Intermittent fasting has become an increasingly popular way to lose weight, improve health, and simplify eating habits. The basic idea is to restrict eating to certain windows of time, allowing for an extended fast period throughout the day or week. But can you still eat junk food during your eating window and receive the touted benefits of intermittent fasting? Let’s take a look at what the research shows.
What is Intermittent Fasting?
Intermittent fasting involves alternating between periods of fasting and eating. Rather than restricting what foods you eat, it restricts when you eat them. There are several popular intermittent fasting schedules:
- 16/8 method: Fast for 16 hours per day, restrict eating to an 8-hour window.
- 5:2 diet: Eat normally 5 days per week, restrict calories to 500–600 per day the other 2 days.
- Alternate day fasting: Fast every other day, eat normally on alternate days.
- Periodic fasting: Fast 1–2 days per week and eat normally the other 5–6 days.
The most common approach is the 16/8 method, where you skip breakfast and restrict eating to the afternoon and evening. The idea is that the extended daily fasting period activates beneficial changes at the cellular level that enhance fat burning and metabolic health.
Does Intermittent Fasting Lead to Weight Loss?
Numerous studies show that intermittent fasting can be an effective strategy for weight loss. In a review of 40 studies, intermittent fasting was found to cause weight loss of 3–8% over 3–24 weeks (1).
Other studies show that intermittent fasting could help retain muscle mass during weight loss, especially when combined with resistance training (2, 3).
There are several reasons why intermittent fasting may promote weight loss:
- Lower calorie intake: Reduced eating windows naturally lead to fewer calories consumed.
- Decreased appetite: Intermittent fasting suppresses ghrelin, the hunger hormone.
- Enhanced fat burning: Fasting triggers metabolic adaptations that favor fat as an energy source.
That said, responses vary by individual. Some struggle with intense hunger when fasting. But overall, research indicates intermittent fasting can be a useful strategy for weight management.
- Studies show intermittent fasting leads to 3–8% weight loss over 3–24 weeks.
- It may help retain muscle mass when losing weight.
- Intermittent fasting suppresses appetite and enhances fat burning to facilitate weight loss.
Does Intermittent Fasting Improve Metabolic Health?
Emerging research shows intermittent fasting positively affects metabolic health markers like blood sugar, insulin sensitivity, and cholesterol levels.
In one study, intermittent fasting lowered insulin levels by 11–57% more than daily calorie restriction (4). Another study found that fasting triggered a cellular stress response that improved neuronal resistance to toxins linked to neurodegenerative diseases like Alzheimer’s (5).
Animal studies reveal that intermittent fasting protects against obesity, diabetes, heart disease, neurodegeneration, and cancer. However, human research is still in early stages (6).
That said, current evidence indicates that intermittent fasting may:
- Improve insulin sensitivity
- Lower blood sugar levels
- Reduce markers of inflammation
- Enhance brain function
- Extend lifespan
By giving cells prolonged breaks from food intake, intermittent fasting allows the body to shift from sugar-burning to fat-burning while removing waste products. This supports metabolic health and longevity.
- Intermittent fasting may improve blood sugar control, heart health markers, brain function and longevity.
- Animal studies show it protects against obesity, neurodegeneration, diabetes and cancer.
- It provides metabolic benefits by allowing cells prolonged breaks from food intake.
Can You Eat Junk Food in Your Eating Window?
While research clearly shows that intermittent fasting can be beneficial, most studies have involved overall healthy eating in addition to fasting periods.
Very little research examines the effects of eating junk foods within the eating window. However, a few things indicate it may diminish the benefits:
If most calories come from junk foods during feeding periods, it increases the risk of nutrient deficiencies. Nutrient-dense whole foods contain important vitamins, minerals, antioxidants, and fiber missing from chips, soda, candy, and other junk foods.
Blood Sugar Spikes
Junk foods like candy and soda spike blood sugar rapidly. This leads to high insulin levels that may diminish the insulin-sensitizing benefits of fasting periods.
