Intermittent fasting, your thyroid, and your immune system

Photo: David Billings/Unsplash. Design: BOOST Thyroid.

Photo: David Billings/Unsplash. Design: BOOST Thyroid.

Give your digestive system time to rest and recover

The digestive system has been trained to work only occasionally, yet often it works around the clock — even during sleep processing food.

Most people start eating soon after waking up, and continue until just before bed. This puts immense pressure not only on the digestive system, but on the entire body.

During evolution, humans didn’t eat all the time because food was not freely available, and the body is built to prosper without an overworked digestive system.

Research has shown that periods of fasting can help regulate and maintain healthy weight, and fasting has a direct impact on thyroid and immune health.

What is intermittent fasting?

During intermittent fasting (IF), periods of eating and fasting change in frequency and duration. Both reduced night time eating and prolonged nightly fasting intervals improve health.

Intermittent fasting has a major impact on our metabolism and it does so by regulating a few things:

  1. Circadian rhythm — read in detail here (1, 2).

  2. Bacterial composition of the gut/intestine (AKA the gut microbiome) — the health of gut and the rest of the body is determined by the amount and type of bacteria residing in the gut. This helps digestion and builds up a healthy immune system. The gut microbiome is regulated by the type of food people eat and the frequency of eating (3–6). Extended time when our digestion is at rest might help to reduce leaky gut and autoimmune flare-ups (7).

  3. Sleep and activity periods (8) — many research studies have shown that nighttime eating leads to a lack of sleep and poor sleep quality (9, 10), desyncs the circadian rhythm, and can cause overweight and obesity (11, 12).

There are several intermittent fasting protocols, and people usually choose them depending on what personally fits their lifestyle the best.

The most common protocols:

  1. 16 : 8 — fasting for 16 hours and eating during 8 hours a day. People mostly do this by removing either breakfast or dinner from their food intake; starting to eat around 10AM and stopping to eat around 6PM. This type of fasting can help improve heart health and regulate weight (13).

  2. Alternate day fasting — eat one day as much as needed and the next day fasting by only drinking calorie restricted fluids. Research has shown that this can help achieve a healthy weight, regulate insulin and blood sugar, and reduce inflammation in the body — if done in moderation (14, 15, 16).

  3. 5 : 2 — eat about ¼ (quarter) of total daily calorie needs on the two non-consecutive days a week. Eat as usual the other five days of the week (17- 19). Research has shown that this diet might help with regulating weight and reducing the amount of molecules triggering hunger feeling (20, 21).

Research has shown that consuming more of total daily calories earlier in the day, and not consuming foods in evenings and nighttime, is associated with better health and better regulation of weight (22–24).

Intermittent fasting and the thyroid

Fasting has an impact on the thyroid. Since fasting directly impacts metabolism and the way body uses energy. Thyroid hormones drop when intermittent fasting. It causes a drop in thyroid hormone T3 and an increase in reverse T3 (rT3). Hormone T3 is crucial for the body to use energy, but during fasting the body wants to conserve as much energy as possible, because it’s unclear when food will be eaten again.

The drop of T3 hormone levels is caused by a lower rate of conversion from T4 to T3, and it balances back once the normal feeding patterns continue (25, 26).

Fasting has some benefits — it seems levothyroxine taken during the fasting state is more efficient and better absorbed than when taken during normal diet days (27).

Intermittent fasting and Hashimoto’s

The immune system is closely regulated by diet and metabolism — this determines how many (and what type of) immune cells will be produced by the body (28), and a faulty metabolism can trigger problems with the immune system (29, 30).

Both allergies and autoimmune disorders come from a deregulated metabolism and faulty immune cells (31).

Intermittent fasting can reduce inflammation — it’s been shown to do so for other autoimmune conditions such as multiple sclerosis (MS). It usually helps with reducing flare-ups, meaning it needs to happen before the disease worsens (32).

Intermittent fasting is also helpful for rheumatoid arthritis (RA) — reducing both inflammation and pain (33). In type 1 diabetes (T1D), intermittent fasting helps with regulating blood sugar (34).

