Fasting has gained immense popularity as a health trend, celebrated for its diverse benefits. However, fasting is not a one-size-fits-all solution. There are people and occasions for which fasting can backfire. After all, fasting is stressful to your body. While it’s medicine to some, it’s harmful to others.
Signs and Reasons Intermittent Fasting Is Right for You
You’re Keto-Adapted and Can Tap into Your Fat Stores Easily
Some people enter ketosis easily, while others struggle with worse keto flu due to their genetics and metabolic tendencies to do better on carbohydrates.
If you enter ketosis easily or are already keto-adapted or fat-adapted, your body can easily break down stored fats into energy and relies minimally on carbs. You can generally achieve this by having gone through keto-adaptation for at least a month and can consistently maintain blood ketones of 0.5 – 3.0 millimolar.
Because being in ketosis typically suppresses your appetite, you will feel steady energy and not be very hungry as you fast. This makes it much easier to stick to your fasting window.
On the other hand, if attempting ketosis makes you feel absolutely miserable and your body can’t produce enough ketones even a few weeks into a strict ketogenic diet, chances are fasting may not be right for you.
Fasting Makes You Feel Better and Improves Your Health Markers
Fasting can make you feel great…or not. Some people experience improvements in important health markers, while others things go the opposite direction. If you are in the former group, then intermittent fasting could be great for you.
Improved Heart Rate Variability (HRV)
Heart rate variability (HRV), the variety of length of each of your heartbeats, measures your rest-and-digest nervous system activity. Higher HRV typically means you have less stress, more stress resilience, and lower all-cause mortality.
Eating naturally increases some stress hormones and inflammation markers, which is why some people experience a drop when they fast. Normally, your body should be able to regulate these post-meal responses and quickly return the inflammation and stress hormones to pre-meal levels.
However, some people struggle more with the regulation, and it tends to manifest as higher cardiovascular risk and excess body fat. Many studies find that fasting improves HRV for some people, especially those with pre-existing cardiovascular risk.
A study monitored 58 individuals fasting during the month of Ramadan from sun up to sun down. Researchers tracked HRV and found that it increased significantly during afternoons. Participants also experienced decreased stress.
However, in some others, especially women and those with pre-existing high stress, it increases fight-or-flight responses and lowers their HRV.
Given that heart rate monitors and wearables that measure HRV are easily accessible, you can monitor your body’s response to intermittent fasting and adjust accordingly.
For some people, fasting improves blood sugar control and leptin levels, so they eat less and lose weight. Fasting and shortened eating windows can also be a great tool to achieve a caloric deficit and lose body fat.
In real life, however, some people become insatiably hungry, end up eating more, or engage in binging behavior when they start intermittent fasting.
In people with a thrifty phenotype, the increase in ghrelin also came with a reduction in energy expenditure. If your goal is to lose weight, it’s important to pay attention to these signs and track your caloric intake, at least initially.
If you notice a decrease in blood pressure after beginning intermittent fasting, you’ll know you’re on the right track.
An observational study tracked 1,422 participants while fasting from four to 21 days. While there was no change in heart rate over any fasting period, there was a reduction in blood pressure.
No matter the gender, fasting longer had more positive effects on blood pressure. Fasting reduces both systolic and diastolic blood pressure. Another study of 82 individuals fasting for the holy month of Ramadan determined that the monthly fast significantly reduces heart disease risk.
While fasting can be highly beneficial for your heart, you’ll need to pay close attention. Not getting enough micronutrients or the physical stress of fasting could have negative effects on your heart.
A meta-analysis determined that overall intermittent fasting can:
- Reduce your leptin levels which can enhance your metabolism and reduce your appetite.
- Increase adiponectin which is associated with increased insulin sensitivity due to improved glucose metabolism.
- Increase Human Growth Hormone (HGH), which can boost metabolism and muscle growth.
Therefore, fasting can work well in people with elevated leptin or leptin resistance.
Fasting seems to reduce sex hormones in both genders, especially in lean people. However, some men find fasting to be more anabolic, which makes sense because it supports GH and insulin growth factor functions.
In people with sex hormone imbalances due to excess body fat or leptin resistance, fasting may be very beneficial for their hormone balance.
Fasting can affect your mood and mental health in either direction, depending on your tendency. If fasting seems to improve your mood and cognition, then it’s likely fasting is right for you.
Obesity and other metabolic disorders correlate strongly with mood disorders. Often, fasting, caloric restriction, and weight loss relieve both metabolic and mood issues. In a meta-analysis of 11 studies and 1436 participants, intermittent fasting significantly improved mood and mental health.
Some people may feel euphoria from elevated ketone bodies in their blood since ketone bodies such as beta-hydroxybutyrate have similar structures to gamma-hydroxybutyrate.
On the other hand, some people become more irritable, anxious, tired, brain-fogged, and moody with intermittent fasting. Your attitude about fasting can affect your mood, but in the end, if you feel like it’s really making you feel unlike yourself, then fasting is not for you.
