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Intermittent Fasting Can Boost Your Life and Focus

Walker, Graham, MD

doi: 10.1097/01.EEM.0000547208.29212.47

Dr. Walkeris an emergency physician at Kaiser San Francisco. He is the developer and co-creator of MDCalc (, a medical calculator for clinical scores, equations, and risk stratifications, which also has an app (, and The NNT (, a number-needed-to-treat tool to communicate benefit and harm. Follow him on Twitter @grahamwalker, and read his past columns at



I feel like I've joined a cult. There's no savior to follow, and the main thing you need to do is actually not drink the Kool-Aid. There are some rules, but they're mostly from endocrine physiology.

I'm talking about intermittent fasting. I feel like a Crossfitting, gluten-free vegan; I can't stop proselytizing about it. I feel the best I've felt in years, I've lost 10 pounds, my focus has improved dramatically, and most importantly, my body is in the best shape it's ever been. And I'm eating whatever I want.

First some background. Absolute fasts withhold food and drink; intermittent fasting is bursts of food-only fasting frequently or regularly. For me, this means 16:8 fasting; I eat from noon to 8 p.m. (eight hours) and then consume no calories (but drink sparkling water and green tea) from 8 p.m. to midnight the following day (16 hours). You can get benefits from even shorter (12 hours) or less frequent (a few times a week) or longer fasts (24 hours of fasting, then eating regularly).

I never thought I'd be able to fast. I have had the worst heartburn and gastritis since I was 12, and I figured I'd have to eat something or my stomach would tear itself apart. I'm also known for getting hangry when I haven't eaten, but what's the worst that could happen?

It was not easy at first. Day four, I was stuck at home on a conference call thinking to myself, “I can't do this. There's food everywhere!” But after that I gained so much confidence that I could fast and nothing bad would happen that I barely even notice it now, 12 weeks in.

I started focusing on macronutrients, eating controlled amounts of proteins, carbs, and fats while working out five days a week. I'd put on some muscle but also fat at the same time. The longer you fast, the more ketones you produce, but I eat plenty of carbs when I break my fast, mostly from fruit. But that's one of the reasons I don't mind intermittent fasting; there's no category of food I have to avoid.

I learned most of this from a wonderful book, The Obesity Code by Jason Fung, MD, a nephrologist and a medical expert on fasting. My take on his work:

  • Insulin is the root of most obesity and type 2 diabetes. Genetics play some part, but insulin is an anabolic hormone and makes you gain weight.
  • Insulin is triggered by eating calories (in particular, unrefined carbohydrates and proteins and less so by eating fat).
  • We've spent years worrying about what we eat but absolutely no energy thinking about when we eat.
  • If your body is constantly exposed to boluses of insulin (from consuming calories frequently), it's going to downregulate its insulin receptors (hi, homeostasis) and lead to insulin resistance. It'll also change your body weight set point, and make you put on weight that is hard to lose.
  • Your body doesn't know what a calorie is; it's a hoax that there's a “universal nutrition currency.” No one thinks that 200 calories of kale have the same effect on you as 200 calories of doughnut. Your body responds to carbs, proteins, and fats, not calories.
  • What you eat still matters, but it's much more important that you eat whole foods instead of processed junk.
  • Build up to longer fasts. Start with 12 hours by eating from 7 a.m. to 7 p.m. and slowly push breakfast further and further ahead until you make your fast longer.

I boiled all this down to a few simple rules for myself:

  • Don't eat breakfast. This was hard; breakfast food is my favorite, but now I just eat it for dinner when I want.
  • Eat whatever you want, as long as it's not processed garbage. I eat tons of fruit, whole fat plain yogurt without guilt, and add butter to my food. It's satisfying to eat fatty, delicious food.
  • Avoid refined carbs and sugar: I'm not a saint; I still break my fast with a sweet and creamy chai tea, but otherwise try to avoid foods with extra sugar.
  • Eat meals; don't snack all day. I end up eating three meals in my eight hours—a big lunch, a smaller treat halfway through (a cup of yogurt, some hummus, or some fruit), and then a big dinner. I get so much fuller on so much less food (because it's full of healthy fat!)
  • Stop drinking sugary caffeinated beverages. Sipping on a latte was spiking my insulin levels with every sip.

I'm not a strict intermittent faster but a social one. I'll have a beer with dinner. I'll cut my fasting hours if I'm having dinner with a friend. I'm not going to be that guy who refuses to eat after 8 p.m. when everyone is eating at 8:15.

I've convinced some colleagues to give this a try, and I challenge you as well. Next month: A Q&A about intermittent fasting during shifts and overnights and eating well no matter your schedule.

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