There is not yet any peer-reviewed research on Hashimoto’s and intermittent fasting.

What are the health benefits of intermittent fasting?

When comparing intermittent fasting with a regular diet, there seems to be no difference in the weight loss (35), but it seems that intermittent fasting is easier to follow than a typical diet protocol because it doesn’t demand a daily restriction, and people usually eat less calories in the day after fasting.

Intermittent fasting can be helpful for health; as research shows that in people with excess weight intermittent fasting may:

  1. Improve eating behavior (18)

  2. Reduce inflammation and ageing of the body (36, 34)

  3. Improve mood (37)

  4. Regulate insulin (38)

  5. Regulate weight and reduce weight-related health problems (39)

  6. Improve memory and brain activity (40)

However, there are some health problems intermittent fasting can predispose:

  1. Insulin resistance (41)

  2. Reduced muscle size — long-term intermittent fasting can lead to reduced muscle size and mass (42).

  3. Low basal metabolic rate (BMR) — the amount of calories we use during the time we are at rest. BMR might be reduced during the longer bouts of intermittent fasting, meaning that the body will use less calories than expected throughout the day (43).

  4. Change menstrual pattern — can cause longer and less regular menstrual cycles (18).

If you have thyroid issues, learn how to safely implement intermittent fasting into your lifestyle.

Longer periods of food restriction restrict energy for some basic bodily processes. This is when the liver and muscles to start using energy from fat instead of sugars. Flexible metabolism can use and switch between using fat and sugars to obtain energy, and it’s a hallmark of a healthy body. Less flexible metabolism has a higher likelihood to suffer from insulin resistance (44). The less flexible your metabolism is, the more likely you’ll experience problems when doing intermittent fasting.

How long should you fast for?

Research has looked into intervals from two to six months, and have shown that IF together with exercise does help reach weight loss goals (45) and reduce inflammation (16).

However longer periods of fasting will lock our bodies in a lower calorie spending state (starvation state), which will not help with weight loss in a longer period of time (46).

Before you start intermittent fasting

As with many other approaches to health, intermittent fasting may benefit some, and not others. There are some crucial research points missing in order to fully understand why some people benefit , while others don’t. In general:

  1. Menstrual cycle and hormone fluctuations might make it harder for intermittent fasting to be successful, and in some cases IF can de-regulate your menstrual cycle.

  2. If your heart and vessels aren’t healthy, you should get a clearance from your medical doctor before you start

  3. If you are pregnant, planning to get pregnant, or breastfeeding, fasting will put an immense pressure on your body — talk to your doctor before trying it

Fasting can help you become healthier and regulate your weight, but it’s a long-term commitment and will require you to modify some of your routines. If you decide to try it out, make sure you discuss it with your healthcare provider — make sure there are no underlying conditions that would cause problems Be patient, it might take a few weeks before you feel or see results.

Track diet and symptoms in a free to use BOOST Thyroid app, and observe how your health improves through time.


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  16. Yang W, et al. Alternate-day fasting protects the livers of mice against high-fat diet–induced inflammation associated with the suppression of Toll-like receptor 4/nuclear factor κB signaling, 2016

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  32. Kim do Y, et al. Inflammation-mediated memory dysfunction and effects of a ketogenic diet in a murine model of multiple sclerosis, 2012

  33. Muller H, et al. Fasting followed by vegetarian diet in patients with rheumatoid arthritis: a systematic review, 2001

  34. Brandhorst S, et al. A periodic diet that mimics fasting promotes multi-System regeneration, enhanced cognitive performance, and healthspan, 2015

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  40. Cignarella F, et al. Intermittent Fasting Confers Protection in CNS Autoimmunity by Altering the Gut Microbiota, 2018

  41. Salgin B, et al. Effects of prolonged fasting and sustained lipolysis on insulin secretion and insulin sensitivity in normal subjects, 2009

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  46. Bhutani S, et al. Alternate day fasting and endurance exercise combine to reduce body weight and favorably alter plasma lipids in obese humans, 201/.