegenerative diseases. Although this evidence has mainly come from animal studies, there is a growing body of research which explores the impact of intermittent fasting on lifespan and disease in humans.

    This article looks at the benefits of intermittent fasting on health and longevity in humans, supported by the most relevant clinical studies.

    What is intermittent fasting?

    Intermittent fasting is a structured regime of calorie restriction which aims to change the way your body gets energy. A standard Western diet usually involves eating 3 meals spread throughout the day, often with snacks in between. In this case, the body uses glucose from your meals for energy and any calories which are not used immediately are converted into triglycerides and stored in your fat cells.

    However, when you fast, these triglycerides become the energy source instead of glucose. They are released from the fat cells and broken down to fatty acids and glycerol. The liver converts the fatty acids to ketone bodies, which provide energy to your cells. Interestingly, ketone bodies offer more than just an energy source. They have major effects on cell and organ function and have been shown to influence ageing and health [2].

    What are the benefits of intermittent fasting?

    Intermittent Fasting for Longevity

    To accumulate data on the impact of intermittent fasting on humans will take years, or even lifetimes to gather. However, epidemiological research indicates that caloric restriction may have contributed to increased life expectancy and a reduced risk of chronic diseases in certain populations, already. The Japanese island of Okinawa is at the centre of research on Intermittent Fasting, with studies on their health and lifestyle dating back more than 50 years. These studies showed that the Okinawan people eat a lower number of calories than other Japanese and Western populations.

    For example, in the late sixties, it was noted that Okinawan school children consumed only 62% of the calories of other Japanese children, and in the seventies the Japan National Nutrition Survey found that Okinawan adults had a lower calorie intake than the rest of the Japanese population; 83% of the average.

    Interestingly, death rates from heart disease, cancer and cerebral vascular disease were found to be only 60-70% of that of the Japan average and both average and maximum lifespan in the Okinawan population are increased compared to Japanese and American populations. Not only do Okinawans live longer, but they remain healthier for longer too, with almost a decade of disability- free life expectancy beyond what typical Western populations experience. These findings are considered a result of long-term calorie restriction in combination with their healthy diet [3].

    Intermittent Fasting for Obesity and Diabetes

    Intermittent fasting is associated with lower rates of obesity and type 2 diabetes. Using the Okinawan population as a reference, the average body mass index (BMI) of adult Okinawans remained stable at 21kg/m2 during their lifetime, with no weight gain in early adulthood which is often seen in other populations.3 More recent studies support intermittent fasting as a method for weight loss. A recent study looked at the effect of fasting during Ramadan on the weight and metabolic profile of 27 healthy Muslim male participants, who fasted between dawn and sunset daily for one month.

    The results showed reductions in weight, fat mass, BMI, and waist circumference that were all clinically significant. It also showed an improvement in insulin sensitivity and reduced insulin resistance. Insulin resistance is associated with a range of diseases, including type 2 diabetes, heart disease, high blood pressure, polycystic ovarian syndrome, high cholesterol, fatty liver disease and certain types of cancer [4]. Furthermore, two recent studies showed that fasting for 24 hours, three times a week reversed insulin resistance in patients with pre-diabetes or type 2 diabetes [5].

    It is important to note that not all clinical studies on intermittent fasting show the same results. A 12- month study which compared alternate- day fasting with a control group, found that both groups lost weight but there were no changes in insulin sensitivity in either [6]. More studies are needed to look at the use of intermittent fasting in the reversal of type 2 diabetes, however, given that weight loss itself is one of the most important interventions for reversing or improving type 2 diabetes, intermittent fasting could be a useful method to achieve this.

    Intermittent Fasting for Cardiovascular Disease

    According to the World Health Organisation, cardiovascular diseases are the leading cause of death globally, taking an estimated 17.9 million lives each year. More than four out of five cardiovascular deaths are due to heart attacks and strokes, and one third of these deaths occur prematurely in people under 70 years of age. Certain risk factors increase a person’s risk of developing cardiovascular disease.

    These include high blood pressure, high cholesterol levels, type 2 diabetes and being overweight or obese. Intermittent fasting has been shown to improve all these risk factors and offers a lifestyle intervention which could significantly reduce a person’s risk of having a heart attack or stroke. For example, an American study looked at the risk of developing atherosclerosis in a group of people who had followed a calorie restricted diet for 6 years, compared to a similar group of people who ate a standard American diet.

