Intermittent Fasting: Insights from a Clinical Dietician

In recent years, intermittent fasting (IF) has gained popularity as a means to lose weight. In 2023 alone, roughly 38 million Americans tried intermittent fasting. While social media is filled with success stories of weight loss, not all researchers are convinced it’s the right approach for everyone. Most notably, the relationship between IF and heart health is complex. Research on its heart-health related benefits versus potential risks is mixed.

What is Intermittent Fasting?

According to Brandy Sarrette, RDN, LDN, a clinical dietician with Baystate Health, “Intermittent fasting is an eating plan that switches between fasting and eating on a set schedule. The idea is that restricting your eating or caloric intake to certain hours of the day or days of the week causes your body to burn fat for fuel.”

The structure of intermittent fasting plans can vary dramatically but generally involves skipping one meal per day.

Popular intermittent fasting plan options include:

• Time-restricted eating: The most popular plan, this approach allows for consuming food of any type or amount to a set six- to eight-hour window each day.
• Fast-mimicking diets: This approach, typically lasting five days, involves consuming a very low-calorie, nutrient-restricted diet for a short period.
• Multi-day fasts: Common in many cultures, often part of a religious practice, multi-day fasts involve abstaining from food for more than 24 hours. In most cases, water or fruit and vegetable juices can be consumed to avoid dehydration and other health issues.

Benefits and Risks of Intermittent Fasting

Primarily embraced for its ability to help people lose weight, intermittent fasting is purported to have many other benefits. But, as Sarrette cautions, “Despite its popularity, intermittent fasting is not well studied, and the long-term effects are unknown. What we do know is that factors like medical history, medications, metabolism, and exercise all impact how well a diet practice does or doesn’t work. There is no guarantee that the benefits enjoyed by one person engaging in intermittent fasting will be appreciated by all.”

Some of the reported benefits of intermittent fasting include:

• Weight Loss: Intermittent fasting has been found to help weight loss thanks to reducing calorie intake and boosting fat burning.
• Reduced Diabetes Risk: Intermittent fasting may improve insulin sensitivity and lower blood sugar levels, potentially decreasing the risk of or even reversing type 2 diabetes.
• Heart Health: Intermittent fasting may improve various heart disease risk factors, including blood pressure, cholesterol levels, and inflammatory markers (substances in the blood that indicate inflammation in the body). However, it is not recommended for people with heart disease or cancer.
• Brain Health: Some studies suggest intermittent fasting may support brain health and potentially reduce the risk of Alzheimer’s disease, Huntington’s disease, and Parkinson’s disease.

On the flip side, a study examining the connection between heart health and intermittent fasting revealed several concerning findings:

• People who followed a pattern of eating all their food across less than 8 hours per day had a 91% higher risk of death due to cardiovascular disease.
• The increased risk of cardiovascular death was also seen in people living with heart disease or cancer.
• Among people with existing cardiovascular disease, an eating duration of no less than 8 but less than 10 hours per day was also associated with a 66% higher risk of death from heart disease or stroke.

Choosing the Right Intermittent Fasting Schedule for You

As Sarrette notes, the benefits and impact of intermittent fasting aren’t fully understood. “If you have a chronic medical condition like diabetes, heart disease, high blood pressure, or kidney stones it’s important to consult with your doctor or a registered dietitian before beginning any restricted eating plan.”

She stresses that fasting is meant to be done intermittently. “When done day after day, week to week, or month to month, fasting can work to decrease your metabolism and make it hard to lose weight.”

Here’s her advice for healthy individuals considering a fast:

• Multi-day fasts are best done only two to four times per year, or every three to six months.
• Fast-mimicking diets should be done only once or twice a month for a five-day period. Fasting period should be separated by a week or more.
• Time-restricted eating can be done two to three days per week. The allowed eating time frame should be between eight and ten hours. Avoid skipping the same meal day after day.
• Avoid skipping breakfast unless the fast extends beyond 24 hours.
• If foods are permitted, choose a variety of minimally processed, healthy foods and keep meals as balanced as possible.
• When a fast is over, reintroduce foods in small portions and focus on light, healthy foods.

Is Intermittent Fasting Right for You?

As nice as it would be to have a one-size-fits-all solution to weight loss and better heart health, Sarrette says intermittent fasting is not it.

“Our bodies are complex, and how they respond to dietary changes is unique to each of us,” she says. “Intermittent fasting may be the answer for those with insulin resistance, but it may put others with heart disease and other conditions at risk. It’s important to weigh your options carefully.”

If you’re curious about intermittent fasting, connect with your doctor or a registered dietitian to review your health history and reasons for considering IF. They can advise you on the safest approach to achieving your goals and preserving your health.