5.7 Intermittent Fasting & Time Restricted Feeding (TRF)

Fasting is a centuries old traditional approach to managing different conditions. In recent years science has provided the evidence into the physiological and metabolic mechanisms whereby using fasting patterns can be clinically useful. This means that we don’t just consider the types of foods (high alkaline load, anti-inflammatory etc.) in functional nutrition but also the times we eat it in order to optimise metabolism and the endocrine and immune system responses that follow.

There’s now a wealth of clinical evidence showing that the fasting state improves:

• Obesity
• Type 2 diabetes
• Metabolic syndrome
• Insulin resistance
• Fatty liver
• High blood pressure
• High cholesterol
• High triglycerides

The main metabolic drivers when in the fasted state are reduction in pro-inflammatory markers. Studies have even shown that the fasted state can improve cognitive processes linked to dementias and Alzheimer’s disease, which has also been described as Type 3 diabetes, as well as alter the gut microbiome positively influencing immune states such as Multiple Sclerosis (MS). Please read the following articles, which are part of this module content:

• Farris et al (2012) Intermittent fasting during Ramadan attenuates proinflammatory cytokines and immune cells in healthy subjects. Nutr Res 32:947-955 Full paper
• Mattson et al (2018) Intermittent metabolic switching, neuroplasticity and brain health. Nat Rev Neurosci 19:63-80 Full paper

On the face of it, fasting seems simple – just avoid food! However, this is not necessarily the most appropriate way to address health when taking into account metabolic and physiological requirements, as well as the social and cultural aspects of eating food. The mere abstinence from food for long periods of time (anymore than 12 hours for many people) can lead to symptoms and conditions related to low blood sugar levels, hormonal imbalance and nutrient deficiencies if the metabolic system is not used to being in the fasted state. Remember that the liver needs a number of dietary acquired vitamins, minerals and nutrients for Phase 1 and 2 biotransformation pathways, as well as all cellular ATP production relying on dietary water-soluble B vitamins.

So what is the modern fasting message? How can you safely fast to produce positive long-term metabolic changes without perturbing the system and metabolism in the short term?

Fed versus fasted state

Your body is designed to smoothly transition between two different and opposing states: ‘Fed’ and ‘Fasted’.

In the fed state, with sugar present in the blood, insulin is elevated, and this signals to your body to store excess calories in your fat cells. In the presence of insulin, the burning of fat is halted, while the body burns glucose (from your last meal) instead.

In the fasted state, insulin is low (while glucagon and growth hormone, opposing hormones to insulin, are elevated). The body starts mobilising stored body fat from your fat cells and burning this fat for energy (instead of glucose). The practical importance of all this? You can only burn stored body fat while in the fasted state and you can only store more body fat while in the fed state (Figure 5.4).

Insulin resistance

Unfortunately, over time we seem to be spending less and less of our time in the fasted state and more and more time in the fed state. As a result, our bodies and our cells spend less time mobilising and burning stored body fat for energy, and the glucose-burning pathways are overused. Eventually, insulin is high all the time and the body avoids burning stored body fat, relying mostly on glucose.

Over time, this chronic exposure of the cells to increased levels of insulin in the blood leads to insulin resistance where the body secretes even more insulin but the message at the cells to take up blood sugar for energy is blunted. This instead promotes fat storage and suppresses fat mobilisation from the adipocytes leading to weight gain and a state of chronic inflammation with pro-inflammatory markers released from metabolically active adipocytes.

Someone with insulin resistance is burning predominately glucose on the cellular level to produce ATP, and they rarely ever get the opportunity to burn any body fat. When these people run out of glucose from their last meal, instead of easily transitioning over to the fasted state to burn fat, they become hungry for more glucose (i.e. crave carbohydrates), as their bodies and cells have decreased capacity for mobilising and burning fat for energy; that is to say these people create ATP from inefficient glycolysis pathways in the cells rather than the more metabolically efficient mitochondrial oxidative phosphorylation pathways. Eventually, the fat-burning pathways are down-regulated further exacerbating the problem. Remind yourself about cellular ATP production pathways in Module 3 Part 1.

