22nd of March 2014

Intermittent Fasting

What is Intermittent Fasting?

IF is very simple – you alternate periods of not eating (fasting) with eating. Everyone fasts in their sleep. IF just extends this naturally occurring fast to periods during the day as well. The focus on when you eat, not what you eat. It also simplifies your decisions, preventing decision fatigue, which is a problem on some diets. There is also a suggestion that going without food for extended periods is natural and therefore makes sense from an evolutionary perspective – Dr Michael Eades writes: “intermittent fasting was probably the way Paleolithic man ate. We modern humans have become acculturated to the three-square meals per day regimen. Animals in the wild, particularly carnivorous animals, don’t eat thrice per day. They eat when they make a kill. I would imagine that Paleolithic man did the same.”

Who is Intermittent Fasting good for?

– Leangains and Warrior Diet are good for guys looking to get very lean while maintaining muscle mass and strength
– 5:2 and Alternate Fast Day are good for people looking to lose weight without making drastic changes in what you can and can’t eat

Who is Intermittent Fasting not good for?

– Leangains isn’t for people with large amounts of fat to lose (and who don’t lift weights)
– Brad Pilon specifically says Eat Stop Eat is not suitable for diabetics.
– Endurance athletes will find it difficult to train for long periods of time while on a fasting diet.

Why does Intermittent Fasting work?

Intermittent fasting works because it affects 3 crucial hormones:

Most IF diets also advocate eating similar foods to those on paleo and slow-carb diets (lean protein, vegetables) and maintaining a caloric deficit.

How do I do Intermittent Fasting?

There are a number of different methods of IF, called protocols. Each has a different fasting period and ‘feeding window’. Fasting windows vary from 12 to 72 hours. Some popular methods are outlined below.

1. Eat Stop Eat
This protocol involves a 24 hour fast on two non-consecutive days per week. Example: eat normally on monday, and finish dinner at 8pm. Then don’t eat again until 8pm tuesday. Eating normally on wednesday, you finish dinner at 7.30pm: don’t eat again until 7.30pm on thursday. Eat normally friday, saturday and sunday. Eat Stop Eat is designed to be flexible, so you can alter your fast days as and when you need. You can buy the ebook here ($9.99) or read an interview with the author, Brad Pilon, here. Also advocates lifting weights 3x per week to avoid losing muscle mass. ESE does nto advocate any actual changes in the food you eat. Quote from Eat Step Eat, pg 60: “the key to making Eat Stop Eat work for you is self-control. After you have completed your fast, it is important that you go back to eating as you normally would. Remind yourself that you do not need to reward yourself with extra-large helpings or extra desserts…While most diet programs ask you to give up certain foods, all I’m asking you to do is keep eating the way you normally eat. If you want to improve your nutrition while living the Eat Stop Eat lifestyle, go ahead.”

2. 16/8 aka Leangains
Popularized by Martin Berkhan of is a good overview with plenty of links. It is what is says: fast for 16 hours per day, and have an 8 hour feeding window during which 3 meals are eaten.e.g. no breakfast lunch at noon, a smaller meal at around 4pm, and dinner at 8pm – although Martin advocates breaking your fast immediately after working out, which can make this difficult to schedule if you can’t work out in the middle of the day.

Used in conjunction with weightlifting, you can build muscle while minimizing fat gain, or lose fat without losing muscle (both of which are difficult on a traditional bulk-then-cut bodybuilding diet cycle). On workout days, carbs are favoured over fat; on rest days, the opposite. Protein intake is moderate on all days (more information here). More calories can be eaten on workout days than rest days – but you still need to count calories and not go crazy. Martin also did a detailed interview where he answers many common questions about IF. This approach seems to be more suitable for those who are already lean and are looking to shed the last few pounds without losing any muscle – in this guide Martin says “You should be fairly lean when attempting this approach, as lower body fat means better insulin sensitivity. This seems to work synergistically with the fasting and tends to improve results on my recomposition plan. I would say 10-12% body fat is an appropriate starting point to pull this off with the greatest efficiency.”

3. Warrior Diet
This is a 20 hour daily fast with a 4 hour feeding window during which a large meal is eaten. However, there is a key difference with the warrior diet, which is that you are allowed to eat small portions of fruits, vegetables, broth and milk during the ‘fasting’ period. It’s more of an ‘undereating’ phase than a true fasting phase. The full book here outlines 3 phases: detox, high fat, and concluding fat loss. Detailed diet plans start on page 17 of the book. Long interview with the creator, Ori Hefmekler, here.

