What Are the Long-Term Effects of Intermittent Fasting? Here’s What the Experts Say

Intermittent fasting (IF) can have both positive and negative long-term effects. Over the long term, IF offers numerous health benefits, such as improved body composition, enhanced cognition, reduced risk of diseases, and even anti-aging benefits (1). However, if not done correctly, prolonged fasting regimens can also lead to side effects like nutritional deficiencies, muscle loss, and impaired metabolic function.

Intermittent fasting is more of an eating pattern than a specific diet plan. It involves alternating between eating and fasting windows. The most popular IF plan is the 16/8 method, where you fast for 16 hours and eat all your meals within an eight-hour window. Other common IF styles include alternate-day fasting and one-meal-a-day (OMAD).

While all this sounds great, in my experience as a nutritionist,  I have seen many cases of malnutrition, dehydration, and even eating disorders caused by practicing IF long-term. Stay with me as I take you over the positive and negative effects of long-term fasting. Get the best tips to maintain IF successfully in the long term and reap the maximum benefits.

7 Long-Term Benefits of Intermittent Fasting

Here are the advantages of adding IF to your routine:

1. Improved body composition

Intermittent fasting can help you lose weight, especially when combined with a balanced diet plan and some physical activity. Over the long term, this can help improve your body composition.

Traditional calorie-restricting diets can also help you lose weight, but they are difficult to follow long-term. Intermittent fasting is an easier way to reduce your calorie intake.

The small eating window in IF encourages weight loss, as you tend to consume fewer calories in the day. This creates a calorie deficit needed to boost fat burning. Intermittent fasting also helps regulate hormones, boost metabolism, promote fat loss, and preserve lean body mass.

2. Enhances metabolic health 

Long-term intermittent fasting has a positive impact on metabolic health and associated disorders such as diabetes (2). Obesity or being overweight increases your risk of developing insulin resistance and diabetes.

If you have been diagnosed with prediabetes, shedding extra weight can significantly reduce your risk of developing full-blown diabetes. Reducing overall calorie intake during intermittent fasting naturally results in decreased insulin production.

In the long run, intermittent fasting improves pancreatic function and boosts glucose metabolism. This helps lower blood sugar levels and enhance insulin sensitivity. Thus, if you follow this eating plan consistently, IF may help prevent insulin resistance.

3. Decreases oxidative stress and inflammation

Free radicals in your body can trigger reactions that result in oxidative damage to your cells. These unstable molecules are associated with numerous chronic conditions and aging.

Intermittent fasting helps reduce oxidative stress and inflammation by enhancing the body’s antioxidant defense mechanisms. IF induces autophagy, a cellular process where damaged components of the cells are broken down and recycled. 

During fasting, the body uses up its glucose stores and shifts to using fat for energy. This metabolic shift activates autophagy pathways. In the long term, this reduces inflammation and offers numerous health benefits, including a reduced risk of diseases and protection against age-related cognitive decline. 

4. Reduces risk of diseases

Long-term intermittent fasting has been linked to improved heart and brain health, as it improves various health markers, such as inflammation, heart rate, blood pressure, and cholesterol levels.

Benefits such as reducing high blood sugar levels, insulin resistance, and inflammation have a positive impact on brain health. Intermittent fasting may even reduce your risk of cancer. 

5. Boosts athletic performance

Weight loss diets often leave you feeling too tired for a workout. IF can lead to some muscle loss alongside fat loss. This raises valid concerns about its effects on muscle mass and athletic performance.

With IF, you can actually preserve muscle mass and even enhance athletic performance, given you maintain an adequate protein intake and lift weights. 

IF tends to be more about when you eat than what you eat. So eat the right foods, and you will reap more benefits. A high-protein diet boosts weight loss and helps build muscle. Strength training puts those muscles to work, building and strengthening them.

IF boosts your energy levels once you become more fat-adapted. The cellular repair processes stimulated by IF promote cellular rejuvenation, which, in turn, enhances the energy-producing capacity of your muscle cells. Over the long term, IF makes your system more efficient at using body fat for energy and endurance.

7. Anti-aging benefits

Longevity refers to living longer and healthier lives. Dietary and lifestyle changes, like intermittent fasting (IF) and calorie restriction, can impact cellular processes and prolong lifespan.

Studies have found that IF activates pathways associated with longevity (3). AMPK, one of these pathways, regulates cellular energy usage, while SIRT1 shields cells from stress and damage. IF-induced autophagy removes damaged cell parts and repairs DNA.

Esteemed Italian biogerontologist Dr. Valter Longo says:

“It’s about reprogramming the body so it enters a slower aging mode, but also rejuvenating it through stem cell-based regeneration (4).”

Dr. Mark Mattson, a neuroscientist, has researched intermittent fasting. According to his findings, fasting for 18 hours triggers a metabolic switch to ketone-based energy. This change may increase longevity. It also enhances stress resistance and reduces the risk of diseases such as cancer and obesity.

7. Enhances cognitive benefits 

IF can make you smarter.

Intermittent fasting helps improve learning, memory, and cognitive function by boosting brain-derived neurotrophic factor (BDNF) production.

BDNF, a protein crucial for neuron health and brain function, fosters neuroplasticity, the brain’s capacity to adapt to experiences. Intermittent fasting (IF) lowers the risk of neurodegenerative conditions like Alzheimer’s by supporting brain protection and flexibility. IF decreases oxidative stress, inflammation, and neuronal loss, all linked to Alzheimer’s.

