Fasting for Weight Loss and its Effect on Blood Sugar

In the last couple days I've received a number of emails and posts about using short periods of fasting for weight loss and its effect on blood sugar.

In a post to my blog I was told that a couple people who are diabetic have been using short periods of fasting with a lot of success and have actually been able to lower the dosages of their medications.

I also received an email from a personal trainer who wants to use Eat Stop Eat with her clients but is worried about the effects that using short periods of fasting for weight loss would have on blood sugar levels. Mostly, she was concerned about hypoglycemia.

Throughout the 24 hour cycles of eating, digestion, and fasting, the amount of glucose in your blood is generally maintained within a range of 70-140 mg/dL (3.9-7.8 mmol/L) as long as you are healthy.

Hypoglycemia is another way of saying "low blood sugar". While many people claim to suffer from low blood sugar, as little as 5-10% of the population actually have a malfunction in their ability to regulate their blood sugar levels. There is no actual "cut off" value for blood glucose levels that truly defines hypoglycemia for all people and purposes.

Research in healthy adults shows that mental efficiency declines slightly but measurably as blood glucose falls below about 65 mg/dL (3.6 mmol/L). However, the precise level of glucose considered low enough to define hypoglycemia is dependent on the age of the person, the health of the person, the measurement method, and the presence or absence of negative effects.

According to the research on using fasting for weight loss, a 24 hour fast should not place you into a hypoglycemic state, and I have not seen any research that has shown a subject going below 3.6 mmol/L blood sugar during a short term fast. However, I'm the first to admit that there is a difference between research and real life, so I decided to experiment on myself.

I took blood glucose measurements on myself during a 24 hour fast. It's important to note that this investigation only had one subject (me) and that testing hypoglycemia with the finger prick technology is prone to misleadingly low numbers. Nonetheless, I couldn't help but be intrigued with what I may find.

(Good for research, bad for my fingers)

I started my experiment at 1:10 pm with a Bowl of Chili, a whole wheat bun an old fashion dip donut and a large coffee with two cream and one sugar, all courtesy of my friends at Tim Hortons ( ~750 Kcals, 40 g fat, 70 g Carbs, 30 g Protein)


I began measuring at 2:15, exactly 45 minutes after my last sip of coffee. The first numbers are my blood glucose in mmol/L, the number in brackets is my blood glucose levels in mg/Dl.


2:15 PM TUES- 5.9 (106.2)

4:15 PM TUES- 4.8 (86.4)

6:15 PM TUES- 4.4 (79.2)

8:15 PM TUES- 4.3 (77.4)

10:15 PM TUES- 4.6 (82.8)

12:15 AM WED- 4.0 (72) (Bed Time)

3:15 AM WED- 4.4 (79.2)

6:15 AM WED- 4.7 (84.6) (Right after I woke up)

8:15 AM WED- 4.6 (82.8)

9:00 AM WED- 4.4 (79.2) (A little bit of hunger here)

10:15 AM WED- 4.1 (73.8)

12:15 PM WED- 3.9 (70.2)

1:30 PM WED- 3.9 (70.2)


At 1:30 I ate my first post-fast meal, which was supposed to be a small chicken salad, but unfortunately it was a 1,200 Kcal Bomb of a salad from Boston Pizza (1,200 Kcals, 86 g Fat, 45 g Carbs, 62 g Protein)...My mistake, I was at a business meeting, but still, I should have known better.


Twenty minutes after finishing my heart attack of a salad, I started recording again.


2:00 PM WED- 5.9 (106.2) 20 minutes after my last bite

2:20 PM WED- 5.8 (104.4) 4o minutes after my last bite

2:45 PM WED- 4.8 (86.4) 65 minutes after my last bite

4:45 PM WED- 4.4 (79.2)


The interesting things I learnt from this study were that my feelings of hunger were not related to my blood sugar levels, nor was any feeling of light-headedness.


I was also very surprised that despite the massive amounts of sugar, my blood sugar never went above 6, and dropped pretty quickly after my meals. It stayed pretty tightly controlled between 3.9 and 5.9.


All in all, this experiment showed me that while I am fasting my blood sugar stays within the normal range. I will repeat the experiment during my next fast and study what happens when I workout while fasted.


BP