jason fung low carb md

The Complete Guide to Fasting: How To Heal Your Body Through Time-Restricted Eating – Jason Fung on Ben Greenfield Fitness

Check out the Ben Greenfield Fitness Episode Page & Show Notes

Key Takeaways

  • Eating breakfast isn’t necessary
    • Around 4-4:30 AM, your body releases “counter-regulatory hormones” (hormones that run counter to insulin: noradrenaline and growth hormone) and activates the sympathetic nervous system. These hormones serve to decrease hunger, activate your body for the day ahead, and stimulate your liver to pump out glucose.
  • When you fast, growth hormone rises—this serves to maintain muscle mass and lean tissue
  • On average, you lose an average of 0.5 lbs. of fat for every day you fast
  • The longest fast ever recorded: In the 1970s, a 456-pound man fasted for 382 days, dropping his body weight to 180 pounds
  • “A little starvation can do more for the average sick man than can the best medicines and the best doctors”Mark Twain


Ben Kicks Things Off

  • “One of the things that tends to be a prevailing theme from diet to diet—in terms of actually moving the dial when it comes to health—are periods of time when you stop stuffing calories into your gaping maw” Ben Greenfield

Should you eat breakfast?

  • Most days, Jason doesn’t
    • “Once you get used to it, it’s not very difficult. Your body takes the calories it needs from stored energy—glycogen and body fat. It’s a completely natural process.”Jason Fung
  • If you’re not hungry, why force down breakfast? There’s no reason to.
  • Ben typically doesn’t eat until ~3 hours after waking

But doesn’t eating breakfast set the circadian rhythm?

  • No!
  • Around 4-4:30 AM, your body releases “counter-regulatory hormones” (hormones that run counter to insulin: noradrenaline and growth hormone) and activates the sympathetic nervous system
    • These hormones serve to decrease hunger, “activate your body for the day ahead,” and stimulate your liver to pump out glucose
      • “You don’t have to eat to get ready for the day; your body’s already done that for you” – Jason Fung
  • Related: “Nothing shuts down growth hormone secretion like eating” Jason Fung

The 5 Physiological Stages of Fasting

  • First, some background:
    • When you eat, levels of insulin rise. This does a few things:
      • Some of the insulin is used to shuttle glucose into cells
      • Lipolysis (the burning of fat) is stopped 
      • Once your liver & muscle glucose stores (glycogen) are full, insulin directs glucose to be stored as body fat
        • Think of your glycogen stores as the kitchen fridge (which has a limited capacity) and your fat stores as the basement freezer (which has an unlimited capacity)
  • 1) The eating/feeding phase (your last meal)
  • 2) The fasting phase (0-6 hours after your last meal)
    • Towards the beginning of your fast, your insulin levels begin to drop, and your body starts to burn stored glycogen for fuel
  • 3) The post-absorptive phase (6-24 hours after your last meal)
    • Once your glycogen stores run out, the body begins breaking down damaged proteins through a process known as gluconeogenesis to provide cells with energy
      • (Thinking ahead, once you eat again, the body starts to replace/regenerate these damaged proteins)
  • 4) The fat-burning stage (24-36 hours after your last meal)
    • Up to this point, fat oxidation (burning) has been steadily increasing
      • Some of the body’s fat is converted to ketones for fuel while other tissues, like muscle, can burn fat directly
    • After ~36 hours, fat oxidation takes over, and protein burning drops to minimal amounts
  • 5) The protein conservation stage
    • At this stage, nearly all of the body’s energy is derived from fat

Caloric Restriction vs. Alternate-Day Fasting As it Relates to Lean Mass Preservation

  • In one study comparing caloric restriction and alternate-daily fasting diets over a period of 32 weeks:
    • The lean mass of the average person in the calorie-restricted group increased by 0.5%
    • The lean mass of the average person in the alternate-daily fasting group increased by 2.2%. In other words, the fasting group was better at preserving lean mass; here’s why:
      • When you fast, growth hormone rises—this serves to maintain muscle mass and lean tissue

Let’s Dive Deeper

  • As mentioned above, once the fasting process starts, the body begins to release counter-regulatory hormones (noradrenaline and growth hormone) and activate the sympathetic nervous system 
    • These hormones give you energy and “pump you up.” Think about it evolutionarily: when hunter-gathers needed to find food, this energy surge helped them to do so.
  • After a prolonged fast, your basal metabolic rate will have risen (after a 4-day water-only fast, your resting energy expenditure increases by ~10%)
    • But, if you just restrict calories by 10-20% (instead of fasting), insulin stays elevated, inhibiting any metabolic rate benefits

Why Most of the Biggest Loser Contestants Regain Weight After the Show

  • When contestants drop their caloric intake to low levels, their metabolic rate drops as well. Then, after the show, once they start eating more, they enter a caloric surplus, causing them to regain the weight.
    • (Many people experience similar struggles when attempting to lose weight—as you decrease calories, metabolic rate slows, causing the weight loss process to stagnate)

How can you maintain your basal metabolic rate when attempting to lose weight?

