Fasting

Fasting is often marketed as a dopamine “reset”, frequently in the same breath as “dopamine detox” claims. The actual evidence, human and animal, is small, comes from very differently designed studies, and does not point in a consistent direction — and one of the clearest direct measurements found dopamine falling during fasting, not rising.

Human evidence

The most direct human evidence comes from a study of fifteen women with obesity, scanned with PET before and after roughly ten days on a very-low-calorie diet. Using a radiotracer sensitive to dopamine D2/D3 receptors, the researchers found a significant decrease in receptor availability in the substantia nigra, with similar but smaller trends in other brain regions¹. This decrease was strongly predicted by the accompanying fall in leptin, a hormone released by fat tissue that signals energy stores to the brain, and this relationship held even before the women had lost much weight¹. A decrease in receptor availability is generally interpreted as consistent with increased dopamine competing for those receptors, though as with the sleep-deprivation research elsewhere in this index, receptor-level changes and dopamine-level changes are not automatically the same thing.

A separate, differently designed study measured plasma serotonin, dopamine and two growth factors in 29 healthy adults across the Ramadan fasting month, taking blood two days before the fast, then on the 14th and 29th days of fasting. Serotonin, brain-derived neurotrophic factor and nerve growth factor all rose significantly as the fasting month went on. Plasma dopamine did not change significantly at any point compared with the pre-fasting control².

Animal evidence

Animal research on fasting and brain neurotransmitters is more extensive, and more direct, than the human literature. In one rat study, thirty animals were split into a control group and an intermittent-fasting group given repeated 24-hour fasts over one, seven or fifteen days, with brain tissue analysed directly for neurotransmitters. Brain dopamine fell significantly after just one day of fasting, alongside decreases in glutamate and aspartate, while norepinephrine and serotonin rose³. The direction and size of these changes fluctuated with how long the fasting continued, and brain-derived neurotrophic factor rose after one day but fell after fifteen days across the brain regions tested³. This rat finding — a fall in brain dopamine, not a rise — runs directly against the popular “fasting boosts dopamine” claim, though a single rodent study should not be read across directly to predict human responses, particularly given how differently the two available human studies behaved from each other.

Mechanism

The clearest proposed mechanism connects fasting to dopamine indirectly, through leptin. Leptin, released by fat tissue in proportion to how much energy is stored, normally suppresses hunger; when calorie intake drops sharply, leptin falls, and the PET study found this fall closely tracked the change in dopamine receptor availability¹. This fits a broader picture in which the brain’s dopamine system is sensitive to signals about energy availability, plausibly as part of motivating food-seeking behaviour when energy stores are low, rather than fasting directly “boosting” dopamine as a reward in itself.

Given that the Ramadan study found no equivalent plasma dopamine change over a full month of a milder fasting pattern, it is plausible that this leptin-linked mechanism requires a more severe or rapid degree of calorie restriction than typical religious or intermittent fasting produces². Until more directly comparable studies exist, the most accurate statement is that fasting’s effect on dopamine in humans depends heavily on how severe and how long the fast is, rather than being a single, predictable effect.

Evidence strength

• CEvidence grade C · Limited or observational human evidence

One small PET study found reduced D2/D3 receptor availability after ten days of severe calorie restriction in women with obesity, associated with falling leptin ¹, while a separate plasma study of a full month of Ramadan fasting found no significant change in dopamine levels ². The two studies used different fasting protocols, populations and measurement methods, and together represent limited, inconsistent human evidence rather than a well-established effect.

Key studies

• Fifteen women with obesity studied before and after roughly ten days of a very-low-calorie diet, using PET with a dopamine D2/D3 receptor tracer; found decreased receptor availability in the substantia nigra, strongly associated with falling leptin concentrations.¹
• Twenty-nine healthy adults (22 women, 7 men) studied before and during the Ramadan fasting month, with plasma serotonin, dopamine, BDNF and NGF measured by ELISA; found serotonin, BDNF and NGF rose significantly with fasting, but plasma dopamine did not change significantly.²
• Thirty male rats split into control and intermittent-fasting (24-hour fasts over 1, 7 or 15 days) groups, with brain neurotransmitters and oxidative stress markers measured directly; found brain dopamine decreased significantly after one day of fasting, while norepinephrine and serotonin increased, with effects fluctuating by fasting duration.³

References

1. 1. Dunn JP, et al. Caloric Restriction-Induced Decreases in Dopamine Receptor Availability are Associated with Leptin Concentration. Obesity (Silver Spring, Md.). 2017. PMID 28944597. doi:10.1002/oby.22023
2. 2. Bastani A, et al. The Effects of Fasting During Ramadan on the Concentration of Serotonin, Dopamine, Brain-Derived Neurotrophic Factor and Nerve Growth Factor. Neurology international. 2017. PMID 28713531. doi:10.4081/ni.2017.7043
3. 3. Abdel-Rahman M, et al. Intermittent fasting alerts neurotransmitters and oxidant/antioxidant status in the brain of rats. Metabolic brain disease. 2024. PMID 39292431. doi:10.1007/s11011-024-01415-7