Intermittent fasting indeed has its downsides, typically in the form of metabolic changes. Depending on how long your body has acclimated to fasting, the effects can be more or less apparent.

You should first gauge your threshold of risk before fasting, then choose a regimen that fits that risk assessment. Starting small, perhaps with a 12-hour fasting period, before attempting a harsher schedule such as a 48-hour fasting. Some real-world data can be viewed in the NIH's Intermittent Fasting Regimen for Health Improvement research paper [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7551479/]. This source includes downside, upside, and regimen assessments ranging from microbiome improvements, weight-loss counter points, and hemoglobin improvements but a decrease of white blood cell parameters.

In terms of side effects, according to Harvard, intermittent fasting can cause a range of them, including hunger, constipation, headaches, lethargy, mood swings, fatigue, bad breath, poor sleep quality, food obsessions and hair loss.

Here are some intermittent fasting regimens: A 12-hour fasting window is recommended by Medical News Today, but according to Mind Body Green, some people may benefit from a longer fasting period of 14-18 hours. Other methods according to Women's Health Magazine, such as the 16:8 method involve fasting for 16 hours and restricting eating to 8 hours, while 48-hour fasts are typically done 1-2 times per month. Keep in mind that during fasting you should have water and other calorie-free beverages available at all times to maintain hydration.
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