  • “Intermittent fasting just happens to be one of the better-studied ways to maintain your basal metabolic rate” Jason Fung

Fasting is a Great Way to Lose Weight

  • “When you don’t eat, you’re going to lose weight. That’s not hard to understand.”Jason Fund
    • “The standard advice to eat less and move more doesn’t work. We all know it, and the studies show it. Why should we not use this tool that we have [fasting], which is honestly the most powerful weight loss tool? You can’t go lower than zero calories. This is the most powerful tool, and we’ve decided not to use it? This is the craziest thing I’ve ever heard.”

Why do people get hungry towards the beginning of a fast, but then, it goes away?

  • Examine ghrelin, the hunger hormone
    • In the average person, ghrelin spikes at breakfast, lunch, and dinner (a learned behavior). But, when you don’t eat, ghrelin levels begin to decrease. Thus, over the day, hunger comes in waves.
  • How can you mitigate the hunger pangs?
    • Keep yourself busy
    • Have tea or coffee
  • Over a longer fast (3+ days), after day 2, hunger dissipates as you begin to switch into a state of ketosis
  • “For anyone listening in, get Jason’s book and try a 24-hour fast. You’d be surprised how easy it gets once you get past the cravings you’d experience when you’d normally have your first meal.”Ben Greenfield

Most Weight Loss from Fasting Comes from Water

  • When you fast, on average, you lose an average of 0.5 lbs. of fat per day (most of the weight loss you experience during a fast comes from water weight)

Has Jason done a prolonged 7+ day fast?

  • No, but he does a lot of 24-hours fasts
  • We don’t quite know how long you have to fast for autophagy to begin, but many of the metabolic benefits of fasting start to occur after ~24 hours
    • (For reference, autophagy is the process by which damaged cells are killed off)

Fasting for Type 2 Diabetics

  • After two weeks of fasting, many of Jason’s diabetic patients can stop taking insulin
  • As diabetics are prone to many diseases, Jason likes to push their limits with long-term fasting
    • “These are high-risk people. If you don’t do anything, they’ll develop a lot of different diseases: heart attacks, strokes, kidney disease, eye disease, amputations—all kinds of things. The rewards are much higher, so we’ll push the envelop more for these patients.” – Jason Fung
  • Jason notes that if you’re a diabetic and decide to do a long-term fast, you MUST adjust your insulin dosage 
  • “When you put diabetics on very low-carb diets and use intermittent fasting, you can reverse the disease because their blood sugar starts coming down … That’s completely opposite to the message that comes from the American Diabetes Association and most doctors, which is that type 2 diabetes is chronic and progressive … That’s not true. As you lose weight, that diabetes goes away.”Jason Fung

How to Break Your Fast

  • After a long-term fast (5+ days), when you eat your first meal, insulin rises, but because you’re malnourished, phosphorus levels in the blood plummet (all the phosphorus is going back into your cells due to the high insulin load—this can lead to heart arrhythmias)
    • That said, your risk of the above occurring is very low (unless you’re incredibly skinny
  • Jason’s advice for breaking a fast:
    • Eat a small meal (and eat it slowly), so your insulin doesn’t spike too much
    • Avoid refined grains and sugar when refeeding. Instead, eat a meal higher in protein.

The Longest Fast Ever Recorded

  • Ben reads an excerpt from Jason’s book:
    • “In the 1970s, a 27-year-old Scottish man started fasting at a weight of 456 pounds. Over the next 382 days, he subsisted on only non-calorie fluids, a daily multivitamin, and various supplements. He set the world record for the longest fast. His body weight decreased to 180 pounds.”

Wrapping Up

  • Fasting isn’t the latest and greatest fad; it’s the oldest dietary intervention in the book!
  • “A little starvation can do more for the average sick man than can the best medicines and the best doctors” Mark Twain

Additional Notes

  • Frequent meals do NOT increase the speed of your metabolism
  • 2-4x a month, Ben does a 24-hour fast from Saturday to Sunday dinner
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