Does Diet Soda Affect Behavior?

It’s a hot summer’s day and you decide to refresh yourself by sipping on a cold diet soda. You think to yourself, “It’s okay to drink this, it’s just diet soda after all.” But, is it okay? Is diet soda as harmless as we think it is?

Diet soda has an impact on behavior and it might not be what you expect it to be.

In this article, we will review animal models used for studying diet soda and take a close look at a few experiments which scientists have done to uncover the workings of diet soda on animal behavior.

What Is Aspartame?

One of diet soda’s key ingredients is aspartame. Aspartame is an artificial sweetener which is used to make soda sweet while keeping the calorie-count to a minimum. Aspartame is also commonly found in other sweetened goods, such as ice cream and cookies. It’s even used to slightly sweeten vitamins and prescription medicines!

Ever since the Food and Drug Administration (FDA) approved this sugar-alternative in 1981 (and then once more in 1987),^[1] aspartame has been quite controversial.

So controversial, in fact, that some large, international retailers (like Woolworths and Marks & Spencer) have removed any aspartame-containing products from their product line, concerned that supplying such items would negatively affect their brand-image!

Even though aspartame has been labeled as “safe” by both the FDA and the European Food Safety Authority, it still remains a cause of concern to many people and thus warrants further scientific attention and research.

Does Aspartame Affect the Brain’s Receptors and Enzymes?

Yes. Aspartame does affect the brain. The impact is more clear in long-term studies than in short-term studies.

According to one study, published by Christian et. al. in the Journal of Pharmacology, Biochemistry and Behavior, aspartame-treated rats (ingesting 250 mg/kg/day of aspartame) had higher concentrations of muscarinic receptors in several brain regions and took longer to complete the T-Maze than the control group did after a 4 month period of aspartame administration.^[2]

So, the researchers trained the two groups of rats (control and aspartame-treated) in the T-Maze 3 times per day for a period of 2 weeks. Regardless of condition, all animals were able to find the piece of chocolate (the reward) within 12 seconds time. Aspartame administration continued over the next 4 months and the animals would be tested again from time to time.

The behavioral differences between the control and aspartame-treated groups became clear at the 90-day mark. After 3 months, the aspartame-treated animals took significantly longer to reach the reward than the controls did, needing 18 ± 4s to reach the reward compared to the controls’ time of 10 ± 1.4s. At the 4 month mark (equal to 120 days of aspartame treatment), the pattern continued. The aspartame-treated rats needed 34 ± 5s to complete the T-maze whereas the control group needed a mere 14 ± 2s. The aspartame-treated rats’ time nearly doubled from the 3-month mark while the control groups’ increased by about 4 seconds.

Then, at the end of the long-term experiment, all animals were anesthetized and sacrificed, in order for the scientists to gather information about the animals’ brain and enzyme concentrations. Muscarinic receptors and the enzyme Na⁺, K⁺-ATPase, because of their involvement in learning in memory, were also closely examined by the scientists. Thus, if aspartame affected learning and memory, it would influence the concentration and distribution of muscarinic receptors and the Na⁺, K⁺-ATPase enzyme.

Once the animals were sacrificed, the researchers analyzed the rats’ brain tissues to assess receptor concentrations in certain brain areas and found the aspartame-treated rats had a greater amount of muscarinic cholinergic receptors than the controls. More specifically, the following brain areas in the aspartame-treated group showed a significant increase in muscarinic cholinergic receptors at the 4-month mark:

• Frontal cortex
• Midcortex
• Posterior cortex
• Hypothalamus
• Hippocampus
• Cerebellum

Na⁺, K⁺-ATPase enzyme activities were reported as being similar across the different brain regions with the only exception being the midbrain which had higher activities in the aspartame-treated rats.

Given the increased time it took for the long-term aspartame-treated rats to complete the T-Maze and their increased concentrations of muscarinic cholinergic receptors, the researchers successfully demonstrated that long-term aspartame supplementation alters the brain’s physiological structure and impairs memory.

Does Aspartame Cause Oxidative Stress to the Brain?

In addition to affecting the brain’s receptors and enzyme levels which are implicated in learning and memory, aspartame has been linked to causing oxidative stress in certain regions of the brain and even altering electroencephalogram (EEG) response.^[3]

Recall that oxidative stress is defined as the imbalance between antioxidants and free radicals and contributes to the development of conditions such as autism, schizophrenia, Alzheimer’s disease, multiple sclerosis, and Parkinson’s disease.

In a 2016 study, researchers tested aspartame exposure in mice, aiming to see how the animals’ behavior changed and whether any of these changes would be indicative of oxidative stress occurring within the animals’ hippocampus.^[4]

The mice belonged to one of several groups, either control or aspartame-treated at 20, 40, 80, or 160 mg/kg supplementation per day for the course of 28 consecutive days.

