The Fast and the Furious! Intermittent Fasting and Caloric Restriction Diets

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What is the current thinking on the risks, benefits and options for intermittent fasting and caloric restriction?

In 2018, The National Institute of Health (NIH) broadly declared that there is insufficient evidence to recommend any type of caloric-restriction or fasting diet. They stated, “A lot more needs to be learned about their effectiveness and safety, especially in older adults.”

Since then, there is mounting evidence that limited forms of caloric restriction and short-term fasting often are beneficial for people of normal or obese weight when combined with exercise. First, let’s start with caloric restriction.

Caloric Restriction

Here are some low risk, high benefit ways to optimize your weight with caloric restriction. Most do not require the unpleasant task of counting calories.

  • In 2020, CALERIE 2 clinical trial results indicate that 11.9% caloric restriction of healthy non-obese people, on average, brings many improvements in aging-related biomarkers without adversely affecting psychological or behavioral outcomes.
  • Dr. Valter Longo, one of the leading researchers in fasting, nutrition and longevity, provides a longevity diet for adults. He recommends only two meals per day for overweight people, consisting of breakfast plus either lunch or dinner along with one low-sugar, under 100 calorie snack. 
  • Eliminating or reducing snacking between meals allows your body to clear out waste and burn fat. Do not eat more after dinner and before bedtime.
  • Consider reducing the number of meals you eat to 19 or 20 per week. Skip a meal when you’ve eaten too much in the previous one, when you don’t feel hungry or as part of a planned regular schedule to keep your weekly calories in line with your ideal body weight. 
  • Keeping the timing of your daily eating to within a 12 hour window (i.e. 7 AM – 7 PM) improves your metabolism. After going about 12 hours without food, your liver’s stores of glycogen — a form of energy — are depleted, and your body uses a process called ketosis to tap into your fat cells for energy. This metabolic switch places stress on your cells — mild at first, but more significant the longer you go without eating — that triggers beneficial adaptations.

With any of these, do not continue if you drop below a healthy normal weight or body fat level. For best results, combine these dietary lifestyle adjustments with eating tasty and nutritious healthy meals.  

Intermittent Fasting Diets

Beyond caloric restriction, there are over 14,000 clinical trials on the benefits of fasting which are being conducted or have completed. It is clearly a space of scientific interest. In 2019, the prestigious New England Journal of Medicine published an article concluding that intermittent fasting may help with obesity, insulin resistance, hypertension, and inflammation. Additionally, longevity expert David Sinclair notes in his 2020 book Lifespan that caloric restriction and intermittent fasting engage your body to slow aging.

However, which is the safest and most effective intermittent fasting diet? There are 5 common types of intermittent fasting diets, each popularized by fitness experts, most without scientific research. Although there are health benefits with temporarily abstaining from food, the NIH is correct because there is not sufficient research to recommend a specific diet yet, given the lack of information on effectiveness and safety controls.

That said, doing nothing and letting our body age naturally, is itself a risk. For those who wish to explore further, here are three leading emerging science-backed options.

  1. Dr. Longo has launched ProLon, a commercial company which offers his 5-day Fast Mimicking Diet, a way to still eat and gain the benefits from a fast.
  2. The 16/8 time-restricted fast (TRF) is showing beneficial results from recent clinical trials. Keeping your daily eating within a 6-10 hour daytime window (i.e. 8 AM – 6 PM) likely provides additional health benefits for your body by causing a metabolic switch inducing 2-6 hours of ketosis to reduce body fat. 
  3. Longevity company Juvenescence provides a Cognitive Switch powder and beverage. Each provide similar benefits as TRF, without having to fast. The core ingredient is a ketone ester that has been proven in mammals to be geroprotective, neuroprotective and cardioprotective. Unfortunately, it does not taste good!

If you are going to pursue TRF or any other intermittent fasting diet, please consult with your primary care physician as ketosis can impact medications and exacerbate some types of existing health conditions. And again, do not continue if you drop below a healthy normal weight or body fat level.

7 Things to Know About Doing Intermittent Fasting

So, what is the experience like while refraining from eating? Here are 7 things to know about intermittent fasting.

  1. After 12 hours, you may not feel hungry. Ketosis suppresses appetite, similar to how eating fat makes you feel full.
  2. It is difficult to eat enough calories on non-fasting days to make up for fasting days.
  3. Intermittent fasting can make you think more clearly, by stimulating brain-derived neurotrophic factor (BDNF). BDNF is responsible for the growth and maintenance of brain cells involved in learning and memory. BDNF levels decrease as people age and are associated with age-related cognitive impairments. Studies show improvement in memory in older adults who practice fasting. 
  4. It is easier to become angry (The Fast and the Furious!) when you are hungry from avoiding eating. Having difficulty sleeping and experiencing low-libido are other potential drawbacks. Exercise helps offset these issues. When fasting, you will have much more energy to workout than you’d expect.
  5. You must still consume liquids when fasting. Coffee, tea and water are healthy choices and the best calorie-free options. Coffee is also an appetite suppressant.
  6. You will lose muscle mass if you do not offset that with eating sufficient protein on non-fasting days and exercising regularly. Dr. Longo’s 5-day Fast Mimicking Diet addresses this issue well. 
  7. After fasting, your body will be more susceptible to sugar spikes, since your glucose processing will take some time to get back on track. This is dangerous to diabetics and others who have trouble regulating blood sugar.

How Does Fasting Make Us Healthier?

Skeptics of fasting note that, “It just makes life seem longer.”

However, when deprived of energy from food, the body has to rearrange its priorities. Dr. Luigi Fontana, director of the Healthy Longevity Research and Clinical Program at the University of Sydney, says one of the ways it does this is by initiating a cellular process called “autophagy,” which roughly translates to “self-devouring.” Basically, the body’s cells start to eat their own dysfunctional proteins, organelles, mitochondria, and any other disease-triggering waste that’s sitting around. Dysfunctional cells interfere with our youthful healthy processes.

Autophagy is a natural function of healthy cells. But research suggests this function breaks down as an organism ages. Fontana says an extended fast seems to stimulate the process of autophagy.

Killing off unhealthy cells is not a bad thing. “Fasting kills cells, but refeeding leads to new cells,” says Dr. Longo.“In mice, we saw a third of white blood cells die, but with refeeding, they not only went back to normal but were healthier — more like [the cells of] young mice,” he says, referring to a 2016 study that appeared in the journal Cancer Cell.

Of note, avoid any sort of long-term fasting. Going weeks without food is extreme and dangerous. There is insufficient research to be worth the risk and the dangers vary significantly per person.


Thanks for reading. As your reward for continuing to focus on your health, please go enjoy something good to eat. It may be your last meal for a while.

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