Diet and microbiota: what about fasting?

In a world where diets are multiplying, intermittent fasting is attracting growing interest. But what does science have to say about its impact on health? intestinal microbiota and the digestive health ?

Here's a series of blogs about three diets and their impact on the gut microbiota. This one is related to fasting.

The other two cover the topics listed in the following links:

• Ketogenic diet
• Mediterranean diet

It should be noted that the reasons for adhering to this type of diet, such as its effects on diabetes and weight loss, are not covered in these blogs. However, a medical conference on the subject is available. Contact us if you're interested.

Definition: what is intermittent fasting?

Intermittent fasting (IF) is a popular method of alternating periods of fasting and eating (3)(11). There are several variations (3)(10)(11) :

• Alternative: alternate days of regular eating with days of complete fasting lasting from 24 to 36 hours; ;
• Modified: alternation between regular eating days and fasting days when consumption represents only 20 to 25% of its usual energy intake (e.g. 5:2);
• Time-limited Fasting: food intake for a given period (e.g. 16 to 20 h of fasting and 4 to 8 h of eating). This type is generally easier for most people to follow, and fewer side effects are reported.

Possible risks of fasting

Like all diets, fasting entails certain risks and possible side effects. Here is a list of some of them (3)(5)(6)(11) :

• Irritability, reduced energy, fatigue;
• Orthostatic hypotension, cardiac arrhythmia;
• Menstrual irregularities;
• Worsening of stomach ulcers.

Contraindications of fasting

There are also contraindications to fasting. Here are a few (1)(3)(11):

• People at risk of malnutrition, underweight;
• Children, teenagers, the elderly;
• Pregnancy and breastfeeding; ;
• Eating disorders (e.g. anorexia, bulimia, hyperphagia);
• Hepatic or renal insufficiency;
• Frequent hypoglycemia.

What does science have to say about the microbiota?

The microbiota is influenced by food intake (so yes, food choice), but also by the timing of food intake (so yes, meal duration, routine and frequency) (4)(7)(8)(12)! Intermittent fasting creates an «intestinal pause» which appears to have certain benefits for the intestinal microbiota (4)(12).

In the majority of studies in healthy subjects benefits are reported on (2)(4)(7)(8)(9)(10)(12):

• The diversity and richness of the microbiota's bacteria (remember: healthy microbiota = diverse and balanced);
• The function of the intestinal barrier (we want it to let only the good nutrients through);
• The immune and inflammatory response;
• The production of short-chain fatty acids (AGCC), which are beneficial for intestinal health.

These benefits have been observed when fasting for less than 2 days (4). The impact on the microbiota of prolonged fasting (i.e. more than 2 days) is not yet known (4).

Various possible mechanisms

Science is still exploring the mechanisms explaining the impact of dietary change on the microbiota. One hypothesis is the change in the circadian cycle (biological rhythm of a 24-hour cycle), which influences the composition of the microbiota and its metabolites (9).

If you're interested in the subject of shift impact, read this article Is it possible to eat well when you work nights?

In addition, during fasting, the increase in intestinal pH alters mucus production and may affect the microbiota ecosystem (9). The observed benefits of fasting may also be due to the increased production of SCFAs (9).

Limits of current science

The scientific literature on intermittent fasting and the microbiota is not perfect. There are few studies in humans (mostly in animals), often with small cohorts (e.g. between 9 and 80 subjects) and short durations (e.g. between 7 days and 22 weeks) (4)(7)(9)(10)(12). In addition, many studies lack control groups, which reduces their reliability (7).

A major challenge is to distinguish the effect of intermittent fasting on weight loss, which is often present in subjects (4)(12)! Food choices during the feeding period also influence results (7).

Heterogeneity studies complicates the interpretation of results, as impacts may vary from one individual to another depending on his or her bacterial profile initial. Several factors influence the microbiota (e.g. power supply, Some studies have not considered them all (7)(8)(10).

Longer-term studies in healthy humans, but also with different conditions such as irritable bowel syndrome (IBS), could provide clearer answers over the next few years.

In conclusion

All in all, fasting could provide interesting benefits for the intestinal microbiota, but the research is still preliminary. Before adopting this diet, it is crucial to consult a nutritionist to make sure it's right for your personal situation and to maximize the benefits while minimizing the risks of digestive problems or other complications related to food intolerances.

References

(1) Albosta and Bakke (2021) Intermittent fasting: is there a role in the treatment of diabetes? A review of the literature and guide for primary care physicians. Clin Diabetes Endocrinol Feb 3;7( 1 ):3. https://pubmed.ncbi.nlm.nih.gov/33531076/

(2) Cadena-Ullauri et al (2024) The effect of intermittent fasting on microbiota as a therapeutic approach in obesity. Front Nutr Apr 25;11:1393292. https://pubmed.ncbi.nlm.nih.gov/38725575/

(3) Harris et al. (2018) Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep 16;2, 507-547. https://pubmed.ncbi.nlm.nih.gov/29419624/

(4) Mohr et al. (2021) Recent advances and health implications of dietary fasting regimens on the gut microbiome. Am J Physiol Gastrointest Liver Physiol May 1;320( 5 ):G847-G863. https://pubmed.ncbi.nlm.nih.gov/33729005/

(5) Morales-Suarez-Varela et al (2021) Intermittent Fasting and the Possible Benefits in Obesity, Diabetes, and Multiple Sclerosis: A Systematic Review of Randomized Clinical Trials. Nutrients Sep 13;13( 9 ):3179. https://pubmed.ncbi.nlm.nih.gov/34579056/

(6) Ojo et al (2022) Role of Intermittent Fasting in the Management of Prediabetes and Type 2 Diabetes Mellitus. Cureus Sep 5;14( 9 ):e28800. https://pubmed.ncbi.nlm.nih.gov/36225474/

(7) Paukkonen et al (2024) The impact of intermittent fasting on gut microbiota: a systematic review of human studies. Front Nutr Feb 12;11:1342787. https://pubmed.ncbi.nlm.nih.gov/38410639/

(8) Popa et al. (2023) A Scoping Review of the Relationship between Intermittent Fasting and the Human Gut Microbiota: Current Knowledge and Future Directions. Nutrients Apr 26;15( 9 ):2095. https://pubmed.ncbi.nlm.nih.gov/37432222/

(9) Pramono et al. (2024) Intermittent fasting modulates human gut microbiota diversity in a phenotype-dependent manner: a systematic review. Biosci Microbiota Food Health 43( 3 ):170-182. https://pubmed.ncbi.nlm.nih.gov/38966051/

(10) Purdel et al. (2023) The Beneficial Effects of Dietary Interventions on Gut Microbiota-An Up-to-Date Critical Review and Future Perspectives. Nutrients Dec 3;15(23):5005. https://pubmed.ncbi.nlm.nih.gov/38068863/

(11) Sutton et al. (2018) Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even Without Weight Loss in Men with Prediabetes. Cell Metab 5;27, 1212-1221. https://pubmed.ncbi.nlm.nih.gov/29754952/

(12) Zeb et al. (2023) Gut Microbiota and Time-Restricted Feeding/Eating: A Targeted Biomarker and Approach in Precision Nutrition. Nutrients Jan 4;15( 2 ):259. https://pubmed.ncbi.nlm.nih.gov/36678130/