Fasting – Catholic Carnivore

Last update: 7 February 2021

Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil. And he fasted forty days and forty nights, and afterward he was hungry. And the tempter came and said to him, “If you are the Son of God, command these stones to become loaves of bread.” But he answered, “It is written,

‘Man shall not live by bread alone, but by every word that proceeds from the mouth of God.’”
Matthew 4:1-4, RSVCE Bible

Just as it is necessary to eat, it is necessary to sometimes not eat. This page is about fasting, both short-term and long-term.

Short-term (intermittent) fasting

This is what the Church traditionally promotes² – with one full meal, and (optionally) two small ones besides. Presently, the only two days where this is required are Ash Wednesday and Good Friday, but don’t let that stop you from voluntarily fasting anyway. Especially if your lifestyle means that you’re basically eating one 16-hour meal every day³.

Intermittent fasting is roughly comparable to caloric restriction⁴ in terms of health benefits associated with weight loss; it’s better than doing nothing⁸, and if you value your health⁹, it’s something to try just in case you’re an abstainer than a moderator type of personality.

Long-term (extended, prolonged) fasting

As the quote from Scripture at the top of the page would imply, by extrapolation from following Christ’s example, some people do attempt to do very prolonged fasts for religious reasons. There is at least one such a case in the medical literature⁵, ending the fasting prematurely due to hypotension and weakness after 36 days, probably due to inadequate sodium intake. This, however, isn’t the longest fast ever recorded. That’s Angus Barbieri’s fast⁶ of 382 days under medical supervision, and more recently there’s also a case study of a 50-day fast¹⁰. Fasting used to be one of the methods for treating obesity⁷ – but don’t expect not to regain everything, if you go back to your old habits afterwards¹¹.

While the physiology of short-term fasting is largely uninteresting, in prolonged starvation, the metabolism shifts towards a fat-burning, protein-sparing mode¹². This is partly similar to a ketogenic diet – gluconeogenesis and ketogenesis are increased, and glycemia lowers. Most tissues, excepting some that cannot, begin to use ketone bodies in place of glucose. Crucially, insulinemia normalizes, which is likely the crucial part in how various diseases are reversed.

Fasting is remarkably safe to try¹³ if you want to lose weight really fast, so long as you don’t actually want to kill yourself.

Modified fasting

In modified fasting, you are consuming some calories, but much less than even in low calorie diets. It is similarly safe as water fasting and effective for a wide variety of illnesses¹⁵, consistent with the idea that the main problem of contemporary populations is energy toxicity. In particular, small amounts of carbohydrate are beneficial to guard against protein loss, given the requirement for gluconeogenesis doesn’t go away in fasting¹⁶, and sodium supplementation will help avoid hyponatremia²⁰.

Dry fasting

Dry fasting is the most extreme form of fasting, where you abstain from drinking as well as eating. There have been few published studies on the topic, but it does not seem harmful for up to five days^17,18. Weight loss is greater, and animal experiments suggest that the rate of fat catabolism is much increased in such a state¹⁹.

Short-term (intermittent) fasting

Long-term (extended, prolonged) fasting

Modified fasting

Dry fasting

References