In a nutshell, intermittent fasting is just that: fasting intermittently.
It’s limiting calorie intake during certain hours/day or days/week. It’s more of an eating pattern than a diet. It limits when to eat, and not so much what to eat. And that’s part of its appeal to people who don’t want to count calories or use their food log to track everything.
Some would say that it’s a more natural way to eat because humans evolved without refrigerators, drive-throughs, or 24-hour convenience stores. We now have access to food (including junk food) all day long, so eating several meals per day plus snacks may be less natural than fasting from time to time. Fasting is one of the oldest healing remedies out there, touted by Hippocrates and Galen, and is part of almost every major religion of the world.
There are lots of variations on this theme. They include:
- 16/8 which is 16 hours of fasting, and eating only within the other 8 hours (for example 12:00 pm. – 8:00 p.m.);
- 5:2 days of fasting, where you eat regularly for five days of the week, then take in just 500-600 calories/day for the other two (non-consecutive) days.
Fasting allows for blood to be shunted toward other areas than the digestive tract, such as the brain, which is why you feel more mentally alert while fasting (think about how hard it is to focus after a large lunch). It also allows the body to repair itself which is why we naturally fast when we are sick.
Is intermittent fasting effective for weight loss?
Intermittent fasting can help to lose weight because it can help you to eat fewer calories, and burn more fat.
Glucose is the body’s main source of energy. When glucose runs out, the body starts using up fat stores. If we keep replenishing our glucose by eating food, then there’s no need to dip into our fat stores. Glucose stores last for about 24 hours. Of course, the less carbohydrates you eat throughout the day, the faster you will deplete your stores while fasting. Don’t expect to go into ketosis unless you are fasting for at least 24 hours, or you are following a ketogenic (extremely low carb) diet.
However, just because you may not be completely depleting your glucose stores before your fast is over, doesn’t mean the fast didn’t do anything…
Fasting also improves insulin sensitivity which is key in weight management. Increased sensitivity to the hormone insulin means we use the glucose in our blood stream for energy. When we are insulin resistant, seen in overweight people, and in extreme cases, type 2 diabetes, less glucose gets used for energy, and more gets stored as fat. Insulin levels begin to fall only 6 hours after a meal.
Fasting also increases growth hormone, which helps preserve muscle mass and bone density, and helps the body to further use fat for energy.
What about good old calorie restriction?
Studies show that the same adaptive effects seen in intermittent fasting (IF) are not seen in day-to-day calorie restriction. IF keeps the body guessing and does not allow it to become resistant to the change. Calorie restriction slows down metabolism, IF does not. It actually increases adrenaline, and therefore energy expenditure.
Sticking with a diet is one of the keys to weight loss success. So, if you can’t stay with a weight-loss diet, you’re less likely to lose the weight and keep it off. IF can also be easier to stick to, as it has an end point, rather than continuously restricting yourself.
Lots of people say they have success with it. But what do the studies say?
According to one review study, intermittent fasting helped people to lose 3-8% of their weight over 3-24 weeks. In this study, people also lost 4-7% of their waist circumference (i.e., belly fat).
Studies of alternate day fasting show 6% decreases in body weight, 11.4% decreases in fat mass, with no change in lean mass. Cholesterol markers improved as well.
Another study found that eating a single meal per day, compared to eating 3 meals per day, with the same caloric intake, resulted in significantly more fat loss with no muscle loss.
There are studies showing similar weight loss in intermittent fasting people and calorie-restricted people, but better markers of insulin sensitivity in the IF groups which may be the key to long-term success.
Before you consider intermittent fasting
Intermittent fasting is not for everyone. People who are underweight, or have eating disorders shouldn’t fast. Neither should women who are pregnant, trying to get pregnant, or are breastfeeding.
Certain medical conditions can be worsened with longer periods of fasting so it is always a good idea to get the go-ahead from your doctor. Also, people taking certain medications can be prone to side effects with intermittent fasting as well.
One of the reasons people drop out of the intermittent fasting eating pattern is that it’s hard to stick with the fasting part. They eat more than the allowed (low-level of) calories when they’re supposed to be fasting. Also, the hours and days of fasting can be very difficult. So having strong social support will be key to those intermittent periods of fasting. Sticking to a (healthy, nutrient-dense) weight loss diet is the key to success, and intermittent fasting can be difficult for many people to stick with.
Intermittent fasting is a weight loss trend that seems to work for some people. It can help to lose weight and reduce belly fat. For the best chance of long-term weight loss success, finding a diet, you can stick with is key. If it sounds right for you, then I encourage you to give it a try!
Start with shorter fasts, and remember that non-calorie beverages like bone broth, coffee and tea are allowed.
What about you – Have you or someone you know tried intermittent fasting? What were the results? Let me know in the comments below.
Recipe (Whole food): Almond Butter Energy Bites
Makes about 12 energy bites
- 1 cup oats
- ⅔ cup almond butter
- ½ cup chocolate chips (semi-sweet and dairy-free if possible)
- ½ cup flax seeds, ground
- 2 tbsp honey
- Combine all ingredients in a medium bowl and stir.
- Using a tablespoon to measure, roll into about 12 energy bites.
- Serve & enjoy!
Tip: You can roll the bites to coat them in cocoa powder for a bit of extra flavour and to prevent them from being too sticky.
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Bhutani S et al. Improvements in coronary heart disease risk indicators by alternate-day fasting involve adipose tissue modulators. Obesity. 2010 Nov; 18(11):2152-9.
Stote KS et al. A controlled trial of reduced meal frequency without calorie restriction in healthy, normal-weight, middle-aged adults. Am J Clin Nutr. 2007 Apr;85(4):981-8
Harvie MN et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers. Int J Obes (Lond.) 2011 May;35(5):714-27.