Intermittent Fasting: the Basics

Intermittent fasting (IF) is a dietary pattern that cycles between periods of unrestricted eating and periods of limited or no caloric intake. In addition to weight loss, purported benefits include reduced inflammation, improved cardiovascular markers, support for brain health, and even potential increase in lifespan. But is IF as safe and effective as the claims would lead you to believe?

The most common variations are:

🌞 Alternate-Day Fasting (ADF): Fast every other day.

📅 The 5:2 Diet: Eat normally five days per week, but greatly restrict caloric intake (~500 calories) on two nonconsecutive days.

⏰ Time-Restricted Eating (TRE): Eat within a set daily window (e.g., 8 hours) and fast the rest of the day.

Here’s a more detailed breakdown.

Intermittent Fasting vs. Ketogenic Diet

Intermittent fasting and the ketogenic diet are both popular for weight loss, but they take different approaches.

• IF focuses on when you eat
• Keto restricts what you eat, particularly carbohydrates. 

The aim of the keto diet is to shift the body into ketosis to burn fat instead of carbohydrates for fuel, but its long-term benefits are unclear, and it may cause side effects. Intermittent fasting can also lead to ketosis, though it’s typically not as sustained as with a ketogenic diet.

Ultimately, both patterns may be effective, but sustainability and safety vary. A registered dietitian can help you choose a personalized, realistic approach based on your health, preferences, and goals.

How does intermittent fasting work?

Proponents of IF tout its ability to force the body to undergo a “metabolic switch” during extended periods of fasting. They claim this leads to fat burning, ketone production, and cellular repair, all central to the health benefits of intermittent fasting. 

See below for the proposed mechanism, and keep reading to learn whether this is what actually happens!

Stage 1	Eating stops, fasting begins
Stage 2	Blood sugar and insulin levels drop Lower insulin allows the body to start accessing stored fat for energy.
Stage 3	Glycogen is burned first The body taps into stored glucose in the liver and muscles for energy.
Stage 4	Gluconeogenesis kicks in The body begins making glucose from non-carbohydrate sources like amino acids.
Stage 5	Fat oxidation begins As fasting continues, the body starts breaking down fat for energy. Fatty acids are released into the bloodstream and transported to the liver.
Stage 6	Ketogenesis begins The liver converts fatty acids into ketones, an alternative fuel for the brain and other organs
Stage 7	Cellular repair is triggered Fasting activates a cellular “clean-up” process where damaged proteins and cells are removed and recycled (autophagy). This may support brain health and reduce risk of chronic diseases over time.
Stage 8	Hormonal and genetic shifts Fasting may increase human growth hormone and influence gene expression related to longevity, inflammation, and metabolism.

But, the below study suggests that it is caloric restriction and not the fasting pattern that drives weight loss changes

Study: calorie counting and intermittent fasting are equally effective for weight loss

A randomized controlled trial published in Annals of Internal Medicine found that time-restricted eating and traditional calorie counting resulted in nearly identical weight loss over a 12-week period among adults with obesity and prediabetes. Participants in both groups consumed the same meals, with the intermittent fasting group eating within a 10-hour window and finishing most of their calories by 1pm, while the calorie-counting group followed a more extended eating window, consuming most of their calories in the evening. 

Despite the differing meal timing, both approaches led to similar weight reduction and no significant differences in metabolic health markers between groups.

These results highlight that regardless of time restriction, the overall quality and quantity of food remain the most important factors for long-term weight loss success.

IF Benefits Beyond Weight Loss

Weight loss is often the primary focus of intermittent fasting, but a recent umbrella review of randomized controlled trials found comprehensive metabolic benefits: 

📏 Reduced Waist Circumference: IF was associated with a significant decrease in abdominal fat, with an average reduction of 1.02 cm in waist circumference.

⬇️ Lower Fat Mass: Participants experienced a decrease in fat mass by about 0.72 kg

📊 Improved Lipid Profile:

• Decreased LDL cholesterol (aka “bad” cholesterol)
• Reduced total cholesterol and triglycerides
• Increased HDL cholesterol (the “good” cholesterol) in individuals with overweight, obesity, or type 2 diabetes

🧁 Better Glycemic Control: Fasting insulin levels dropped significantly, suggesting improved insulin sensitivity.

💪 Preserved Muscle: IF led to an increase in fat-free mass (approximately 1 kg), indicating it may help preserve lean body tissue during weight loss.

🩺 Blood Pressure Effects: IF lowered systolic blood pressure, although not as effectively as traditional continuous calorie restriction.

Potential Risks of IF

The only dietary pattern we fully endorse is an individualized one that satisfies your hunger while meeting your nutritional (and taste!) needs. 

Intermittent fasting is popular for a reason. Setting simple, time-based rules may simplify eating and support reduced caloric intake, which in turn leads to weight-loss. However, it is important to be aware of the potential downsides.

There will probably be adverse events.

Common short-term side effects include headache, dizziness, fatigue, and irritability—especially in the initial weeks. These symptoms are often related to low blood sugar or dehydration. 

One study found a cardiovascular risk.

A recent study raised concerns about a possible link between the popular 16:8 fasting pattern (8-hour eating window) and an increased risk of cardiovascular death. There is no causal link at this time, however, and the research requires further investigation.

Intermittent fasting is not safe for everyone.

Fasting is not safe for pregnancy or breastfeeding. It’s also not advised for individuals with a history of eating disorders, certain metabolic or endocrine conditions, or those taking medications that affect blood sugar or require food intake.

For many people, it’s just not sustainable. When a dietary pattern feels too restrictive or disruptive to daily life, it’s less likely to lead to lasting success.

The Bottom Line

If you’re curious about trying intermittent fasting, or pursuing weight loss, or simply want guidance on how to eat in a way that supports your overall health, the safest and most effective way to begin is with the guidance of a registered dietitian. Book your first session today!