Intermittent fasting (IF) has become increasingly popular over the last decade and was the most researched diet in 2019. IF is an umbrella term for a type of eating pattern in which you eat and fast within specific time windows, compressing the window in which you consume food. This is why it is sometimes referred to as time-restricted feeding (TRF). There are a variety of ways to do IF, for example:
• Alternate day fasting (ADF)
• 5:2 - eating ~500 kcal on two non-consecutive day of the week
• 12:12 - eating within a 12-hour window and fast for 12 hours
• 16:8 - eating in an 8-hour window and fasting in the other 16 hours
These are a more popular methods as they are not as psychologically demanding, but some people may fast for 3 days or longer. Unlike other dieting methods, you don’t restrict what you consume, IF changes when you eat, not what you eat. Unless participants are undertaking IF with weight loss goals, and thus may reduce calorie consumption during feeding hours.
Fasting is no new concept; it has been used as both a religious and a medical practice for thousands of years. For example, people participating in Ramadan religious fasting, can fast from 13-18 hours per day for 30 days[1]. Your body typically goes into fasting mode after 12 hours, so if you’re asleep for that period of time or longer you’re actually already fasting. Exactly when your body goes into fasting state will depend on how much glucose you have in your blood stream and how long it takes to deplete the glycogen stored in your liver at the beginning of the fast. If you typically eat a high carbohydrate diet, it will take longer for your liver glycogen stores to deplete.
What are the benefits of fasting?
Evidence is accumulating that eating in a time-restricted window could help trigger a shift from glucose energy usage to that of fatty acid-derived ketones. This may be beneficial due to increased stress resistance, increased longevity, and decreased incidence of disease including cancer and obesity[2]. One theory for this is the optimisation of hormones and enhanced metabolic efficiency.
As your body shifts to burning fat as its main energy source, it transitions from a fat storage to fat mobilisation state. Fat storage is promoted by the anabolic hormone insulin (storage hormone). An anabolic hormone is one that encourages cellular growth, and insulin promotes the absorption of glucose from the blood, which is then converted into glycogen and/or fats. However, flipping the switch from glucose to ketones, can confer many health benefits as insulin levels lower and glucagon levels rise. Glucagon is the main catabolic hormone of the body and promotes mobilisation of fat. When one eats continuously, blood glucose level are constantly high, meaning the pancreas is always working. It’s thought that the brains’ responsiveness to the hormonal rhythms become less effective and more resistant – like with insulin resistance seen in prediabetic and type 2 diabetic (T2D) individuals. IF could be an effective alternative diet strategy to help improve insulin sensitivity, especially in T2D patients[3][4].
Are there other benefits?
The clinical applications of IF expand far beyond weight loss. The scientific literature paints a clear picture of the positive mechanisms of action and pathways that occur from this eating style, independent of weight loss. These include:
1. Improved cardio-metabolic health: improved lipids profiles by positively influences LDL cholesterol, triglycerides, and total cholesterol[5][6].
2. Improved cognitive function: increased mental sharpness and neurotrophic factors. Animal studies suggest that IF may protect against age-related memory decline through an increase in hippocampal neurogenesis[7].
3. Retention of lean mass is increased following IF regimens for weight loss as compared to continuous calorie restriction regimes in humans.[8] This is especially important for older adults losing weight given the known loss of lean mass that occurs during both ageing and continuous calorie restriction.
4. IF can help fight inflammation in the muscle, intestines (which may enhance motility in the gut), adipose tissue and in blood[9].
5. When we fast, the body induces important cellular repair processes, such as removing waste material from cells, and destruction of cellular waste or damaged tissue, a process called autophagy.[10]
6. Oxidative stress is one of the steps towards ageing and many chronic diseases. Several studies show that IF may confer disease protection by enhancing the body's resistance to oxidative stress[11][12].
7. Unlike many other diets such as a Keto or the Atkins diet, you’re not omitting a whole food group. This is crucial as carbohydrates are essential for human health e.g. for reproductive health in women. Glucose is necessary for the conversion of T4 to T3 in the liver. Without sufficient T3, hypothyroidism results. Hypothyroidism is implicated in mood disorders, reproductive irregularities such as PCOS and amenorrhea, in skin conditions, and in weight gain, among other things.
What are the disadvantages?
Initially people may experience side effects such as headaches, fainting, intense cravings, drowsiness, irritability and a reduced ability to concentrate during the first fast days, but these side effects often dissipate after doing IF a few times. The more fasting you do, the better the body can adapt. Studies are still in their infancy, so it is not clear which IF protocol is best, how long and how often it should be done for to reap the most benefits. Long term studies are not yet available so it is unclear if benefits will last five or ten years down the line, once fasting has ceased. Additionally, studies are predominantly in overweight and obese subjects, so there still remains a need for randomised controlled trials (RCTs) in normal weight subjects. Saying this, findings look promising, which has really sparked scientific interest in the benefits of short-term fasting approaches over the last decade.
Who should not fast?
IF is not suitable for everyone and should be done with careful consideration. People with advanced diabetes or who are on certain medications, people with a history of eating disorders or disordered eating, and pregnant or breastfeeding women should not attempt intermittent fasting unless under the close supervision of a physician who can monitor them. You may want to proceed with caution if you have a performance oriented or client-facing job, or until you have adapted to your new IF plan and overcome the initial periods of low performance. If you compete in sports/athletics it could result in low performance and should be discussed with your coach if you wish to begin IF.
Who may this be especially appropriate for?
IF may be especially useful for those who have trouble with hunger cues as it may be a way of pressing the reset button, not just for T2D patients with insulin resistance, but also shift workers, those with trouble sleeping, or people with metabolic disease needing to reset their circadian regulation of appetite.
Considerations for someone starting IF
It is important to ensure you are nourishing your body prior to starting a fast to help prevent feeling ‘hangry’ whilst fasting. This involves consuming nutrient dense, unprocessed diet rich in fruits and vegetables, whole grains (all three being fibrous), sufficient lean protein and healthy fats. These food groups have been linked to improved health benefits by boosting important antioxidants and nutrients.[13][14][15][16] Depending on how long you fast, you may need to ease your way back into a caloric intake. The gut is in a rested state, so be conscious about what you eat after fasting, as the meal you break your fast is most important. Cooked vegetables, and broths can help acclimate your body and digestive system to eating as internal mechanisms come back online. But again, it should include fibrous complex carbs, lean protein and healthy fats. Eating too many high GI carbohydrates after fasting can cause your blood sugar levels to become too high, so opt for lower GI carbohydrates such as bulgar wheat, brown rice or whole grain bread.
Limiting your eating window is not a reason to binge eat — especially on unhealthy foods, as you will not reap the benefits of IF. In fact, eating unwholesome junk food in a time-restricted eating window whilst IF may put you at risk of deficiencies in key nutrients such as calcium, iron, protein, and fibre, all of which are essential for normal biological function. Aim to maintain a healthy number of calories for your body during the fast so you do not “rebound eat”.
Summary
The most important thing is breaking your fast with fresh, unprocessed, nutrient-dense whole foods, prioritising healthy sources of protein and fats, all while limiting junk food that may negate the benefits of IF. Lastly, be sure to get lots of fluids in during the hours you’re not eating solid food to stay hydrated during the process! It’s important to be properly informed before changing your eating habits, so be sure to consult your GP before making any drastic changes to your diet.
By Abigail Attenborough
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