A diagnostic fast refers to prolonged fasting from 8–72 hours (depending on age) conducted under observation to facilitate the investigation of a health complication, usually hypoglycemia. Many people may also fast as part of a medical procedure or a check-up, such as preceding a colonoscopy or surgery. Fasting may also be part of a religious ritual.
This is probably a good time to mention that while I have practiced intermittent fasting consistently for the last year, I'm not fanatical about my diet. I work on building healthy habits that guide my behavior 90% of the time, so that I can do whatever I feel like during the other 10%. If I come over to your house to watch the football game and we order pizza at 11pm, guess what? I don't care that it's outside my feeding period, I'm eating it.
Third, you've probably already fasted many times, even though you don't know it. Have you ever slept in late on the weekends and then had a late brunch? Some people do this every weekend. In situations like these, we often eat dinner the night before and then don't eat until 11am or noon or even later. There's your 16–hour fast and you didn't even think about it.
Carbs are only problematic when eaten excessively, beyond what your body requires. As long as you are eating low calories then carb intake will make very little difference in respect to fat loss. In other words, two diets of equal calories with varying levels of carb intake will result in the same level of fat loss. If you are still unsure then you can read this brilliant article by Lyle McDonald, Low Carbohydrate Diets Have No Metabolic Advantage. In addition, carbs in the main meal serve very important functions such as increasing leptin (improved satiety and metabolic rate), refilling depleted muscle glycogen stores and triggering the release of serotonin which will improve your quality of sleep and make you feel good.
A systematic review of 40 studies found that intermittent fasting was effective for weight loss, with a typical loss of 7-11 pounds over 10 weeks.  There was much variability in the studies, ranging in size from 4 to 334 subjects, and followed from 2 to 104 weeks. It is important to note that different study designs and methods of intermittent fasting were used, and participant characteristics differed (lean vs. obese). Half of the studies were controlled trials comparing the fasting group to a comparison group and/or a control group (either continuous calorie restriction or usual lifestyle), with the other half examining an intermittent fasting group alone. A brief summary of their findings:
If you're looking to lose weight, this isn't a problem. And even if you're happy with your weight, this won't prove to be too much of an issue if you follow the daily fasting or weekly fasting schedules. However, if you're fasting for 24 hours per day on multiple days per week, then it's going to be very difficult to eat enough of your feast days to make up for that.
Lowers insulin levels: Intermittent fasting acts on insulin in two ways. First, it boosts your adiponectin levels, which helps restore insulin sensitivity to prevent weight gain and diabetes. Second, fasting decreases your fasting insulin levels. Lowered insulin is the cue your body needs to make the switch to burning stored fat instead of glucose.
And there are many plans, because there are many schools of thought about what’s considered fasting, what food or drink should be consumed during eating windows, and how long these windows should last. Here are a few of the most common methods: Intermittent fasting means eating normally four or five days a week and consuming minimal calories on two or three nonconsecutive days. Time-restricted eating requires setting a window for consuming food, such as 7 a.m. to 7 p.m, and having nothing but noncaloric liquids the rest of the time. The “fasting mimicking diet” involves eating a small number of calories daily for a specific period of time — say five consecutive days every other month. These protocols all have benefits and drawbacks. One approach may work well for some while being entirely unsustainable for others.
Getting back to intermittent fasting, many studies have confirmed the health benefits of calorie restriction, and it seems clear that eating less is part of the equation if you want to live longer. Interestingly, research4 has shown that life-long calorie restriction in mice "significantly changes the overall structure of the gut microbiota" in ways that promote longevity. So one reason why calorie restriction may lengthen lifespan appears to be due to the positive effect it has on gut microbiota.
Along with obligatory prayer, it is one of the greatest obligations of a Bahá'í. In the first half of the 20th century, Shoghi Effendi, explains: "It is essentially a period of meditation and prayer, of spiritual recuperation, during which the believer must strive to make the necessary readjustments in his inner life, and to refresh and reinvigorate the spiritual forces latent in his soul. Its significance and purpose are, therefore, fundamentally spiritual in character. Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires."