What to Know Before Starting Intermittent Fasting: 5 Scientific Considerations

Intermittent fasting, or IF, has gained lots of traction for its potential health benefits as a diet pattern. Some IF-ers report more energy, weight loss, and metabolic health improvements. (1) But intermittent fasting isn’t for everyone. There are a few health considerations you might want to ponder before you get started. 

Some research suggests that folks with specific health conditions, like those that impact digestion or blood sugar or those in specific life stages, should be cautious before beginning intermittent fasting. (4) Let’s break down the research-backed considerations to help you decide if intermittent fasting is right for you and your lifestyle.

Medical Conditions and Medications: Who Should Avoid Intermittent Fasting?

Before jumping on the intermittent fasting bandwagon, you should consider your personal medical history and current medications and as always, talk to your doctor. Particularly, those with diabetes, kidney stones, or gastroesophageal reflux disease should approach fasting cautiously and receive personalized advice before beginning to fast. (4)(5)

Plus, if you’re on medications for heart disease, blood sugar, high blood pressure, or drugs that require food for proper absorption, intermittent fasting may affect their function or have negative side effects. (6) And anyone with a history of an eating disorder should avoid intermittent fasting, because any type of diet plan can trigger unhealthy patterns related to food restriction and binge eating. (7)

Health Conditions That Might Make Intermittent Fasting a Bad Idea

If you’re taking medications for any health condition, including heart disease, diabetes, high blood pressure, or drugs like insulin and blood thinners may require regular food intake for proper absorption, intermittent fasting may not be right for you.  (6) 

Here are other health conditions that might make intermittent fasting the wrong approach for you:

• Diabetes: Fasting impacts blood sugar levels, leading to dangerous drops or unpredictable spikes.  (4)
• Kidney Stones: Fasting may increase the risk of stone formation by affecting hydration status and dietary intake.(5)
• Underweight Individuals: Fasting can lead to further weight loss, so it should be done under medical supervision and with regular healthcare check ins. (8)
• History of Eating Disorders: Fasting may lead to unhealthy eating patterns, making it unsafe for those with past or present eating disorders. (7)

The Impact of Intermittent Fasting on Metabolism

One of the most widely discussed benefits of intermittent fasting is its impact on metabolism. Research suggests that fasting kick-starts fat-burning through a process called ketosis. That’s when the body burns fat for energy in the absence of carbohydrates. (1)

Additionally, intermittent fasting may improve insulin sensitivity, potentially reducing the risk of developing type 2 diabetes by lowering blood sugar levels and promoting better glucose control. (9)

However, some noteworthy research points out that when head-to-head with other types of calorie controlled diets, fasting wasn’t associated with any more significant weight loss than other types of calorie-restricted diet plans. (10) 

This means that if you aren’t a good candidate for fasting, any type of diet that resonates with you can still work to help you achieve your weight loss goals. 

Another metabolic benefit is autophagy, a process of cellular repair and regeneration that happens with any type of calorie restriction. (11) This process has been linked to longevity and may protect against age-related diseases by clearing out damaged cells. (11) 

Hormonal Effects: Why Fasting May Be Different for Women

Women’s bodies are highly sensitive to changes in calorie intake, which is why intermittent fasting can affect female hormones like estrogen and progesterone. (12) 

Studies show that prolonged fasting might be disruptive to women’s reproductive hormone balance, potentially leading to irregular menstrual cycles and unpleasant symptoms. (12) Importantly, women trying to conceive may want to avoid long fasting windows, as caloric restriction can negatively affect fertility. (12)

For women who are pregnant or breastfeeding, fasting can lead to nutrient deficiencies and insufficient calorie intake, which may impact the health of both mother and baby. (13) If you are a woman in this stage of life, consult your healthcare team before you attempt intermittent fasting. 

Even outside of pregnancy, women with hormonal imbalances such as polycystic ovary syndrome (PCOS) should be cautious with fasting. It may exacerbate symptoms. (13)

Ultimately, some research suggests that women may benefit from shorter fasting windows, like 12 to 14 hours instead of a more restrictive 16 hours schedule. (14)

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About the Experts:

Caroline Thomason is a is a dietitian and diabetes educator combining her love of nutrition with the power of making better health easy to understand. With 12 years in the industry, she’s published in 40+ publications, a CPG consultant and advisor, a speaker, broadcast spokesperson, and recipe developer.

Joanna Gregg, RD is a is a food data curator at MyFitnessPal. She earned her bachelor’s degree in Nutrition from Georgia State University and obtained her Master’s degree from The University of Nebraska.

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Cortisol and Stress: The Science of Fasting and Stress Response

Intermittent fasting can also affect your cortisol levels, a hormone responsible for managing your stress response in the body. (15) 

Long fasting periods can increase cortisol production, especially if you’re already stressed or sleep-deprived. (15) Elevated cortisol levels seem to be associated with an increase in weight, particularly in the abdominal area, which may counteract the weight loss benefits associated with fasting. (16)

Even more so, high cortisol levels might cause you to stress eat once your fasting window ends, creating a cycle of overeating followed by restrictive fasting. (17) This is one form of disordered eating, and if you find yourself falling into this pattern, fasting may not be a good fit for you. (17)

Overall, if you’re prone to stress or have difficulty managing stress-related cravings, intermittent fasting may not be the best approach.

Blood Sugar Sensitivity: Who Should Be Careful?

If you have blood sugar concerns, such as diabetes or hypoglycemia, you need to be particularly careful with intermittent fasting. (4) While fasting might improve your insulin sensitivity, it can be risky for those with diabetes, because prolonged fasting can lead to dangerously low blood sugar levels. (9)

Especially if you take insulin or other medications that affect blood glucose, fasting may increase your risk of hypoglycemia, which can cause serious symptoms like dizziness, brain fog, confusion, or fainting. (18)

Even for those without diabetes, long fasting periods can cause blood sugar dips, leading to irritability or general feelings of fatigue. (19) If you have a blood sugar-related condition, consider shorter fasting windows or speak with a healthcare professional to find the safest approach for you.

The Bottom Line: Is Intermittent Fasting Right for You?

While intermittent fasting has become popular for its potential benefits on metabolic health, it’s not right for everyone. Before you jump in, consider how fasting might affect your hormones, metabolism, stress levels, and blood sugar regulation. 

With any new diet, consult a healthcare provider like a dietitian or your doctor. If fasting isn’t the right fit for you, there are endless options for your nutrition plan to help you reach your goals instead. 

And if you decide to try it, be sure to check out the Intermittent Fasting Tracker inside the MyFitnessPal app. 

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