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One Piece of the Pie

November 20, 2018

 

Each year at Thanksgiving, I find myself with a heaping plate of absolutely everything available to me. Time and time again I work my way through the plate and end up full beyond belief. However, when the desserts start coming out, I can’t help but to snag a slice of sweetness. Despite what my stomach is telling me, and regardless of the fact that I could easily save a piece and eat it later, I still help myself to the sugary treat… but why?

First let’s begin by distinguishing between hunger and appetite. There are several definitions in circulation among scholars, but, generally, hunger has been defined as the conscious sensation urging us to eat and appetite refers to food intake, selection, motivation and preference.

Researchers traditionally break the neuroendocrinology of appetite into 3 parts: feeding and energy expenditure, glucose control and hedonic value. The first two parts factor in a model that looks at energy intake, energy expenditure and our body’s drive to keep our blood glucose levels in check. When I begin to fill my plate with food at the Thanksgiving table, this is due to the hormone ghrelin being released as a result of low blood glucose levels. Ghrelin is released from the stomach and stimulates hunger. As I’m eating delicious mashed potatoes, stuffing, and green beans, two other prominent hormones, cholecystokinin and peptide YY, detect the food and act on the hypothalamus and vagus nerve to start to suppress hunger. Leptin will then be released, further reducing appetite and even pleasure, causing the thought of more food consumption to be aversive. 

However, these hormones are intertwined with another path: the reward circuit. Activation in the reward circuit, which has been linked to addictive drug use and compulsive behaviors, is involved when we eat particularly palatable foods, like a slice of pumpkin pie. Studies show that even the sight of foods high in sugars, fats and sodium activate our reward circuit. The main culprit scientists attribute these feelings of pleasure with is the neurotransmitter dopamine. Neurons in the part of the brain called the arcuate will project to orexin neurons, which are located in the lateral hypothalamus of the brain. These neurons project to two major areas of the reward circuit involved in dopamine release: the ventral tegmental area and the nucleus accumbens. So when the cookies and pies are assembled in front of me at the table, you best believe my brain is glowing with dopamine activity.

It is possible to become addicted to the pleasure derived from eating sugary and fatty foods. Amazing right? You can get a rush from delicious foods! While this can be great in doses, overeating these types of foods can lead to a desensitization of the dopamine release. This is very similar to what occurs with drug addiction. Over-activation of these chemicals, whether it be dopamine or a substance that mimics it, will cause a reduction in the chemical receptors. This is our brain’s attempt at moderation, however, it means we don’t just want but require more of the substance to derive the same pleasure. The reward circuit could be why it is very difficult to stop eating the chips once we’ve dipped our hand into the bag once.

So how do we combat our body and brain to prevent finishing the the family size bag of chips? The short answer is there is no magical solution. Scientists are looking into the role of gut microbiota and how this interacts with our hormones and appetite. It is possible that the foods we eat influence the microbiomes responsible for digestion, and this can affect the types of food we crave. Researchers have also found that when the body is in a fasting state, ghrelin levels are not only elevated, but we are also more sensitive to reward and impulsive feeding behaviors. This suggests we are more likely to reach for less nutritious foods on an empty stomach. Orexin is the primary player in the connection between reward and appetite. While knocking out receptors that sense this hormone prevent the temptation of eating high fat diets, it could also be the reason why we have five star restaurants and how they can even stay in business. All that to say, I think overdoing it one day of the year to make your brain (if not your stomach) happy can be worth it. 

 

 

 

 

References:

Berridge, K et al.“Food reward: Brain substrates of wanting and liking” Neuroscience and Biobehavioral Reviews vol. 20,1 (1996): 1-25. 

Blundell, J et al. “Appetite control: methodological aspects of the evaluation of foods” Obesity reviews : an official journal of the International Association for the Study of Obesity vol. 11,3 (2010): 251-70.

Choi, D.L et al. “Orexin signaling in the paraventricular thalamic nucleus modulates mesolimbic dopamine and hedonic feeding in the rat” Neuroscience vol. 210 (2012): 243-248.

Hernandez, L et al. “Food reward and cocaine increase extracellular dopamine in the nucleus accumbent as measured by microdialysis” Life Sciences vol. 42,18 (1988): 1705-1712.

Ralevski, E., et al. “Ghrelin is related to personality differences in reward sensitivity and impulsivity” Alcohol and Alcoholism vol. 53,1 (2017): 1-5

Volkow, N et al. “Reward, dopamine and the control of food intake” Trends in Cognitive Sciences vol. 15, 1 (2011): 37-46.

Wise, R. “Role of brain dopamine in food reward and reinforcement” Philosophical Transactions of the Royal Society B vol. 361,1471 (2006).

 

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About Author

Siena is a Neuroscience undergraduate at the Georgia Institute of Technology. Her passions include education, writing and sustainability. When she's not studying or volunteering, she is preoccupied with a new DIY activity or art project.

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.