Fasting and Your Health

Natalie Ledesma is a board-certified specialist in oncology nutrition, Natalie is the founder of Natalie Ledesma Integrative & Functional Nutrition Consulting, the clinical nutrition specialist at Smith Integrative Oncology in San Francisco, an integrative and functional dietitian for Radiant Women’s Health, and the founding dietitian for the nutrition program at the University of California, San Francisco Helen Diller Family Comprehensive Cancer Center. This article is an excerpt from Natalie Ledesma’s monthly newsletter, reprinted with her permission. 

Fasting has become one of the most talked-about topics in health. You’ve probably heard that it can support healing, improve metabolism, or even reduce cancer risk.

But what’s actually happening in your body when you fast?

And more importantly, does any of this apply to you, especially if you’re navigating illness, recovery, or supporting someone who is?

Let’s break it down in a way that feels clear, grounded, and actually helpful.

What Happens in Your Body When You Fast

When you go without food for a period of time, your body shifts into a different mode. Instead of focusing on digestion and energy storage, it begins to prioritize repair, balance, and efficiency.

Autophagy: Your Body’s Built-In Cleanup System

One of the most talked-about processes is autophagy, which means “self-eating.”

This is your body’s way of clearing out damaged cells, broken proteins, and dysfunctional components. It’s a natural recycling system that helps keep your cells functioning well.

Research suggests fasting can activate autophagy and help remove cells with DNA damage. Some studies suggest this may play a role in cancer prevention by clearing oncogenic factors and damaged cellular components. However, its role in existing tumors is more complex and depends on the specific context.

The takeaway is simple: your body already has powerful systems for repair. Fasting may help activate them, but it is not the only way.

Metabolic Shifts: Switching Fuel Sources

During fasting, your body transitions from using stored glucose to burning fat for energy.

This “metabolic switch” lowers levels of insulin and IGF-1, hormones that influence cell growth and proliferation. These changes are part of why researchers are interested in fasting for long-term health.

Clinical studies have shown improvements in:

• Blood sugar and insulin levels
• Cholesterol and triglycerides
• Blood pressure
• Body composition

In some cases, structured fasting protocols like the fasting-mimicking diet have resulted in reductions in abdominal fat without loss of lean muscle mass.

Inflammation and Oxidative Stress

Chronic inflammation is linked to many health conditions.

Fasting has been shown to reduce inflammatory markers such as CRP, TNF-alpha, and IL-6. It may also lower oxidative stress, which can damage cells over time.

Together, these changes may help create a more stable internal environment for healing and overall health.

Gut Health and the Microbiome

Your digestive system is constantly working. Fasting gives it a break.

That pause may help improve microbial diversity in the gut and reduce bacteria associated with inflammation and metabolic disease. A healthier gut can support immune function and overall well-being.

What the Research Says About Fasting and Cancer

Fasting is not a treatment for cancer, but it is being studied as a potential supportive strategy alongside standard therapies.

Clinical Evidence

A NIH-indexed study found that a fasting-mimicking diet was safe and feasible for certain cancer patients and led to meaningful biological changes, including reduced blood glucose and growth factor levels, along with improved immune activity associated with antitumor response.

Reference:
Vernieri C, et al. Fasting-Mimicking Diet Is Safe and Reshapes Metabolism and Antitumor Immunity in Patients with Cancer. Cancer Discovery. 2022. PMID: 34789537.

Additional research suggests fasting may:

• Lower IGF-1, a hormone linked to cancer cell growth
• Improve the body’s response to treatment
• Reduce side effects like nausea in some patients
• Influence immune activity within tumors

Breast Cancer Survivorship

The Women’s Healthy Eating and Living (WHEL) Study found that women who fasted less than 13 hours overnight had a 36 percent higher risk of breast cancer recurrence and a 22 percent higher risk of mortality compared to those who fasted longer overnight.

Longer overnight fasting was also associated with better blood sugar control and improved sleep.

Differential Stress Resistance

One of the most promising areas of research is something called differential stress resistance.

This means fasting may help protect healthy cells from chemotherapy while making cancer cells more vulnerable to it.

Some patients who follow modified fasting protocols around treatment report fewer side effects, especially nausea. However, this should only be done under medical guidance.

Beyond Cancer: Metabolic and Liver Health

Fasting is also being studied for its broader health benefits.

Metabolic Health

Intermittent fasting and fasting-mimicking diets have shown improvements in:

• Insulin sensitivity
• Cholesterol levels
• Blood pressure
• Body weight

These benefits are especially relevant for people at risk for metabolic syndrome or type 2 diabetes.

Liver Health

Fasting may also support liver health, particularly in conditions like fatty liver disease.

Studies have shown reductions in liver fat, improved liver enzymes, and decreased inflammation following fasting interventions.

Who Should Not Fast (or Should Use Caution)

Fasting is not appropriate for everyone.

It is generally not recommended for:

• Individuals who are pregnant or breastfeeding
• Children and adolescents
• Anyone with a history of eating disorders
• Individuals who are underweight or malnourished
• People with type 1 diabetes or those taking insulin or certain medications that affect blood sugar
• Anyone who requires consistent caloric intake without medical supervision

A Note for Oncology Patients

If you are currently undergoing chemotherapy, radiation, or other cancer treatments, fasting may offer potential benefits in some cases, but it should never be done without guidance.

Nutritional needs during treatment are highly individualized. Your body may need consistent nourishment to maintain weight, manage side effects, and support recovery.

If you are considering fasting, it should be discussed with your oncology team and a registered dietitian who understands your care plan.

The goal is always the same: support your body, not stress it.

A Final Thought

Fasting is one of the most exciting and evolving areas of nutrition research.

But it is not a shortcut or a one-size-fits-all solution.

Your body is already doing an incredible amount of work to protect and heal you.

The question is not whether fasting works.

It is whether it supports you, right now.

And sometimes, the most powerful thing you can do is not restrict, but nourish.

That counts too. 💛

If you’re interested in working directly with Natalie, you can reach out to her on her website, https://natalieledesma.com/, or consider joining her Nat Pak.

Nat Pack offers accountability, community building, personal connection, and loads of cutting edge nutrition discussion. This membership includes virtual 30-minute discussions twice monthly; meetings will be on Wednesdays at 12pm pacific time. These sessions will be recorded and available to members.