Alzheimer’s is characterized by amyloid-beta plaques and tau protein accumulation. Intermittent fasting triggers autophagy, reducing amyloid-beta plaque and tau pathology, thus enhancing cognitive abilities.

Side Effects of Long-Term Intermittent Fasting

While intermittent fasting does offer some amazing health benefits, some people might experience side effects such as fatigue, headaches, mood changes, and dehydration. Most of these side effects are temporary and can be handled by adjusting your timing, food choices, and fluid intake.

Dr. Pam Taub, a cardiologist at UC San Diego Health, warns that intermittent fasting isn’t for everyone. She recommends seeking medical advice before changing diet or lifestyle (5). 

Nutrition professor Dr. Krista Varady warns individuals over 65 not to practice intermittent fasting. The same goes for pregnant women and breastfeeding moms (6). She has written books such as The Every-Other-Day Diet and The Fastest Diet, which offer insights into the benefits and risks associated with extreme fasting practices.

Prolonged adherence to fasting regimens can give rise to other health issues, such as:

1. Nutritional deficiencies

Frequent or extended fasting periods can lead to considerably reduced food consumption. This can lead to insufficient nutrient intake, resulting in micro- and macronutrient deficiencies. Some of the vitamin deficiencies associated with long-term IF are vitamins B12, D, and E. You should also watch out for iron, calcium, and omega-3 fatty acid deficiencies.

2. Impaired metabolic function

Excessively restrictive fasting regimens with irregular eating patterns can backfire. Prolonged fasting may disrupt your metabolic function and hormone regulation. This can upset the balance of insulin levels and blood sugar regulation. Thyroid functions can also be affected. Over time, this can raise the risk of metabolic disorders, including insulin resistance and thyroid dysfunction.

3. Muscle loss

If prolonged fasting is combined with increased physical activity or strength training, you need adequate protein intake to avoid losing muscle. You should have sufficient protein regularly during your eating periods to reduce muscle loss associated with long-term fasting regimens.

4. Reduced energy levels

Chronic calorie restriction over the long term can result in decreased energy levels and lethargy. You must supply your body with the nutrients required to support your metabolic needs. If you experience fatigue or feel sluggish, this can negatively impact your daily functioning, mood, productivity, and quality of life.

5. Eating disorders

Some people may develop unhealthy relationships with food and eating disorders due to IF. These can lead to guilt, anxiety, and even depression.

Long-Term Sustainability of Intermittent Fasting

Here are some tips to help you avoid the unwanted side effects of intermittent fasting and stay on course with your regimen.

1. Gradual adaptation

Start slow, with a short fasting window of 10 to 12 hours. Gradually increase this to 14, 16, or even 18 hours. Once your body adapts to the fasting regimen, you can even experiment with OMAD or alternate-day fasting. This will help you stay on track and minimize the side effects.

2. Seek support

Find a fasting buddy or join an online intermittent fasting community. This will offer you the much-needed motivation, accountability, and support for continuing your fasting journey.

3. Stay hydrated

It is essential to stay hydrated during your fasting windows. Dry fasting, which involves avoiding both food and water, is extremely dangerous and should only be attempted under strict medical supervision.

Depending on your physical activity and local climate, you can aim for three to four liters of water daily. Increase your fluid intake by adding smoothies and soups to your diet. You can still enjoy herbal teas and other non-caloric beverages like lemon or mint-infused water without breaking your fast.

Health writer and BBC presenter Dr. Michael Mosley says (7):

“Whatever diet you decide to follow, it is vital that you get enough protein (at least 50-60g a day); otherwise, you will lose muscle. You also need to be sure you are getting enough of all the other essential nutrients — you should avoid going on one of those crazy cabbage soup or green juice diets, for example.”

5. Listen to your body

You do not have to stick to your chosen IF fasting windows rigidly. 

Be a little flexible. Pay attention to your hunger cues and energy levels throughout the day. If you feel excessively hungry or fatigued, you may need to adjust your fasting window. Listen to your body’s signals and find a fasting routine that works for you.

Wrapping Up

IF offers a wide range of long-term health benefits, including sustained weight management, improved metabolic health, preservation of muscle mass, and even potential anti-aging effects. 

With long-term IF, you can also end up with a host of unwanted side effects. This includes malnutrition, muscle loss, and the development of eating disorders. It’s best to get guidance from your physician and a nutritionist before making any drastic changes to your diet.

References:

1. Rafael de Cabo, Mark P. Mattson. (2019).” Effects of intermittent fasting on health, aging, and disease.” New England Journal of Medicine: Vol. 381 No. 26.
2. Mark P. Mattson, Valter D. Longo, and Michelle Harvied. (2017). “Impact of intermittent fasting on health and disease processes.” Ageing Research Reviews: Vol 39, Pages 46-58.
3. Valter D. Longo et al. (2021). “Intermittent and periodic fasting, longevity and disease.” Nat Aging. Jan 1(1): 47–59.
4. Robert Perkins. (2015). “Diet that mimics fasting appears to slow aging.”  USC Today.
5. Alice Callaha. (2024). “Is Intermittent Fasting Bad for Your Heart? Here’s What We Know.” The New York Times.
6. Longevity by Design. (2022). “Does Human Research on Intermittent Fasting Support Longevity?”  InsideTracker.
7. Francis Blagburn. (2019). “The gospel according to Michael Mosley.” The Telegraph.

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