Gut Health Disruption
A healthy gut microbiome is linked to reduced inflammation, improved immunity and lower disease risk. An unhealthy diet high in junk food disrupts the gut microbiome, which may negate some benefits of intermittent fasting.
Junk foods like processed snacks and sodas contain pro-inflammatory ingredients that may trigger inflammatory pathways deactivated during fasting periods.
While research is limited, these factors indicate that a diet high in junk foods would reduce the health benefits of intermittent fasting. It may still lead to weight loss due to the calorie restriction. However, other advantages would likely be diminished.
- Eating junk foods during feeding windows may increase nutrient deficiencies.
- It can spike blood sugar, disrupting the insulin-sensitizing effects.
- Junk foods disrupt gut health and promote inflammation.
- Overall, a diet high in junk food will likely reduce the health benefits of intermittent fasting.
Healthy Food Choices to Make While Intermittent Fasting
While you can eat any foods within your eating window, junk foods high in carbs, fat, and sugar may diminish the benefits.
Focus on getting nutrients from the following healthy whole foods:
- Fatty fish
- Lean poultry
- Nuts and seeds
- Leafy greens
- Bell peppers
- Summer squash
- Brown rice
- Olive oil
- Coconut oil
- Nuts and seeds
Fruits, dairy foods, and starchy veggies can fit into a healthy intermittent fasting eating plan but control portions to avoid sugar and carb overload.
Prioritize protein, fiber, and nutrients to feel satisfied while optimizing health benefits.
- Choose proteins, healthy fats, non-starchy veggies, and whole grains.
- Limit sugar, refined grains, and heavily processed foods.
- Include nutrient-dense foods to optimize nutrition and satisfaction.
Sample Meal Plan
Here is a sample healthy meal plan for one day of intermittent fasting using the 16/8 method:
Morning (Fasting Period)
- Black coffee
- Herbal tea
- Tuna salad with spinach and avocado
- Sparkling water
- Dinner: Veggie omelet with kale and mushrooms with a side salad
- Dessert: Greek yogurt with blueberries and walnuts
- Beverages: Herbal tea, water
This sample meal plan provides balanced nutrition using wholesome foods that are high in fiber, protein, and nutrients.
- Drink water, coffee, and tea during the fasting period.
- Break the fast with a protein- and veggie-rich meal.
- Include wholesome proteins, fats, fruits, veggies, and grains in evening meals.
- Minimize sugar and refined carbs like bread, pasta, and desserts.
The Bottom Line
Research shows that intermittent fasting can improve metabolism, reduce inflammation, enhance brain function, and support weight loss.
While you can consume junk foods within your eating window, this is likely to reduce the health benefits. Limit sugar, refined carbs, and processed foods. Instead, focus on proteins, healthy fats, fruits, veggies, and whole grains to maximize nutrition.
With the right food choices, intermittent fasting can be a sustainable lifestyle that nurtures excellent health.
Frequently Asked Questions
Can I drink coffee while intermittent fasting?
Yes, black coffee and other zero-calorie beverages like tea and water will not break your fast. Focus on avoiding cream, sugar, and high-calorie coffee drinks containing milk and syrups.
What if I feel weak or dizzy while fasting?
Hunger and low energy are common side effects when getting used to intermittent fasting. Make sure to drink plenty of fluids. Consider shorter fasting periods if you feel faint or unwell, and break your fast by eating a healthy snack.
Can I exercise while intermittent fasting?
Yes, you can and should continue exercising and stay active even on fast days. Just listen to your body and take a break if you feel low on energy.
Does intermittent fasting slow your metabolism?
No, research actually shows intermittent fasting boosts metabolism via hormonal changes that optimize fat burning. It may lower metabolic rate slightly on fast days.
- Intermittent fasting can aid weight loss and improve metabolic health.
- Eating junk foods may diminish these benefits by spiking blood sugar, disrupting gut health, and promoting inflammation.
- Focus your diet on nutrient-dense whole foods high in fiber, protein, and healthy fat.
- Limit refined carbs and sugars to maximize health gains while intermittent fasting.