4. Alternate Fast Day
Based on a study from the University of Chicago, the basic principle is that you eat what you want (“ad libitum”) one day, and the next day you are limited to 25% of your daily caloric needs – usually 400-600 calories, all in one meal at lunchtime. This study showed that following this protocol led to fat loss as well as improved levels of LDL cholesterol, triacylglycerol concentrations and blood pressure. Another study supports this method, demonstrating that it leads to increased fat oxidation. During the ‘fast’ day you can either eat all the calories in one go, or split over 2-3 small meals.

5. 5:2 Diet

Similar to the Alternate Fast Day, but easier: eat what you want 5 days per week, and on 2 non-consecutive days, limit intake to around 600 calories. You should also avoid fast-release carbs and refined sugars, eating primarily protein, plants and vegetables. Once you’ve reached your target weight, switch to fasting 1 day per week. The main rules of the diet are listed here. And here’s a good overview of the literature on 5:2 from the UK NHS. As per AFD, on fast days you can choose the number of meals you have.

Frequently Asked Questions:

If I don’t eat regularly, won’t my body go into starvation mode and slow my metabolism down?
No – you’ll be fine. This study showed that even after 72 hours of fasting, metabolic rate did not change significantly.

Can I eat whatever I want during my feeding windows?
No. You still need to be achieving an overall caloric deficit. “Don’t be stupid and scarf down a box of doughnuts.” (source) The 5:2 Diet recommends sticking mainly to protein, vegetables, and plants, and avoiding refined sugars and fast-release carbs. Leangains advises higher carb intake on workout days, because it’s geared towards guys who lift weights – but still recommends “good carbs” like sweet potatoes, oatmeal, etc. to do this, as well as plenty of lean protein and vegetables – and eating low carb, moderate fat, high protein on rest days, with total calories 20% below maintenance level. Here is a good overview of general Leangains eating principles.

What do I eat during fasting periods?
Nothing (obviously). You can drink water, coffee, diet soda, calorie free sweeteners, and small amount (<20g) of sugar-free gum. On 5:2 and ADF diets, you are permitted a small amount of calories on ‘fast’ days - 7 example recipes for meals around 250 calories.

Isn’t breakfast the most important meal of the day?
No – insulin sensitivity is usually higher at breakfast time, but that’s because you’ve been fasting for the previous 6-10 hours (while you’ve been asleep). If you feel hungry in the morning, that’s probably due to the release of ghrelin (see above).

Don’t I have to eat 5-6 smaller meals to lose weight?
No. (source) This was due to the idea when you eat, your metabolism increases due to the Thermic Effect of Food (TEF). It was proposed that eating more frequently would maintain a higher TEF, and therefore a higher metabolism, burning more calories throughout the day. It has since been found that TEF is linked to the total number of calories consumed, not the frequency with which you eat. This study concludes that the number of meals you eat isn’t a factor in weight loss.

Will fasting make me weaker in the gym?
No. One study showed that a 3.5 day fast “did not influence isometric, anaerobic capacity or aerobic endurance”. However, performing endurance activity (1 hour plus) will be more difficult in a fasted state.

Will fasting make me lose muscle?
Almost certainly not. This is one of the common fasting myths busted here. Essentially, there have been no studies that really looked at this in detail. However, studies have shown that small amounts of protein, on an empty stomach, are still releasing amino acids into the bloodstream after 6-7 hours – so it’s not unreasonable to think that after a large, protein-rich meal, amino acids are released into the bloodstream for at least 16-24 hours, meaning no muscle loss.

Does eating large meals at night lead to fat gain?
No. This study tracked women during Ramadan, and those women ate exactly the same amount of calories in exactly the same macronutrient split, and it made no difference if that was in 3 meals or all in one meal during the evening. In fact, this study compared two meals patterns, one with a large meal in the morning, and the other with a large meal in the evening. The AM group lost more weight, but the extra weight was in the form of muscle mass. Eating a large meal in the evening helped retain muscle mass and burn bodyfat.

Why do I get headaches when I fast?
It might just be dehydration – make sure you drink enough water. This may also be withdrawal symptoms from a sugar addiction according to Eat Stop Eat.

What supplements do I need to take?
Leangains advocates 10g of BCAAs before working out (with workouts done in a fasted state).

More Resources

Nerd Fitness Beginners Guide to Intermittent Fasting
Intermittent Fasting 101,Intermittent Fasting 201 and FAQs
James Clear: Beginners Guide to Intermittent Fasting and FAQs
Leangains subreddit
Leangains guide to intermittent fasting
Long article from BBC about health benefits
Pros and cons of IF with a number of references
Introduction to IF with pros and cons of different protocols
10 common fasting myths debunked (from
Dr Eades is skeptical of IF
Leangains macronutrient and calorie calculator

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