Time Restricted Feeding for the Endurance Athlete

April 19, 2018

Since 1950, obesity rates in the adult population in the Western World have increased from 13% to 35%. Adolescent obesity is sadly not far behind, with 18% of teenagers being obese. Aside obesity, 75% of the US population is considered overweight (BMI 25~30). Excess weight increases the risk of cardiovascular diseases, cancer and brain diseases. Obesity and overweight have become the epidemic of our time and we are in fact living in a time bomb. Higher life expectancy achieved through medicine and pharmaceutical approaches are “patches”, and as more of the obese population reaches the old age, the higher the pressure in our medical system. Combine this with the inverted population pyramids seen across the Western World, with more elderly people benefiting from the health system than those contributing in their working age with their taxes, and we are facing a social disaster.

We are constantly exposed to the notion of “being fat”, so it’s not unexpected that whenever we talk about fasting nowadays we assume we are talking about _yet another new fashionable diet that probably a celebrity or two have made popular and has become all the rage for weight loss. When I mention to somebody the notion of fasting on a daily basis, or intermittent fasting (IF), I always get a strange look. Fasting does not sound good, and scientists have been using instead the term time restricted feeding (TRF). TRF is also known as Intermittent Energy Restriction (IER) or Intermittent Fasting (IF).

Thinking about society is all good and fine, but why do I care? There is a higher risk in my father’s family of heart / cancer / brain problems, and since I became 40 I have been very conscious about this. I don’t know about you, but I am scared of getting old and having to take 11 different pills daily and visit the doctor every few weeks. That’s not really quality of life for the elderly. We live longer, not better. I pro-actively manage my health to age gracefully, and this is also why I went back to sports and exercising regularly. Having read about TRF/IF, I decided to try it a few months ago. Since then, I have been going deeper into the existing research publications in this topic, and also seeing the results first hand in myself.

Many people do TRF/IF for weight loss (including celebrities). I think of TRF as a way to keep me healthy, not lose weight. I went from overweight (220lbs / 99kg) to healthy weight (160 lbs / 74 kg) by balancing my level of exercise and diet. This post summarizes my findings regarding the alleged health benefits of TRF, and to relate to how TRF interacts with exercise.

When Rather than What

TRF is not about what we eat, but when we eat. In TRF, we eat the same food and amounts we would otherwise consume, but instead of eating the food over 16 hours, from dawn till dusk, and fasting for 8 hours while we sleep, we restrict eating to a relatively narrow window of 4 to 8 hours, and fast for the rest of the day. In the fitness world, the 16:8 diet introduced by Martin Berkhan of Lean Gains, is the most popular form of TRF, although others exist such as alternate day fasting, or restricting feeding to 500 kcal two days per week. Whatever the fast/feed protocol, in TRF the body gets the same energy and the same nutrients it would otherwise, just within a shorter time window (the feeding period aka the feast). TRF is not a diet restricting energy or some food groups. Eating a balanced diet rich in macro-nutrients is as important as ever.

What we know is that TRF has benefits in the following areas based on studies on animals and humans:

  • Preventing heart diseases and diabetes,
  • Reducing adipose tissue, specially visceral fat around the abdomen,
  • Removing damaged molecules and tumors (cancer),
  • Preventing Alzheimer, Parkinson’s, and other brain diseases,
  • Slowing aging.

There is a caveat. Although there is already a large volume of academic publications covering the benefits of TRF, it’s still not yet well understood its full impact on humans. With that caveat in mind, let’s dive in.

Eating, Insulin, and Tissue Inflammation

Obesity promotes inflammation of the local tissues. And all major diseases, including cardiovascular, diabetes, cancer, brain diseases, involve chronic inflammation of the local tissues. In fact, inflammation is recognized as a contributing factor for tumor cells growth. Excess eating causes inflammation.

As we eat, we ingest carbohydrates that get synthesized as glucose in the digestive track. First, we refill the liver “easy energy” reserves, as glycogen, where we can store up to 500 kcal. Then, as more glucose comes in from the digestive system, the pancreas starts releasing the insulin hormone. The insulin hormone is the energy transport for glucose into our cells, and it signals our cells to respond to the glucose. The glucose is either burnt immediately in the cells, stored for later use as glycogen in the muscles, or transformed into fat. These reactions all require insulin to happen. What’s important to remember is that if our cells are not using the glucose, the insulin hormone promotes its absorption from the blood into fat.

When our diet includes consuming a lot of simple sugars, especially fructose, we become less sensitive to insulin and develop insulin resistance. In individuals with insulin resistance, the body cells respond no longer to the insulin being released in the blood stream, glucose levels remain high in the blood, blood insulin also continues to rise, and eventually glucose gets absorbed into fat cells. This condition is called metabolic syndrome, which causes high belly fat, high blood pressure, and leads to cardiovascular diseases and type II diabetes. This is why insulin resistance is associated with excess weight and obesity, and people who become obese develop insulin resistance, which in turn makes it much harder for them to lose weight. Excess eating causes insulin release, and insulin causes chronic inflammation in individuals who are overweight and have developed insulin resistance.

Circadian Rhythms, Eating and More Inflammation

Circadian rhythms are 24 hour oscillations in activity and metabolism, mainly regulated by light. Circadian clocks help us coordinate our active / rest periods within a 24 hour daily cycle, switching between light and darkness. Anyone who has suffered jet lag or worked night shifts knows the effect of confusing our circadian clock. In our modern society we are eating throughout the day, also confusing our circadian system.

Unstable circadian rhythms cause reduced glucose tolerance, increased blood pressure, and resistance to insulin. A confused circadian system also decreases the effect of the hormone leptin, which is responsible for feeling full after eating.