The behavioral tests revealed that aspartame provided a temporary performance boost in the short-term, but caused long-term deficits and damage and a subsequent reduction in performance.

For example, repeated aspartame administration at 20 and 40 mg/kg caused the animals to perform slightly better than the control group in the Y-Maze test, having a higher percentage of alteration. On the other hand, dosage levels at 80 and 160 mg/kg performed worse than the control group, acquiring a lower percentage of alteration score and thus indicative of a lesser willingness to explore.

A similar pattern was observed while conducting Radial-Arm Maze memory tasks on these mice. The mice in the 20 and 40 mg/kg conditions showed higher arm entries before committing a working memory error than the control group did, while the 80 and 160 mg/kg mice performed at the levels of the controls during long-term administration of aspartame.

Then, the animals were sacrificed and their brain matter was analyzed. The aspartame-fed animals had significantly higher levels of

• superoxide dismutase (at dosage levels 80 and 160 mg/kg)
• nitric oxide (at dosage levels of 40, 80, and 160 mg/kg)

Increased superoxide dismutase suggests that the animals had an increase in free radicals and were making an attempt to defend against those superoxide anions. Such activity is associated with cell membrane damage and the alteration of cellular DNA and proteins. While increased nitric oxide suggests that its synthesis occurred in reaction to inflammatory injury. High nitric oxide levels are associated with neurotoxicity and can cause cell death by means of damaging proteins and DNA, including mitochondrial DNA.

Can Aspartame Cause Depression?

Growing concerns have urged researchers to study the links between aspartame and depression.^[5]

A recent study, focusing on the relationship between aspartame on depression in humans, showed that when healthy university students received a high (25 mg/kg of body weight) or low (10 mg/kg of body weight) dose per day for 8 days straight, they showed signs of irritability and depression.

Furthermore, in general medical practice, individuals with unipolar depression are clearly instructed to steer away from aspartame. Aspartame is not encouraged for patients with mood disorders, given their heightened sensitivity to its effects.

The theory behind aspartame’s effects on mood and depression have to do with the physiological structure of the brain. Mood affects decision-making and emotional control relies on the serotonin pathways in the brain, which includes circuits to the hippocampus. As demonstrated throughout this article, the hippocampus is affected by aspartame which, in turn, affects mood and depressive behaviors.

If Aspartame Negatively Impacts Memory, Can It Lead to Alzheimer’s?

No one knows. Research still needs to be conducted regarding the relationship between aspartame and Alzheimer’s disease. But, the question here is: if aspartame can be implicated in memory loss, then would it be possible for its effects to extend and eventually include the pathogenesis of serious brain disorders like Alzheimer’s disease?

After all, as mentioned earlier, high aspartame ingestion can be implicated to oxidative stress as revealed by increased markers. So, logically, it is possible to extend the aspartame-linked oxidative stress to include the pathogenesis of brain disorders. But, long-term studies demonstrating this are sparse.

One study, following the effect of daily aspartame ingestion (divided into groups receiving 0.625, 1.875, or 5.625 mg/kg subcutaneously) over the span of 2 weeks, demonstrated that memory loss and oxidative stress were related.

Using a Morris Water Maze, researchers revealed that the mice in the 5.625 mg/kg aspartame condition were not as quick to find the maze’s submerged platform in the first trial.^[6] The mice in this condition also had significantly greater latency time (averaged across all trials done during the experiment) locating the submerged platform, indicating that there is some kind of problem with the animals’ spatial navigation abilities.

In both the 1.875 and 5.625 mg/kg conditions, nitric oxide and malondialdehyde levels increased by 43.8% and 18.6% respectively (an indication of increased oxidative stress) while glucose levels decreased by 22.5%. In recent literature, a connection between the brain’s glucose deprivation and Alzheimer’s is beginning to come to light.^[7] So, there may be a connection there. But, more research is needed.

Aspartame’s Metabolic Properties and Their Effect on Behavior

Aspartame is broken down in the following ways when metabolized in the gastrointestinal tract:

• 50% phenylalanine
• 40% aspartic acid
• 10% methanol

One major area of concern regarding the research done up to this point in time is that rodents’ metabolism rates of aspartame are different than humans’ due to physiological variations. More specifically, rodents have high liver folate content and thus will not have metabolic acidosis as a result of methanol poisoning the way that humans do (recall that methanol is one of the metabolites of aspartame).

Currently, to address this issue, research is beginning to take a turn, establishing the use of a folate-deficient model to test the effects of aspartame on rodents. One method for inducing folate deficiency is via the administration of methotrexate (MTX).

