By Sarah Ahmed (Pitzer College) and Lea Herbert (Pitzer College) [Edited by Lars Schmitz, as part of BIOL 167 “Sensory Evolution”, an upper division class at the W.M. Keck Science Department. Written for educational purposes only].
“It’s simple, if it jiggles, it’s fat.”-Arnold Schwarzenegger
If fat did not exist, would everyone be in a happier place? Humans would be all muscle zero percent body fat! Imagine a world filled with millions of Arnold Schwarzeneggers …uhh maybe not. But what is the evolutionary reason for our cravings of fatty foods? Well, the desire to eat high volume of fats may stem from the evolutionary advantage of storing calories. Fat is one of the few energy sources that can be stored for a long period of time.In a recent paper, Sanne Boesveldt and Johan Lundström (2014) proposed that humans have the capability to detect olfactory-based fat content in food from a distance. “Olfaction is great because it can detect good from poisonous food before contact in the mouth, thus one step removal from having to taste it. The sense is able to orientate us to food sources and create strategies for foraging.” explains neuroscience Professor Alan Jones (Pitzer College). It is not surprising then that olfaction is widespread among animals, with hundreds of receptors devoted to picking up specific odors, especially in mammals.
Boesveldt and Lundström think that the ability to smell fat content from afar would be advantageous, as it would maximize the likelihood of finding a source of calories. Thus, research showing that animals have the ability to identify triglycerides (derived from glycerol and three fatty acids) and fatty foods via olfaction, may be indicative of the larger evolutionary favorability towards calorie detections and energy storage.For example, there is a correlation between the olfactory sense and the preference for high fat content foods in mice (Kinney and Antill, 1996). Albino male mice were provided food with high and low fat content for four days. The mice were then split into two control groups and one group lacking the olfactory sense. This experimental group had their olfactory nerve bilaterally sectioned—a technique which allows for complete recovery, as the olfactory neurons are able to regenerate and recover within at least 30 days. After this surgery, all groups were given access to the high and low fat content food to experimentally determine the olfactory sense in fatty food preference. Both control groups preferred the fatty food, while the olfactory nerve-sectioned mice were unable to distinguish between the two. Once the experimental group’s olfactory sense recovered, they also began to prefer the fatty food showing that smell plays a role in identifying fat content.
Subsequent papers documented human ability to detect fatty acids orally, but Boesveldt and Lundström wanted to take it one step further. Can humans smell fat, even when presented with a real food product where the smell of fatty acids is masked by other odors? To explore this idea, they conducted experiments to test if humans are able to identify gradations of fat in a common food item—milk.
They conducted their experiment with two groups—from the US and Netherlands. Average milk consumption is 31% higher in Netherlands in comparison to US. Participants were asked to smell samples of skim (S), medium (M) and fat (F) milk made from milk powder and rank the intensity and pleasantness of each sample. American participants rated the pleasantness of each sample differently, as individuals sensed that with increasing fat content, the intensity increased and the smell became less pleasant. On the other hand, Dutch participants did not smell a significant distinction between each sample’s intensity and pleasantness.Samples were then presented as triplets, in combinations of SSM, SSF, FMM, and participants were asked to identify the odd sample out. The skim, medium and fat milk samples contained fat concentrations of about 0.125%, 1.36% and 2.6%. Both population groups were able to distinguish between skim/medium samples and skim/fat samples, while it was difficult for the majority to distinguish between medium/fat samples.
Next, Boesveldt and Lundström tested to see if there was a correlation between BMI or daily dairy consumption and detecting fats. Referring to the BMI scale, 30 participants deemed normal and 30 participants considered overweight were recruited.The BMI participants were able to distinguish between medium and fat milk but had trouble differing between the skim and medium samples. Between weight and habitual dairy consumption, no correlation was found for enhanced performance of “olfactory fat discrimination.” From this experiment, Boesveldt and Lundström concluded that humans have the capability of detecting even minute differences of fats in food. Participants were 40-55% accurate during all three trials which is better than expected by chance alone. Even though the sample from the Netherlands had a much higher dairy consumption in relation to the Americans, both groups showed similar results, indicating that higher intake of dairy does not make an individual more sensitive for detecting fats. Because all participants had prior experience with milk, it cannot be concluded whether olfactory fat detection is a learned trait or based upon experience and exposure.
Interestingly, triglycerides (=dietary fats) are not volatile, as they don’t evaporate at room temperature (Boesveldt and Lundström, 2014). Therefore, the fats from the milk could not have been the smell that was detected by the participants. At this point we don’t know what exact scent helps us to discriminate the fat content of milk by olfaction.
From further research we can learn what exact volatile components of the milk samples caused the chemical signal detection by the nose. Some hypotheses are that triglycerides may act as a carrier for other volatile components, or that the triglycerides may have interacted with other ingredients in the milk, altering the olfactory perception.
The fact that participants were able to distinguish fat content in milk samples just by smelling is pretty cool but further research is needed to understand the biological aspect of this amazing phenomenon.
Boesveldt S., Lundström, J. N. 2014. Detecting fat content of food from a distance: olfactory-based fat discrimination in humans. PLoS ONE 9(1): e85977. (DOI:10.1371/journal.pone.0085977).
Kinney, N. E., Antill, R.W. 1996. Role of olfaction in the formation of preference for high-fat foods in mice. Physiol & Behav 59(3): 475-78. (DOI: 10.1016/0031-9384(95)02086-1).