    Atherosclerosis is a thickening or hardening of the arteries caused by a build-up or plaque in the lining of the artery which can cause heart attacks and strokes if left untreated. The study found that the fasting group had markedly lower levels in several parameters including, cholesterol, fasting glucose and insulin and blood pressure readings, in comparison to the group eating a standard American diet.

    It also showed that the carotid IMT, a measure used to diagnose the extent of atherosclerosis in the carotid artery, was 40% less in the fasting group [7]. Another recent study looked at calorie restriction in non- obese adults, with or without added exercise. It showed that in just 6 months, their cholesterol levels and blood pressure had improved, and they had significantly reduced their risk of having a heart attack or stroke in the next 10 years [8].

    Other possible health benefits of intermittent fasting

    Studies show that intermittent fasting can reduce the likelihood of getting cancer and suppresses the growth of many types of tumours in rodents. Clinical trials are currently underway to look at the impact of intermittent fasting on cancer in humans. Although it’s early days, there have been some promising findings. For example, several case studies involving patients with glioblastoma suggest that intermittent fasting can suppress tumour growth and extend survival [9].

    Similarly, early evidence suggests that alternate day fasting can delay the onset and progression of neurodegenerative diseases such as Alzheimer’s and Parkinson’s Disease. Furthermore, epidemiological data shows that excessive calorie intake, especially in middle age can increase the risk of these diseases [10].

    Excessive calorie intake, especially in middle age, can increase the risk of developing neurodegenerative diseases, such as Alzheimer’s and Parkinson’s Disease.10 However, there is strong early evidence that alternate day fasting can delay the onset and progression of these diseases [11, 12]

    Other early evidence suggests improved outcomes in other diseases such as asthma and multiple sclerosis.

    How do you put Intermittent Fasting into practice?

    Intermittent Fasting can be tailored to your preference, but there are some common regimens that are recommended, as follows:

    1) Time restricted eating.

    Over a few months, aim to reduce the time window in which you consume food each day, with the goal of fasting for 16-18 hours daily. An example of this is to fast between 18:00 and 10:00.

    2) 5:2 intermittent- fasting

    On two days per week, reduce the number of calories you eat, aiming for 500-700 calories per day. You should aim to introduce this regimen slowly. This could look like:

    Week 1: Eat 1000 calories only, 1 day per week

    Week2: Eat 1000 calories only, 2 days per week

    Week 3: Eat 750 calories only, 2 days per week

    Week 4. Eat 500 calories only, 2 days per week.

    There can be some side effects of intermittent fasting, which should resolve after one month. These include hunger, irritability and reduced concentration during periods of food restriction.

    There are also some groups of people for whom intermittent fasting may not be appropriate such as pregnant women, those with a history of eating disorders or people which diabetes who are on insulin.

    To summarise, the research indicates the benefits of intermittent fasting are broad, with a growing body of evidence to support its use in managing chronic disease, weight loss and increasing longevity. More research needs to be done to truly understand what it can offer but it is an exciting field to follow.

    We hope this has been a useful introduction to the benefits of Intermittent Fasting. If you would like to discuss how intermittent fasting could help you, please book holistic a Well Woman or Well Man check with one of our doctors who would be happy to help you to get a full and comprehensive baseline of your health and explore intermitted fasting as an option to reach your health goals.

    Written by Dr Jessica Howitt, a One5 Health GP, Lifestyle Medic & Certified Health Coach. Jessica has a strong interest in preventative and lifestyle medicine, she is available for private consultations every Tuesday and Wednesday at One5 Health.


    1. Weindruch R, Sohal RS. Caloric intake and aging. N Engl J Med 1997;337:986-994.
    2. N Engl J Med 2019; 381:2541-2551 DOI: 10.1056/NEJMra1905136
    3. Biogerontology 2006 Jun;7(3):173-7. doi: 10.1007/s10522-006-9008-z.
    4. American Journal of Lifestyle Medicine, vol. 15, 2: pp. 200-206
    5. BMJ Case Rep 2018;2018:bcr-2017-221854. Cell Metab 2018;27(6):1212-1221.e3.
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    7. Proc Natl Acad Sci U S A 2004;101:6659-6663.
    8. Atherosclerosis 2009;203:206-213
    9. Front Nutr 2018;5:20-20.
    10. Nat Rev Neurol 2018;14:168-181.
    11. Nat Rev Neurosci 2018;19:63-80.
    12. Cell Metab 2018;27:1176-1199.

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