In a behavioral experiment using Wistar Albino rats, researchers compared three groups: a control group, MTX-treated (folate-deficient) rats, and rats that were MTX-treated and on a diet of aspartame. The control group and the MTX-treated rats (not on an aspartame diet) showed no significant differences in terms of performance in an Open Field test, as measured by the following parameters:

• Ambulation (both in peripheral and central square)
• Rearing
• Immobilization
• Fecal bolus
• Grooming

However, MTX-treated rats that were given aspartame showed significant differences in all of these parameters.

The MTX-treated rats that ingested aspartame showed a decrease in ambulation in both the peripheral and central squares, decreased time spent rearing and grooming, and roughly tripled the amount of time spent immobilization compared to the control group.

Such dramatic results demonstrate the importance of considering the metabolic capacities of animal models used for conducting research.

Do Aspartame’s Effects Vary from Population to Population?

How aspartame’s effects vary from organism to organism is an area of research that still needs to be further investigated.

Some efforts have been made to identify whether males and females react differently to aspartame.

For example, Collison et al. demonstrated that although both genders are affected by aspartame, there seems to be more of an effect on males than on females.^[8]

Beginning in utero, the researchers began exposing the male and female mice to aspartame. By 17 weeks of age, only the males began to show increased weight gain and decreased insulin sensitivity and both genders had raised fasting glucose levels.

When it came to their performance during the acquisition training period of the Morris Water Maze, the males had longer escape latencies than controls while females’ spatial learning had no significant difference from that of the controls.

Both genders showed increased time spent floating directionless and thigmotactic behavior, as well as a decrease in time spent searching for the maze’s target quadrant.

Furthermore, both males and females had their reference memory impaired during a probe test, signifying that aspartame-fed mice spend less time searching for the water maze’s platform.

Based on this research, it is clear that aspartame affects the genders similarly in some ways and different in others. Therefore, more research needs to be conducted to reveal how aspartame interacts with not just gender, but other genotypes as well, including autism, attention deficit disorder, and other neuropsychiatric conditions.

Conclusion

Aspartame, although labeled legally safe, is still a questionable supplement and thus requires further scientific attention. The toll of artificially sweetened drinks is already beginning to show in human studies. For example, self-reported depression is correlated with ingestion of diet, artificially-sweetened drinks. Drinkers of diet drinks (including diet sodas, diet fruit drinks, and diet sweetened iced-tea) were at higher risk of depression than the individuals that drank the same drinks in their non-diet version.^[9]

Although impacts on memory (especially in relation to long-term, habitual consumption of aspartame) have been noted, as well as changes in brain composition and physiological markers of oxidative stress, there is still much that remains unknown about how aspartame affects the human body and mind across various populations.

Future research needs to focus on making use of the correct metabolic models (i.e. folate-deficient models), establishing aspartame’s long-term effects on cognition, and making such findings easily accessible to the general public.

References

1. Food and Drug Administration. “Food additive approval process followed for aspartame.” Report to the Honorable Howard M. Metzenbaum, US Senate. United States Genera Accounting Office (1987).
2. Christian, Brandon, et al. “Chronic aspartame affects T-maze performance, brain cholinergic receptors and Na+, K+-ATPase in rats.” Pharmacology biochemistry and behavior 78.1 (2004): 121-127.
3. Choudhary, Arbind Kumar, Lognatahan Sundareswaran, and Rathinasamy Sheela Devi. “Effects of aspartame on the evaluation of electrophysiological responses in Wistar albino rats.” Journal of Taibah University for Science 10.4 (2016): 505-512.
4. Onaolapo, Adejoke Y., Olakunle J. Onaolapo, and Polycarp U. Nwoha. “Aspartame and the hippocampus: revealing a bi-directional, dose/time-dependent behavioural and morphological shift in mice.” Neurobiology of learning and memory 139 (2017): 76-88.
5. Choudhary, Arbind Kumar, and Yeong Yeh Lee. “Neurophysiological symptoms and aspartame: What is the connection?.” Nutritional neuroscience (2017): 1-11.
6. Abdel-Salam, O. M. E., et al. “Studies on the effects of aspartame on memory and oxidative stress in brain of mice.” Eur Rev Med Pharmacol Sci 16.15 (2012): 2092-2101.
7. Lauretti, Elisabetta, and Domenico Pratico. “Glucose deprivation increases tau phosphorylation via P38 mitogen‐activated protein kinase.” Aging cell 14.6 (2015): 1067-1074
8. Collison, Kate S., et al. “Gender dimorphism in aspartame-induced impairment of spatial cognition and insulin sensitivity.” PLoS One 7.4 (2012): e31570.
9. Guo, Xuguang, et al. “Sweetened beverages, coffee, and tea and depression risk among older US adults.” PloS one 9.4 (2014): e94715.