It Works With Your Lifestyle
Many people love fasting because it works well with their lifestyles, and not eating provides some convenience.
For example, some entrepreneurs enjoy fasting on Mondays when they’re less hungry after the weekend indulgences. It works for them because they’re busy on Mondays and not eating allows them to focus on work and be very productive. Others prefer to fast when they fly or travel.
To begin, choose an eating window that works best with your lifestyle and goals.
Because intermittent fasting provides flexibility, it allows you to hold out to better, healthier, and more balanced meals as opposed to airport foods and takeouts.
On the other hand, if you are an athlete or work a very physically-demanding job, fasting might be harder to fit into your lifestyle.
You Sleep Better While Fasting
Some people experience better sleep on intermittent fasting, while others cannot sleep or experience significant sleep disruptions. A literature review explored the effects of intermittent fasting on sleep and found mixed results.
While intermittent fasting didn’t seem to change the quality or duration of sleep, it did improve other factors. For some participants, it took a shorter amount of time to fall asleep, and the amount of time spent in bed asleep increased.
Another literature review determined that intermittent fasting may reset the circadian rhythm, a complex system of pacemakers in your brain and other parts of your body that influences your circadian rhythm. They are heavily influenced by factors such as when we eat. Maintaining a consistent eating schedule can help keep your circadian rhythm more consistent.
Research also tells us that intermittent fasting can result in weight loss, which may improve sleep quality and duration, especially in people with excess weight.
Signs and Reasons Intermittent Fasting Is Contraindicated for You
Intermittent fasting isn’t a good fit for everyone. This section of the article will cover when intermittent fasting is best avoided.
History of Disordered Eating or if Fasting Causes Binging
Because intermittent fasting involves deliberate periods of not eating, it can potentially trigger or exacerbate disordered eating and related thought patterns.
A 12-month study of 2,762 adolescents and young adults examined intermittent fasting and eating behavior. Those fasting intermittently in the 12 previous months were more likely also to experience disordered eating. This was particularly true for women.
Additionally, even without a history of disordered eating, if you notice that you tend to binge eat after breaking your fast, this is unhealthy behavior. It’s best to stop intermittent fasting and focus on healthier eating behaviors.
As we mentioned earlier, intermittent fasting has mixed results regarding sleep. Some can fall asleep faster, toss and turn less, while others notice no difference.
An observational study of 1,422 people fasting between four and 21 days explored the effects of fasting on sleep. Among the subjects, about 15 percent experienced sleep disturbances, surpassing all other side effects of fasting in terms of reported occurrences.
Pay attention to how your eating window affects your sleep since meal timing impacts your circadian rhythm. Pushing your window back too late can throw off your circadian rhythm, sleep quality, and stress levels. It’s best to stop eating four hours before your usual bedtime, so keep that in mind while planning your eating window.
Worsening of Any Health Markers
Intermittent fasting doesn’t always have a positive effect on your hormone levels. A literature review exploring the effects of fasting on hormones determined that fasting can decrease androgen markers, like testosterone, in both men and women.
This decrease in androgen markers could have different effects on health. For women with polycystic ovarian syndrome (PCOS), fasting could help improve menstruation and fertility by treating excessive androgens.
In obese men, fasting does not affect testosterone levels, and weight loss may have a net positive effect on testosterone. However, in some lean males, fasting might lower androgens, potentially impacting metabolic health and libido in a negative way.
The jury is still out on exactly how fasting affects bone health.
A recent 2023 literature review exploring the effects of fasting on bone health determined that there are no negative effects. In fact, the review suggests that it may even protect against bone loss while losing weight. However, researchers need more investigation to fully understand its effects.
It is important to note that other factors play a role in maintaining optimal bone health, such as:
- Vitamin D
Fasting without attention to these factors could lead to low bone density over time. If this is a concern for you, it’s best to avoid fasting. Instead, eat regularly and ensure that you are getting the proper supplementation for healthy bones.
While we already talked about how intermittent fasting can support a healthy heart, it’s still something you should monitor. Fasting can lead to an electrolyte imbalance, making the heart unstable and more likely to experience arrhythmia.
A loss in micronutrients can also impact heart health. A clinical trial of 38 participants followed a 5:2 intermittent fast. Overall, the following micronutrients were lower than recommended intake:
Optimal heart health requires all of these micronutrients. If you have an ongoing heart condition, work with your doctor to monitor your levels and get advice on whether fasting is a good fit for you.
Major Changes in Mood, Stress, or Energy Levels
It’s normal to have changes in your mood, stress, and energy levels when you start fasting. Your body is adjusting to your new eating schedule. It’s not okay, however, if this continues and negatively affects your daily life.
A drop in blood sugar levels can contribute to feeling more irritable or anxious throughout the fasting period. A study of 52 women fasting for 18 hours found that irritability increased while fasting compared to not fasting.
Fasting may also increase your stress hormones including cortisol, adrenaline, and some other catecholamines. Some people thrive on this, while for others it can be detrimental.
