Intermittent fasting (IF) is a form of dieting that does not particularly require certain foods or even restrict what you eat like other diet trends—the focus is on when you eat. IF is characterized by periods of fasting and feeding, and just like every diet, there are considerations to weigh when deciding whether this is the right strategy to enhance weight and fat loss. Disclaimer: This article is for informational purposes only. Please consult a medical professional before beginning any diet or eating plan.

Stages of Fasting

The premise of IF revolves around the fed-fast cycle, which involves changes in metabolism and hormones. The fed state occurs during the first few hours after eating. The body is processing what has been consumed, insulin is released to allow glucose into cells, and glucose is being converted into glycogen for storage. Ghrelin, the hunger hormone, is suppressed, and leptin, the hormone signalling the feeling of fullness, is released. This period of time is influenced by the composition and the size of the meal (Link, 2021)

After 3 to 4 hours post-meal, the body shifts to the early fasting state, which could last up to 18 hours. During this time, blood sugar drops, so glycogen is broken down for fuel. When glycogen stores have been used, which typically occurs toward the end of this stage, the body resorts to other sources for fuel, including breaking down fats into triglycerides and muscle protein into amino acids (Link, 2021).

Around hour 18 starts the fasting state, lasting up to around 2 days. The body is using triglycerides from fats and amino acids from protein to convert to glucose for energy. The process of breaking down fats results in ketosis, which is the basis of the keto diet. The keto diet leads to fats primarily being used as an energy source by intensely restricting carbohydrate intake. Whether someone doing IF experiences ketosis during the fasting state depends on the size and composition of what was previously eaten (Link, 2021).

After about 2 days of fasting, the body enters long-term fasting, or the state of starvation. This state is not recommended unless done under medical supervision. Insulin levels decline, glucose is made by the kidneys to keep the brain functioning, ketone levels rise from fat breakdown, and branched-chain amino acids (BCAAs) are used in order to conserve muscle (Link, 2021).


Types of Intermittent Fasting

There are different strategies of IF that exist, differing in the durations of each cycle and how many times per week or per month to engage in fasting. The best method is one that you have discussed with your healthcare provider and determined could lead to maximum benefit while minimizing any health risks.

One source suggested that IF may not be as beneficial for women as it is for men because of women’s bodies being more sensitive to calorie restriction. The source referenced a study in which women’s blood sugar regulation worsened compared to men after three weeks of intermittent fasting. The source also noted calorie restriction can lead to changes in hormones that could in turn lead to irregular menstruation, infertility, poor bone health, and other effects. Should one choose to engage in IF, a different approach should be considered for women, with shorter fasting periods and fewer fasting days (Coyle, 2018).

Again, there are several strategies of IF, and it would be best to do more research and present those of interest to your healthcare provider for discussion. Three common IF approaches you might read or hear about include:

  • The 16/8 Method (a.k.a the Leangains Method): 16 hour fast and 8 hour window to consume food. Women should consider starting with a shorter fasting period, perhaps 14 hours.
  • Eat-stop-eat (a.ka. The 24 Hour Protocol): 24 hour full fast once or twice weekly. (Again, start with a shorter fasting period and work up as tolerated. Maximum of twice weekly fasting days for women.)
  • The 5:2 Diet (a.k.a “The Fast Diet”): 5 days of normal eating and the other two days (not consecutive days) restrict calorie intake to 25% of normal (~500-600 calories). (Coyle, 2018)

Caveats of Intermittent Fasting and Why it is Still a Form of Dieting

Sources claim that IF has many health benefits other than decreased calorie intake that leads to weight loss. When done right, it could result in weight loss and potentially an improvement in some health conditions. Yet, the bottom line is that it is still a diet.

During the “feeding” intervals, you still have to be mindful of what you are eating and how much. It is not a free-for-all period where you can eat whatever you want. This is not the diet to go binging on junk food the minute the feeding cycle starts either. The resulting blood sugar spike and crash would not be safe or healthy (Hardick, 2020). Eating too much will sabotage the process, as will eating foods with low nutritional value. Just as importantly, eating too little can not only put your body into storage mode, but it could be downright dangerous and have serious health consequences. There is a fine line that is tread between fasting to burn fat and starvation. Breaking fasting periods can also sabotage weight loss goals as well. Even the smallest source of calories such as coffee creamer could take your body out of a fasting state (Hardick, 2020).

Lastly, while doing IF, the timing and intensity of exercise may need to be altered. If exercise is too intense on fasting days or around fasting periods, this could affect intensity levels, the hunger level afterward, and your safety while doing the workout (Shiffer, 2020). One thing to consider if you should choose to do IF is to have a workout buddy or personal trainer and a source of quick glucose, just in case blood sugar levels tank there would be help onsite.

There are those who have had success losing weight and also managing some disease states by doing IF. It is important to be as knowledgeable as possible and understand the risks and benefits of IF, as well as having a conversation with a medical professional first. At UFit, we do not suggest a particular diet, but we would like you to know certain diet trends do exist and what science (or lack thereof) is behind them. Most importantly, the more you understand and listen to your own body, the healthier relationship you are able to build and maintain.