← Huberman Lab

Effects of Fasting & Time Restricted Eating on Fat Loss & Health

Episode 41 Oct 11, 2021 2h 22m 26 insights
This episode I discuss the science and practice of fasting also called time-restricted feeding. I review the data on how limiting food intake to specific portions of every 24-hour cycle (or fasting longer) impacts weight loss, fat loss specifically, liver health, mental focus, muscle, longevity and more. I explain how "fasted" is contextual, and relates to blood glucose levels and their downstream effects, and how the depth of fasting can be adjusted with behaviors such as different types of exercise, or with glucose disposal agents. I also discuss the optimal fasting protocol: and both the absolute (non-negotiable) and variable (contextual) features of a fasting/time-restricted-feeding protocol that will allow you to get the most benefits. I also discuss what does and does not break a fast, the effects of fasting on hormones like testosterone and cortisol and on fertility. I also review how different feeding windows of 8 or 10 or 4 hours differentially impact the effects of fasting, and why the classic 8 hour feeding window came to be but also might be ideal. I discuss mechanisms and offer tools to discern the optimal fasting duration and timing for you.  Read the full show notes for this episode at hubermanlab.com.
Actionable Insights

1. Avoid Food Post-Waking

Do not ingest any food for at least the first 60 minutes after waking up each day. This practice is metabolically beneficial and supports health, weight maintenance, and weight loss.

2. Fast Before Bedtime

Avoid ingesting any food or liquid calories for two, and ideally three, hours prior to your bedtime. This is crucial for liver health, metabolic health, and maximizing the benefits of sleep-related fasting.

3. Consistent Daytime Eating Window

Place your feeding window during the most active phase of your day (daytime for humans) and maintain its placement consistently across the week. This anchors the body’s gene systems, provides a regular circadian rhythm, and prevents health disruptions caused by shifting eating times.

4. Target an 8-Hour Feeding Window

Aim for an 8-hour feeding window as a general target for time-restricted feeding. This window length has shown benefits across various health parameters, including inflammation, weight loss, fat loss, insulin sensitivity, and blood pressure, and is generally easier to adhere to.

5. Adjust Feeding Window for Reality

If your goal is an 8-hour feeding window, select a 6 or 7-hour target, and for a 10-hour goal, aim for 8 or 9 hours. This accounts for the common tendency for people to underestimate their actual feeding duration by 1-2 hours due to small caloric intakes outside the strict window.

6. Transition Gradually to TRF

Allow a transition period of 1 week to 10 days when shifting your feeding window, moving it by about an hour each day. This helps your hormone systems adjust, reducing overwhelming hunger and irritability, and preventing hormone imbalances.

7. Optimize Light Exposure

Get as much sunlight and bright light as safely possible in your eyes during the daytime, and avoid bright light in the middle of the night. This is the primary method to entrain your circadian clock genes, which are vital for mood, metabolic function, and overall health.

8. Walk After Meals

Take a 20-30 minute light walk after dinner or any other meal. This accelerates gastric emptying and glucose clearing from your system, helping your body transition more quickly from a fed to a fasted state.

9. Afternoon/Evening HIIT for Glucose

Engage in high-intensity interval training (HIIT) in the afternoon or evening to lower blood glucose. This can help accelerate the transition to a fasted state, provided it doesn’t disrupt your sleep.

10. Ingest Salt During Fast

If feeling lightheaded, shaky, or hungry during a fast, ingest a small pinch (e.g., half teaspoon) of sea salt or table salt in water. This stabilizes blood volume, offsets low blood sugar symptoms, and can reduce feelings of hunger, especially when consuming caffeine.

11. Protein Intake for Muscle Growth

If your main interest is maintaining or building muscle, ingest protein earlier in the day, ideally before 10 a.m. if you wake at 7 a.m. This supports muscle tissue maintenance and growth due to enhanced protein synthesis pathways regulated by clock genes.

12. Support Liver Health with TRF

Restrict your feeding to a particular window each 24-hour cycle to enhance liver health. This reduces inflammatory markers and prevents fatty deposits by allowing sufficient periods without food in the digestive tract.

13. Increase Brown Fat with TRF

Engage in time-restricted feeding to positively increase brown fat stores. Brown fat creates a thermogenic effect, helps reduce other fat, and is correlated with a lack of non-alcoholic fatty liver disease.

14. Monitor Short Feeding Windows

Be cautious with very short feeding windows (e.g., 4-6 hours) as they can lead to overeating relative to metabolic needs and potentially increase body weight. Ensure caloric intake aligns with your goals if pursuing such a window.

15. TRF and Fertility/Hormones

Women aiming to maintain ovulatory cycles or get pregnant, and men concerned about fertility, should avoid feeding windows that are too short (less than 8 hours). Sufficient leptin signaling and overall food intake are crucial for reproductive health.

16. Use Glucose Disposal Agents Cautiously

Approach glucose disposal agents like berberine (OTC) or metformin (prescription) with extreme caution. These can dramatically reduce blood glucose, potentially causing hypoglycemia and headaches if blood glucose is already low or if dosage/timing is incorrect.

17. Consider a Continuous Glucose Monitor

Consider using a continuous glucose monitor (CGM) to understand how different foods, supplements, and exercise impact your individual blood glucose levels. This provides objective data for optimizing your feeding strategy.

18. Artificial Sweeteners in Moderation

Ingest artificial or plant-based non-sugar sweeteners (e.g., stevia) in moderation during a fast. They generally have minimal impact on blood glucose and do not typically ‘break’ a fast, but individual reactions and potential gut microbiome effects (in excess) vary.

19. Lemon/Lime Juice for Acidity

Drink water with squeezed lemon or lime juice to mildly lower blood glucose levels. The acidity can help if you feel overwhelmed by sweetness or want to gently reduce blood sugar.

20. Avoid Alcohol While Fasting

Do not consume alcohol, even small amounts like a half glass of wine, during your fasting window, especially after a meal. Even one gram of sugar can disrupt circadian genes related to fasting and sleep when blood glucose is elevated.

21. Use Fasting Tracking Apps

Utilize zero-cost resources like the My Circadian Clock website or the Zero app to log and track your feeding and fasting windows. These tools help monitor progress, provide averages, and offer valuable information.

22. Take Athletic Greens Daily

Take Athletic Greens once or twice a day to cover basic nutritional needs, address potential deficiencies, and support microbiome health with probiotics. It also provides essential vitamins like D3 and K2.

23. Use Element Electrolyte Drink

Dissolve one packet of Element in 16-32 ounces of water upon waking and during physical exercise. This ensures proper hydration and adequate electrolyte intake (sodium, magnesium, potassium) vital for optimal brain and body function.

24. Utilize Waking Up App for Rest

Use the Waking Up meditation app for various meditation programs, mindfulness training, and non-sleep deep rest (NSDR) protocols. Short sessions of NSDR can greatly restore cognitive and physical energy.

25. Be Aware of Sauna Blood Glucose Spike

Be aware that sauna use can temporarily increase blood glucose due to dehydration, as the concentration of sugar in your blood goes up. This is an observation for awareness, not necessarily a reason to avoid sauna if its other benefits are valued.

26. Avoid Swallowing Toothpaste

Avoid swallowing toothpaste while brushing your teeth during a fast. This is a precaution to prevent accidental caloric intake that could technically break a fast, though likely minimal unless swallowed.