A literature review examined the effects of fasting on cortisol. Researchers determined that fasting increases cortisol levels and cortisol secretion. Greater symptoms of stress are associated with higher cortisol levels.
Fatigue may also be a struggle when fasting, both physical and mental. Ramadan intermittent fasting didn’t affect aerobic performance much in elite judo athletes. However, it did increase their fatigue scores.
Fatigue is common because you are experiencing a drop in your food intake. It tends to be particularly pronounced while adapting to your new eating schedule.
If any of these major changes make you unable to carry out your daily activities, it may not be a good fit for you.
Feeling hungry is a sign that your body needs more food. While your body can generally adapt over time, intermittent fasting can cause you to feel much hungrier than usual. Extreme hunger could indicate a deeper issue, such as blood sugar control.
Menstrual Cycle Changes
Some women continue their cycles normally during fasting, but a good number will start to experience abnormalities when they start fasting.
A study of 80 female college students fasting during Ramadan tracked menstrual abnormalities. People who fasted for over 15 days had more irregular, short periods or abnormally heavy bleeding. [R35]
If you start to have irregular cycles or major changes in your menstruation once you introduce fasting (without a pregnancy), then fasting might be contraindicated for you.
Loss of Libido
Fasting may impact libido because of changes to hormone levels, energy availability, and overall physiological stress on the body.
A study of 45 men fasting Ramadan tracked their sexual health throughout the month. Participants experienced an overall reduction in sexual desire and frequency of intercourse. [R36]
If you’re concerned about how fasting might affect your libido, it’s a good idea to listen to your body and prioritize your overall well-being.
If you’re losing more hair more than usual, you should avoid fasting. Losing weight quickly or following strict diets can cause a condition called telogen effluvium, known to make hair fall out all over the scalp.
The physiological stress accompanying intermittent fasting could potentially contribute to excessive hair shedding.
In addition to the stress aspect, it’s crucial to recognize that maintaining a balanced diet is paramount. Not getting enough vitamins and nutrients can also make your hair fall out.
Pregnancy and Breastfeeding
Intermittent fasting is not recommended while pregnant or breastfeeding. Breastfeeding and growing a baby requires more calories and nutrients.
An animal study explored the effects of fasting on pregnancy. Researchers found that fasting during pregnancy can harm the transport of amino acids in the placenta and lead to babies with low birth weights.
A clinical review found that fasting does not affect the micronutrients in breast milk. Another study, however, did find that it may lower the quantity of breast milk output.
Intermittent fasting may not be suitable if you’re dealing with specific health conditions. Those with chronic diseases such as diabetes could face an increased risk of adverse events during fasting. Similarly, conditions like:
- Coronary artery disease
- Unstable angina
- Heart failure
- Atrial fibrillation
- Prior heart attack
- Prior stroke or transient ischemic attack
- Most cancers
- Chronic obstructive pulmonary disease
- Pulmonary embolism
- Peripheral vascular thromboembolism
- Chronic kidney disease
These conditions might magnify the potential risks associated with intermittent fasting. If you have one of these conditions, talk to a doctor before starting fasting.
Children and Teenagers Still Growing
Growing children and teenagers have extra daily calorie needs to support their development.
From a research standpoint, we don’t know whether it’s safe for children and teenagers to grow up too fast. Because children are growing and developing, most people agree that extreme fasting could affect their nutrition.
There is also the research discussed earlier regarding disordered eating. Approach fasting with extreme caution, as it may increase the likelihood of engaging in disordered eating behavior.
That said, it’s generally accepted in Islam that children should begin fully fasting in the month of Ramadan once they reach puberty. It’s also not uncommon for them to begin fasting a day or two throughout the month at a much younger age.
It’s important to prioritize a balanced and nutritious diet for children to ensure they receive adequate nutrients for their growth and overall health.
Depending on the type of ulcers you suffer from, fasting may or may not be a good fit.
If you have stomach ulcers, current research suggests that you can proceed with caution as long as they are uncomplicated.
A literature review explored the effects of intermittent fasting on stomach ulcers caused by Helicobacter pylori. Although fasting increased H. pylori concentrations, it did not put subjects at risk of developing ulcers.
If duodenal ulcers are a problem for you, it’s probably best to avoid intermittent fasting. A literature review looked at how fasting during Ramadan affects gastrointestinal problems. It found that people with duodenal ulcers had a higher chance of bleeding.
Intermittent fasting offers a range of health benefits, but it’s not a one-size-fits-all solution.
If you’re already doing the keto diet, intermittent fasting is a convenient way to create a caloric deficit without much hunger. It can also boost your mood and support many other aspects of your health.
However, if your health conditions prevent fasting or if you feel worse while fasting, respect your body’s needs. Some people fare better eating regularly.
Before embarking on this fasting adventure, prioritize your individual circumstances and listen to your body. Talk to a health professional to determine whether it’s a good strategy for you.
Always keep your well-being at the center of it all.
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