"If you eat something it's not fasting"

Fasting communities tend to be full of fasting pedantry. A lot of people seem to be more concerned with the semantics of the term 'fasting' than the biochemistry of actual *fasting*. * Your body doesn't respond to micro-fluctuations in nutrients at the systemic level--it would be insane if it did. For example, it generally take about 1-2 hours for insulin to act throughout the system and start lowering blood sugar \[[1](https://i1.wp.com/optimisingnutrition.com/wp-content/uploads/2015/12/image0011.png)\]. Your body isn't going to launch a 1-2 hour process in response to ingesting 5 Cal (especially considering 5 Cal provide less than \~4 min of the typical body's energy requirements \[assuming 2000 Cal TDEE\]). If you're interested in mechanistic details, it takes β-cells about 30 min of elevated blood glucose levels to start secreting insulin (through a fascinating electrical/chemical pathway involving building up of a −50 mV drop across the membrane \[[2](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6028526/)\]). * Your body's cells don't magically know whether or not you've eaten something. The presence of nutrients is translated to the periphery through circulatory signals (elevated blood glucose, hormones like insulin, etc.). Absent these signals, there is no way for your body to know that you've broken your fast or not (and this goes for autophagy too). If something (like an artificial sweetener) doesn't increase blood sugar or trigger any hormone release, *it's not breaking your fast* as far as your body is concerned. Luckily, sweeteners like aspartame (diet coke) have been extensively studied and don't affect hormones or blood sugar \[[3](https://pubmed.ncbi.nlm.nih.gov/2923074/), [4](https://pubmed.ncbi.nlm.nih.gov/20303371/)\]. The amount of protein that aspartame breaks down into from a single can of diet coke (\~200mg) is negligible--accidentally eating a single housefly would give your body 5x more protein (\~1g) \[[5](http://recipeofhealth.com/nutrition-calories/fly-824324rb)\]. There may be other minor effects to eating artificial sweeteners (like killing gut bacteria) or triggering cephalic insulin response (but this response is so small & transitory--triggered by just thoughts of food--that it's not worth considering). * It's estimated that the body contains about **2kg of gut bacteria** \[[6](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4191858/#R2)\]. Just think for a moment about how huge a 2kg container of yogurt is. As you fast this bacteria dies from lack of nutrients and is digested providing you with a continuous trickle of energy. The background level of nutrition provided by the dying gut biome is releasing *magnitudes* more calories than a 5 Cal tablespoon of sauerkraut, but according to fasting pedants the latter breaks your fast and the former doesn't. If you're fasting for spiritual reasons and the idea of a pure fast is important to you, then by all means go for it. If you're fasting for weight loss or the health benefits of fasting, there is no need to be pedantic about it. EDIT: spelling/grammar