As a result, an overweight “sugar burner” who stops eating for a few hours will crave refined carbohydrates when their blood sugar levels drop from their last meal instead of seamlessly transitioning to the fasted state and mobilising and burning stored body fat. They will spend most of the day trapped in a cycle of eating every few hours, spiking glucose, and then becoming hungry again when blood sugar drops.

Fat adaptation

Humans have the ability to become ‘fat-adapted’ and improve their ability to fuel themselves with stored body fat instead of glucose. However, this takes time and practice, and your body has to do a number of things to slowly up-regulate (or increase) the fat-burning pathways.

This includes improving insulin sensitivity to lower insulin and promote fat mobilisation into free fatty acids from the adipocytes as well as up-regulating the mitochondrial fat-burning pathways. There are several ways to improve ‘fat adaptation’ (or the ability to successfully burn stored body fat for energy) and insulin sensitivity, some of which we have already discussed:

• Paleo diets and to a certain extent low GI diets. Avoiding refined sugar and high starch carbohydrate foods improves the body’s ability to utilise fat for energy rather than glucose, as there is more fat and less glucose available at all times, even in the fed state.
• High-intensity exercise depletes glucose and glycogen rapidly, forcing the body to switch over and utilise more fat for fuel. Exercise also improves insulin sensitivity.
• Caloric restriction. Eating fewer calories also equals less glucose available for fuel, so the body is more frequently forced to rely on stored body fat for fuel. However, simply avoiding high calorie foods can lead to avoiding nutrient dense food, like nuts and seeds, so to follow a straight forward calorie restriction diet without considering the foods involved is not normally recommended unless carefully considering the foods that contain the calories thereby maximising nutrient density from eating contained portions of whole, natural, unprocessed, real foods found in nature (avoiding processed foods completely) to feel full. Remember that:

Maximum nutrient density = maximum satiety

• Intermittent fasting, and spending more time in the fasted state – gives the body more ‘practice’ at burning fat.

Figure 5.6 summarise the effects of diet on insulin levels and fat adaptation:

We have already discussed the Paleo diet and reduction of high GI foods as way of managing insulin response and system imbalances, so let’s now look in more detail about the timing of food intake and how this can positively impact metabolic and inflammatory imbalances.

Metabolic Exercise with Intermittent Fasting

Intermittent fasting can be used as a strategy for exercising and strengthening the body’s ability to exist in the fasted state, burning fat instead of continually burning glucose (sugar) from the fed state.

Just like anything else, this ability can be strengthened over time with practice. But this ability also atrophies over time with lack of use; just like your muscles atrophy when you break your arm and have to wear a cast for weeks. Spending time in the fasted state is actually a form of exercise – a METABOLIC WORKOUT – in which you train your body to rapidly and efficiently mobilise free fatty acids from adipose stores; something you improve on with the metabolic ‘practice’ of fasting.

Less feeding, more fasting

Training yourself to eat two meals a day (and eliminate snacking) is the goal to achieve a fasting state that can also be incorporated into a way of life compatible with work, family and social life. The easiest and best way to accomplish this is to leverage your natural overnight fasting state by skipping breakfast (having a good quality coffee without milk can help at this stage if you can tolerate caffeine), eating a lighter lunch and larger dinner (comprising of anti-inflammatory, high alkaline and low GI foods) also maximises the body’s natural shifts between sympathetic (“fight or flight”) and parasympathetic (“rest and digest”) nervous system tone. This shift in the autonomic nervous system is part of the body’s natural circadian rhythms, important in regulating 24 hour cycles involved in appetite and sleep.

Typically, the fed state starts when you begin eating and for the next three to five hours your body digests and absorbs the food you just ate. Insulin rises significantly, completely shutting off fat-burning and also triggering excess calories to be stored as fat. After this phase, the body goes into what is known as the post–absorptive state, during which the components of the last meal are still in the circulation. The post–absorptive state lasts until eight to twelve hours after your last meal, which is when you enter the fasted state. It typically takes twelve hours after your last meal to fully enter the fasted state. When you’re in the fasted state your body can burn fat that has been inaccessible during the fed state.

