What are the Differences Between the Fasted and Fed States of the Body?

A nutrition trend that is becoming more popular is to take time away from eating and to enter a fasted state. As with many approaches to health and fitness there is little new here, fasting can be traced back to Ancient Greece and earlier, but of late a number of high level executives and celebrities have adopted the method and swear by the results.

Intermittent Fasting recently had a resurgence thanks to the 5:2 diet, where people would eat normally for five days and then dramatically reduce their calorie intake for two days. This method (and other forms of fasting), when combined with good food choices and exercise is said to reboot the immune system, improve brain function, help  weight loss and bring the body back into balance

In Silicon Valley, this method of eating has been played with and sees executives going days without eating, Phil Libin is among a number of leading business men who push fasting further, “biohacking” to improve mental focus and performance. Perhaps the most famous proponent of fasting, or biohacking is founder of Bulletproof Nutrition, Dave Asprey. His Bulletproof method proposes 18:6, 18hrs fasted with 6 hours of feeding, but starting the day with fats (MCT oil and grass-fed butter or ghee).

What is Fasting?

The uncomplicated view of fasting is abstaining from food and liquids (apart from water, tea and coffee, although some versions would have you abstain from these too) for a given amount of time. There are many protocols that you could follow; – 18hrs of fasting and six of eating, 5:2 which we have mentioned already, day fasts where you would eat nothing for a 24hr period. While these protocols exist, individuals can come up with their own methods and timeframes for feeding and fasting.

What are the Benefits of Fasting?

While conventional wisdom suggests that skipping meals is unsustainable and leads to crash dieting and ultimately failure, making intermittent fasting a lifestyle choice has the potential to yield weight loss results and help to create a healthier body.

When you enter a fasted state, various things happen to and within the body-

Insulin levels drop significantly, which helps to facilitate fat burning. Insulin is the primary hormone that dictates whether or not your body should store or burn energy. When you take in calories, your insulin goes up. Higher levels of insulin equates to greater energy storage. By decreasing insulin, the body will release energy, the energy stored in your fat cells.  With increased growth hormone levels, lower insulin levels and a higher metabolic rate, you should be burning more calories. Essentially, less food = less calories in, high metabolic rate = more calories out[1]

This is unlike the fed state, where insulin levels are high. The body takes energy from food that you have eaten. In essence, the body shuts down the burning of stored food as energy (fat and glycogen)

Another benefit of fasting, the body will begin vital cellular repair, think taking out the rubbish, the body removes waste materials from cells[2]

Through intermittent fasting there is the potential to reduce oxidative stress and inflammation, contributors to ageing and a whole host of ailments [3]

Brain function improves. Intermittent fasting reduces oxidative stress, inflammation, and insulin resistances. Some studies have shown in rats, that there is an increase in the growth of new nerve cells which have the potential to improve brain function. In fruit fly larvae, researchers discovered that the brain responds to nutrient undersupply by reducing synaptic activity.   Fasting can reduce the release of neurotransmitters from synapses in the brain which is a process that consumes a lot of energy. Limiting this activity reduces energy consumption and oxidative damage on the nervous system.

Fasted Vs Fed State for Weight Management

Intermittent fasting can enhance the ability of nerve cells in the repair of DNA, a study revealed that fasting “flipped a regenerative switch” Fasting shifted stem cells from a dormant state to a state of self renewal, triggering stem cell based regeneration of an organ or a system[4] In mice, fasting caused regeneration of the hematopoietic stem cells, essentially fasting kills off old and damaged immune cells, and when the body rebounds, it uses stem cells to create brand new, completely healthy cells[5]

From a training and performance point of view, fasting can have a positive effect on the body and can actually aid the fed state on competition day.  Fasting adds another stressor to the body, training in a fasted state forces the body to adapt, work harder and be stronger by compensating for the lack of food based fuel. When you come to compete, in a fed state, the body is more efficient at using glycogen and will be able to make maximum use of your pre-competition meal.

Learn how to incorporate fasting into your personal training clients eating plans through the body fx programme

What happens in the fed state?

We’ve looked at the fasted state, so what does the fed state look like? The fed state begins 4 hours after you have eaten. In this time, the body is absorbing digested nutrients, using some of them to meet energy needs whilst converting the rest to energy storage. During the fed state there is an increase of glucose which increases the secretion of insulin.

In the liver, excess glucose is stored as glycogen or converted to fatty acids. Whereas a fasted state forces the body to use alternative sources of fuel to keep the body working as it should, the fed state sees a continual “top up” of fuel. Excess nutrients are stored as fat. In the fed state the main source of energy is glucose, while in the fasted state the body uses fat

Should you fast?

From a weight loss perspective, there is good evidence to show that the fasted state yields positive results. You consume fewer calories, whilst increasing your metabolic rate, you force the body to use stored energy in the form of fat and you decrease insulin levels, the hormone that controls the burning of stored energy. In the fed state, insulin levels are high and the food that is eaten is used for energy, so you would rarely ever tap into the energy reserves (fat) apart from while you sleep.

More research needs to be carried out to determine whether fasting does improve brain function, decreases cancer risk, reboots the entire immune system and leads to a longer life, but current evidence suggests that a calorie controlled diet with fasting periods may well lead to a healthier and better functioning you.

A note of warning however, you should not feel sick, dizzy, or overly tired whilst fasting, if you feel any of these then eat something!

Further Reading

What are Mitochondria

Healthy eating tips and tricks