Alder, Kayla
Fast food, slow brain: The effect of diet on brain volume
Faculty Mentor: Dr. Brock Kirwan, Psychology
Introduction
The hippocampus is a brain structure in the medial temporal lobe that mediates the encoding of sensory information into long-term memory (Squire, Clark, & Stark, 2004). Studies have found a relationship between the volume of the hippocampus and behavioral memory tests; that is, the smaller the volume of the hippocampus, the less accurate the performance on memory tests (Rajah, Kromas, Han, & Jens, 2010). Previous research also demonstrates that a high fat and sugar (HFS) diet impairs the hippocampus and affects memory (Francis & Stevenson, 2011).
Specifically, a high fat (HF) diet leads to decreased spatial learning, decreased learning and
memory, and poor spatial learning. In addition, a high sugar (HS) diet also leads to impaired
learning and memory, poor spatial learning and decreased cell proliferation (Zainuddin & Thuret,
2012).
Though many studies have examined the effects of diet on brain morphology in animals, a clearer understanding of how diet affects hippocampal volume in humans is necessary. Therefore, we suggest that HFS affects memory by reducing the hippocampal volume, possibly through decreasing adult neurogenesis (newborn neurons) in the hippocampus (Zainuddin & Thuret, 2012). Diet plays a major part in memory performance, and may do so by directly influencing the volume of the hippocampus. Therefore, the following is the main question guiding this research: Is there a relationship between high fat/sugar (HFS) diet and hippocampal volume? Findings to this question are significant to society because the hippocampus has a compound effect on eating habits. First, hippocampal volume notably influences the ability to remember whether or not one has eaten (Hebben, Corkin, Eichenbaum, & Shedlack, 1985). Secondly, the hippocampus not only maintains episodic memory for whether or not we have eaten, but is involved in intracellular signals for satiety (Francis & Stevenson, 2011). Thus, the hippocampus, and consequently its volume, are critical factors of eating habits and daily diet.
Methods
In order to determine whether or not a HFS diet directly affects hippocampal volume, we
performed a retrospective study on MRI data that had already been collected. MRI brain scans were collected for 158 participants (99 females) as part of other MRI studies. We used scans from only 111 participants (73 females) due to inaccurate volume output for our algorithm for the earlier scans. We collected information from participants with an average age of 21.9 years. The average weight of participants was 147 pounds.
We assessed diet by distributing a simple questionnaire to participants prior to MRI scanning. Participants were asked to rate how often they had consumed each of 25 food and drink items over the last year. Examples of items include ground red meat, salami, sausage, butter, eggs, pizza, French fries, pancakes, ice cream, chocolate, white bread and others. Participants were given five options for how often each of the 25 items were consumed: five or more times per week, three to four times per week, one to two times per week, two to three times per month, and once per month or less. Questionnaires were scored for each food type based on how frequently items were consumed. Each of the five response types above was assigned one of five numbers (1, 3, 8, 16, 20) based on approximations of how often participants consumed food items per month. Because some questions in the survey gave a range for participants to choose from (for example, 3-4 times per week), we chose to standardize these responses by using either the only given number in the question or the greater number in each range. Thus, the most frequent option (five or more times per week) was assigned a score of 20 (because 5 times per week x 4 weeks per month=20), the next most frequent was assigned a 16 (4×4=16), the next most an 8 (2×4=8), the next most a three (3×1=3), until the least frequent option (once per month or less) was assigned a one. These numbers (1, 3, 8, 16, and 20) were assigned in order to more accurately describe the spread in frequency as opposed to a mere ordinal approach of assigning numbers 1-5.
In order to determine hippocampal volumes, we used an advanced software package to define hippocampal volume, as previously utilized in the Kirwan lab. Specifically, we used the
Analysis of Neuroimaging Tools (ANTs) software to warp a template and its mask into individual participants’ scan space (that is, the specific scan of the participant), such that the hippocampal mask is transferred to the scan space for each participant and in each hemisphere (Hunsaker & Amaral, 2014). The mask on the scan space enabled us to extract hippocampal volumes for each subject.
In data analysis, we performed regressions using HFS diet scores and paired hippocampal volumes produced from the ANTS method described above. In order to obtain a total diet score for each participant, we summed the values described above (1, 3, 8, 16, or 20) for each of the 25 food items for each participant. With these scores for each participant we performed a regression of hippocampal volume on HFS diet score. Next, in order to examine more specific possible effects, we assigned food items consumed into two groups: fat and sugar. We then summed individual fat scores for a total fat score and individual sugar scores for a total sugar score. Using total fat and total sugar scores, we performed two more regressions: hippocampal volume on fat score and hippocampal volume on sugar score.
Results
We did not find a significant correlation between HFS diet and hippocampal volume (see Table 1 and Figure 1). We also did not find a significant correlation between fat score and hippocampal volume nor in sugar and hippocampal volume (see Table 1, and Figures 2-3). We can see from the r2 value that only approximately 1% of the variance of hippocampal volume is explained by diet.
Conclusion and Discussion
Contrary to our hypothesis, diet did not predict hippocampal volume and a significant correlation was not found. There are a few reasons possibly explaining this outcome. First, we sampled a young, healthy population and perhaps the effects of diet were not great enough to significantly damage the hippocampus. Second, our measure of diet may have been insensitive as the source was a self-report over the past year. Moreover, the measuring instrument was subject to inaccurate memory, social desirability bias and a lack of awareness from participants. Therefore, we propose that future studies design a more objective or observational method for obtaining individual diet such as food buffets, food journals, or random online assessments as they may reduce some of the error mentioned above. Further, we also suggest that this theory be tested in a middle-aged sample, as opposed to a young adult sample.
References
Francis, H. M., & Stevenson R. J. (2011). Higher reported saturated fat and refined sugar intake
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Hebben, N., Corkin, S., Eichenbaum, H., & Shedlack, K. (1985). Diminished ability to interpret
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Hunsaker M.R., & Amaral D.G. (2014). A semi-automated pipeline for the segmentation of
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Rajah, M. N., Kromas, M., Han, J. E., & Jens, C. P. (2010). Group differences in anterior
hippocampal volume and in the retrieval of spatial and temporal context memory in healthy young versus older adults. Neuropscyhologia, 48(14), 4020-4030.
Squire, L. R., Clark, R. E., & Stark, C. E. L. (2004). The medial temporal lobe. Annual Review of
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Zainuddin, M. S. A., & Thuret, S. (2012). Nutrition, adult hippocampal neurogenesis and mental
health. British Medical Bulletin, 103(1), 89-114.