The FastDay Forum

Has anybody come across any studies that have examined the difference between IF involving “fast” days that include a small intake of food (e.g. 500-600 calories/day) and IF involving fast days that involve a complete abstinence from food?

not specifically, though there are alternate day fasting studies with true fasts and other studies with reduced calories alternate days. Which aspect interests you ?

I'm wondering whether there are there health advantages to eating 500-600 calories on fast days, and whether there are disadvantages to just eating nothing. I find it psychologically easier to just abstain from food completely a couple of days a week.

From what I've read, maximizing the amount of time of the full fast before you break the fast with a little bit of food is better for getting the most out of your fasting days. So, for example, in a ~34 hour fasting day where you go from, say, 10pm on Tuesday night to 8am on Thursday morning eating 500 or 600 kcal, it is better to not snack and eat all 600 kcal in the evening meal on Wednesday rather than eat little bits of 50-200 kcal snacks/mini-meals throughout the day starting on Wednesday morning. From that, it would seem that a full-on fast of zero calories between 10pm on Tuesday to 8am on Thursday would be a little bit better still.

My impression of the 500-600 kcal which turns this into a not-true fast is that it is designed primarily to make the program psychologically-sustainable over the long term. For most people, they can't sustain two true-fasting days per week. If you find it's easier to do so with a full-on fast than it is to count calories on your fasting days, and you find you can sustain it, then by all means go for it.

Some prefer the simplicity of a full-on fast to counting calories. Between not counting calories at all on your fasting days and not worrying about calories on your feed days, you may see the allure of such an approach. It certainly simplifies things.

OK, I had a look ....

Looking at 3 weeks of ADF (alternate days zero calories) in healthy women and borderline overweight men, BMI 22 and 25 respectively, the conclusion of one paper was

Glucose response to a meal was slightly impaired in women after 3 weeks of treatment (p < 0.01), but insulin response was unchanged. However, men had no change in glucose response and a significant reduction in insulin response (p < 0.03). There were no significant changes in the expression of genes involved in mitochondrial biogenesis or fatty acid transport/oxidation, although a trend toward increased CPT1 expression was observed (p < 0.08). SIRT1 mRNA expression was increased after alternate day fasting (p = 0.01). The increased expression in SIRT1 suggests that alternate day fasting may improve stress resistance

The same study group were reported to have ...

lost 2.5 ± 0.5% of their initial body weight (P < 0.001) and 4 ± 1% of their initial fat mass (P < 0.001). Hunger increased on the first day of fasting and remained elevated (P < 0.001). RMR and RQ did not change significantly from baseline to day 21, but RQ decreased on day 22 (P < 0.001), which resulted in an average daily increase in fat oxidation of ≥15 g. Glucose and ghrelin did not change significantly from baseline with alternate-day fasting, whereas fasting insulin decreased 57 ± 4% (P < 0.001).

From a weight loss perspective the 4% fat loss / 2.5 % body weight in 21 days compares favourably with a group of obese alternate day calorie reducers who lost 5.8% of body weight in 8 weeks eating 25% on their calorie reduction days. 0.83 vs 0.73 % of body weight per week. Fat loss was 0.675 kg per week in the latter case, or 12.4%, which at 1.55% per week is slightly better than the 1.33% of the true alternate day fasters however the latter were not obese.

A different study used 300-380 calories of low carb shakes on alternate days for 8 weeks. This one was focussed on health and concluded that ...

Compliance with the ADCR diet was high, symptoms and pulmonary function improved, and oxidative stress and inflammation declined in response to the dietary intervention. These findings demonstrate rapid and sustained beneficial effects of ADCR on the underlying disease process in subjects with asthma, suggesting a novel approach for therapeutic intervention in this disorder.

Thanks for the responses and the information; they're very helpful.

If anybody reading this is involved in IF research, it might be interesting to conduct a study that examines potential differences between a full fast x days/week, a partial fast x days/week that involves y calories eaten all at once, and a partial fast x days/week that involves y calories spread over the course of the day (which is, from where I sit, not a fast, but it would be useful to know if it achieves the same results, or how they differ).