A possible explanation for why our eating affects our circadian clock is that for thousands of years, in the absence of artificial lighting, we were active during the day, and we rested during the night. We would therefore feed during the day, and leave long hours of fasting overnight.

In summary, our modern lifestyles see us eating throughout the day and the night, causing insulin resistance and inflammation, removing the satiety effect of leptin, which leads to further feeding, usually as snacks containing simple sugars, which further increase fat storage and tissue inflammation.

My Personal Fasting Protocol

There are multiple ways to do TRF. To determine which one to use I have analyzed the effect of fasting. As the fast advances, the insulin concentration found in the blood decreases. The insulin concentration decreases as a function of time with a negative exponent, asymptotically converging to about 30 pmol/L. The largest drop in insulin concentration occurs between 16 and 24 hours of fasting, to a level of about 40 pmol/L, which is why the optimum protocol is to fast intra-day, rather than inter-day (such as with the 5:2 diet where 2 days per week the individual restricts energy intake to 500-600 kcal).

The protocol I have adopted is therefore based on Martin Berkhan’s 16:8 fasting:

  • Limit eating to 8 hours. I try ensure this is done during the day, not night.
  • Fast for 16 hours. In my case, 8pm to 12pm.
  • Exercise cardio, in fasted state, around 5-6am. At 10 hours into the fast, we are starting to burn fat, but still have glycogen reserves in the liver. I usually limit my exercise below ventilatory lactate threshold, and whenever I want to do work above lactate threshold or VO2Max work, I load up at dinner time more carbohydrates.
  • I don’t ingest any BCAA, either pre- or post-workout. I don’t feel this is necessary, as the studies show that during fasting the body conserves the fat-free mass, and focuses on burning fat mass. As a result, we get leaner. Personally, I have not experienced any loss of muscle mass during my fasting.
  • I drink coffee pre- and post-workout, till the fast ends. I don’t drink any coffee otherwise during the day. I drink the coffee black, without milk.
  • I drink a lot of water during the whole day, especially sparkling. I don’t use any artificial sweeteners or drink zero calories sodas.

My diet is otherwise quite balanced. My BMR is 1800 kcal/day and I usually burn between 800 and 1200 kcal/day during exercise. My total metabolic rate is 2600-3000 kcal/day, which I break into (2600 kcal numbers) the following macro-nutrients:

  • 80g of fat, mostly nuts.
  • 190g of protein, through yogurt, cottage cheese, eggs, tuna, salmon. I eat limited amounts of animal meat.
  • 280g of complex carbohydrates, through vegetables, beans, grains, and fruit, and I don’t have any issue eating a treat here and there.

I like to eat a “rainbow” of food, fresh vegetables and fruit mostly, to ensure I am getting all the necessary minerals and vitamins. Other than that, I don’t take mineral or vitamin supplements, except sometimes fish oil, calcium and iron.



Autophagy is the cellular self-cleansing process that removes damaged cells. When insulin drops during the fasted state, autophagy increases 5-10 times (which is likely a result of being aligned with the natural circadian rhythm).

As we age, we accumulate damage, and we can no longer recycle damaged cells as easily. One of the benefits of autophagy is therefore the slowing of aging: when we are young, there is a high rate of of autophagy, and as we age, the rate of autophagy decreases. With TRF, we can achieve the rates of autophagy of a younger individual.

Autophagy is reduced in the presence of insulin, so obese or overweight individuals with insulin resistance have suppressed autophagy and age faster.


With lower insulin concentrations, which happen on low blood glucose as a result of the fasting, the use of insulin switches from transporting the glycogen stored in the muscles into glucose that can be burned. In order to produce energy, the body switches to fat breakdown, and small reductions in insulin concentration result in large increase of fat breakdown (lipolysis).

Exercise improves the sensitivity we have to insulin, and as a result the drop of insulin levels, and hence lipolysis is faster in individuals that exercise regularly.

Metabolic Syndrome

Results from animal studies show TRF to be associated with reductions in body weight, total cholesterol, and concentrations of glucose, insulin, as well as with improvements in insulin sensitivity. Human data support the findings of animal studies and demonstrate decreased body weight, lower concentrations of triglycerides, glucose, and low‐density lipoprotein cholesterol, and increased concentrations of high‐density lipoprotein cholesterol. These preliminary findings show promise for the use of TRF in modulating a variety of metabolic disease risk factors.

Neurodegenerative Disorders

TRF increases the levels of the antioxidant enzymes. Studies have shown that TRF can protect neurons against oxidative and metabolic stress in animal models of neurodegenerative disorders, including Alzheimer’s and Parkinson’s diseases. These effects are accentuated by physical exercise and intellectual activity.

Brain-derived neurotrophic factor (BDNF) is up-regulated in hippocampal neurons in response to TRF and exercise, which induces mitochondrial biogenesis. Mitochondrial biogenesis is critical for the formation of synapses in developing hippocampal neurons and the maintenance of synapses in the hippocampus of adult mice.


Tumor cells produce their energy through glyolysis, meaning that they require access to large amounts of glucose in the blood. Because TRF decreases blood glucose, it inhibits and even reverses the growth of a range of tumors, including neuroblastoma, breast, and ovarian cancers. Ketogenesis may also play an important role in suppression of tumor growth as tumor cells are largely unable to use ketones as an energy source. Ketogenic diets rich in fat and protein may potentiate the antitumor effects of TRF.


Unlike many diets, time-restricted feeding is easy to do, and easy to make it a habit. Additionally, TRF does not restrict food groups, and therefore allows a balanced and healthy diet with all required macro-nutrients and nutrients. When TRF is combined with regular exercise and intellectual activity, it is a potent agent for slowing down aging, reducing the risk of cancer and brain diseases, and developing insulin sensitivity.

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