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When I tell people that I'm doing intermittent fasting (aka time-restricted eating) and I try to only eat within a four-hour window every day, I often hear that they can't go that long without eating.  I eat when I'm hungry and don't when I'm not so I find intermittent fasting to be easy for the following reasons:

  • Maintaining low insulin turns on gluconeogenesis and this keeps my blood sugar very stable (no low blood sugar).
  • Low insulin turns on fat metabolism resulting in ketogenesis, and ketones tend to have an appetite suppression effect.
  • The hunger signal is affected by ghrelin, which peaks on a habitual schedule.  Continued intermittent fasting tends to reprogram the ghrelin schedule.
  • After breakfast (my first meal of the day), I'll be hungry again within a hour or two and have a bit more food.

I'm lucky enough to have a drug plan that includes Continuous Glucose Monitors, which is great tool to understand how diet and activity affect my blood sugar.  If its available, I highly recommend using a CGM to get instantaneous feedback on what's happening in the body.  CGMs measure interstitial fluid rather than blood so the absolute value of the reading is somewhat inaccurate.  Some things I've learned (in no particular order):

  • The Dawn Phenomenon happens every morning and what and when I eat the night before has a major impact on its magnitude and duration
  • The Dawn Phenomenon is more pronounced because of glucose intolerance rather than physiological insulin resistance.  I know this because eating carbs will cause my blood sugar to rise and then immediately fall to a normal (low and stable range of  4.5-5.5 mM / 81-99 mg/dl).
  • My blood sugar gradually decreases from its morning high to a normal range by early to mid afternoon.  I generally don't feel any hunger during this time although I might occasionally feel a low-grade hunger that goes away with activity.
  • It takes about 20 minutes of bike riding (a pace where I can feel a slight muscle burn) before I see a slight rise and then a drop in my blood sugar.  Continued hard riding results in normal blood sugar that may drop to the 4.0-4.5 mM (72-81 mg/dl) range.
  • Walking has a different effect on blood sugar.  Increasing my pace tends to increase my blood sugar and slowing tends to reduce it.
  • Regular exercise (biking, walking, etc) significantly improves metabolic markers (HbA1c, HDL-C, Triglycerides, Serum Creatinine, etc)
  • After eating a lot of carbs, I usually see my blood sugar rise very quickly and then fall just as fast.  Depending upon the carb load, my blood sugar might bottom out around 3.9 mM (70 mg/dl) for a short time before returning to a normal range.  I don't find low glucose to be noticeable, unlike in my pre-ketogenic time.
  • As I became more fat-adapted and only ate when I got hungry,  I found that I just kept deferring breakfast to a later time (1 hour later eventually became late afternoon).
  • I sleep better and have better morning blood sugar if I don't eat for couple of hours before bedtime
  • The longer I do intermittent fasting, the longer I can go without feeling hungry so don't expect to immediately be able to fast for 20 hours.
CGM After Breakfast

There are numerous long-term benefits to intermittent fasting and, since fasting lowers insulin with resulting fat metabolism, ketogenic diets naturally complement intermittent fasting.  Intermittent fasting with a ketogenic diet is lifestyle choice and should not be done as a short-term way of losing weight. Some of the benefits of intermittent fasting:

  • improved insulin sensitivity
  • prevention or reversal of low-grade inflammation
  • prevention or reversal of chronic diseases
  • improved longevity and healthspan
  • improved cognition through neurogenesis
  • less medication

Fasting can cause a loss in lean muscle tissue in addition to fat tissue.  Resistance training exercise (weight lifting) is highly anabolic and counteracts muscle loss.  Other physical activities are beneficial but not as effective.  Physcial activity has numerous metabolic benefits in addition to maintaining muscle mass.  The best exercise is the one you will do regularly.