For many people with a three meal a day eating schedule the fasted state overnight is less than twelve hours (see Figure 5.5). For example, eating you last meal of the day around 19.30-20.00pm and eating breakfast at 6.30am means a fasting state of only around eleven hours – not taking into account any food or milk containing drinks consumed after dinner later in the evening. This is one of the reasons why many people who start intermittent fasting will lose fat without changing what they eat, how much they eat, or how often they exercise. Fasting puts your body in a fat burning state that you rarely get to enter during a normal eating schedule.

Eating no calories at the beginning of the day and the majority of your calories later in the day is a concept called a caloric reverse taper and can be used to stimulate the metabolism to burn fat even more efficiently, as well as achieve better quality sleep over night from a higher alertness and activation from sympathetic tone during the day through fasting and light meals then engaging a higher parasympathetic resting tone in the evening from the fed state. This latest science goes against the old adage of “breakfast like a king, lunch like a prince and dinner like a pauper” but the metabolic outcomes of this way of eating calories through the day, as well as the fasting timing, can greatly benefit the health of certain individuals.

How to fast intermittently – Time Restricted Feeding (TRF)

There are a number of ways to actually perform intermittent fasting, but the easiest and most popular varieties involve taking advantage of the natural overnight fast by skipping breakfast and pushing the first meal of the day forward a number of hours. Once past the twelve hour mark from dinner the night before, you are truly in a fasted state and the body begins to rely on stored body fat for fuel.

The longer the body stays in the fasted state, the more metabolic practice the body gets at burning stored body fat and the deeper the fat adaptation will get. In fact, if you can maintain intermittent fasting for twenty to twenty four hours you will achieve a very high rate of lipolysis (breakdown of stored body fat into free fatty acids, available for burning in the cells) and fat oxidation (burning of fat in the mitochondria).

However, twenty four hour fasts are not feasible for many people. The 16:8 diet (or fasting for sixteen hours overnight plus between the first four to six hours of the day and eating in an eight hour window) is just as effective (Figure 5.8). For example, let’s say you get up at 06:00; you would skip breakfast and eat nothing for six hours, then lunch at noon and dinner at 20.00. Snacking inside your eating window is allowed. During the fasts you can still drink non-caloric beverages including water, coffee and/or (without sugar or milk), herbal and fruit teas including green tea.

This 16:8 split (sixteen hours fasting and eight hours eating) can be used a couple of times a week for flexibility in work and social schedules, or even every day.

When first starting out with intermittent fasting, hunger and low energy can predominate. In this case taking “baby steps” by pushing breakfast out by an hour or two at first, then slowly increasing the fasting interval, can help. As time goes by and body metabolism becomes more fat adapted, it is easier and easier to fast. This is identical to exercise in those who are sedentary: it is painful and extremely difficult at first, and then once you are adapted it gets easy and even enjoyable!

Exercise helps greatly with fat adaptation. Glycogen is depleted during sleep and fasting, and will be depleted even further during training, which can further increase insulin sensitivity. This means that a meal immediately following your workout will be stored most efficiently: mostly as glycogen for muscle stores, burned as energy immediately to help with the recovery process, with minimal amounts stored as fat.

Intermittent fasting summary

• Our bodies are evolutionarily designed to go for periods without feeding, i.e. to be in fasted state and use the body’s own energy (fat) reserves.
• Fasting for more than twelve hours can reduce insulin levels, improve insulin sensitivity, reduce pro-inflammatory markers, stimulate autophagy (the body’s natural regeneration process) and stimulate fat burning in mitochondria to increase ATP levels without even changing the types of food that are eaten.
• Using the body’s natural overnight fasting window is the easiest way to increase the fasting state.
• Skipping breakfast, eating nutrient dense foods (i.e. not focussing on calories but more on whole, fresh foods like nuts) and eating within an eight to ten hour window during the day puts the body into a fasted state to burn fat.
• Regular fasting for minimum two days a week can help improve a number of health markers.

Please make sure you’ve read the articles, which contribute to the content of this section:

• Farris et al (2012) Intermittent fasting during Ramadan attenuates pro-inflammatory cytokines and immune cells in healthy subjects. Nutr Res 32:947-955 Full paper
• Mattson et al (2018) Intermittent metabolic switching, neuroplasticity and brain health. Nat Rev Neurosci 19:63-80 Full paper