- Foods such as cow’s milk can influence cognitive skills over the long term, but their short-term effects on cognition are still unclear.
- A new study compared participants’ ability to perform various cognitive tasks shortly after drinking either cow’s milk, juice, or water.
- Participants who had higher fasting glucose performed better on certain cognitive tasks after drinking cow’s milk compared with when they had juice, whereas the opposite was true for participants who had lower fasting glucose.
What we eat is known to influence our cognitive skills, and consuming dairy, in particular, has been associated with improved cognition [1-4]. “There have been epidemiological studies or observational studies looking over the long term showing that people who ingest a higher amount of dairy products have better cognition in the long run,” says Professor Mary Beth Spitznagel of Kent State University.
Rather than focusing only on long-term effects, Spitznagel became interested in studying the short-term effects of a meal on cognition. “We wanted to look at this very acutely, just to see if in the moment what we drink or what we eat is going to impact our thinking skills,” she says. A couple of studies that previously looked at the acute cognitive effects of cow’s milk had contradictory results, with one finding a beneficial effect on cognition and the other finding no effect [5,6].
In a new study, Spitznagel and her colleagues compared the effects of cow’s milk, juice, and water on participants’ ability to perform various cognitive tasks . They found that participants who had higher fasting glucose performed better on certain cognitive tasks after having cow’s milk compared with when they had juice, whereas the opposite was true for participants who had lower fasting glucose.
“There probably is an optimal glucose range for cognitive function,” says Spitznagel. “So people who are starting out lower may need something to push them into that range, whereas the people who start out with that higher fasting glucose need something more balanced to prevent them from being pushed out of that optimal glucose range,” she says. “If you’re on the higher end for fasting blood glucose, milk might be better for your thinking skills, and if you’re on the lower end, maybe juice would be better,” says Spitznagel.
The study highlights the importance of knowing fasting glucose levels even in healthy people. “One of the things I like about this study is that we specifically brought in healthy young people with no problems with glucose tolerance, blood sugar, hypoglycemia, or hyperglycemia,” says Spitznagel. “Potentially it would be important for people even if they consider themselves healthy to know what their typical fasting glucose range is and eat accordingly,” she says. “If you are a student who is about to take a test, and you know that you need to really be focused for that brief period of time, then knowing what your fasting glucose is might actually help you choose what to consume,” says Spitznagel.
Spitznagel became interested in how diet affects cognition a few years ago. “As a clinical neuropsychologist, I’m really interested in finding ways that people can try to improve or optimize their cognitive skills,” she says. “In general I have an interest in non-pharmacological interventions, things like exercise and diet, the minor changes that we might be able to make in our lives that could potentially boost performance on a daily basis,” says Spitznagel.
In 2016, Spitznagel and her colleagues conducted a literature review and found a lot of research on the effects of diet on cognition . But the results of many of the studies were inconsistent. “We were interested in understanding what might be contributing to some of those inconsistencies,” she says. “We noticed in the course of doing that review that the papers that were focused more specifically on people who have impaired fasting glucose or impaired glucose tolerance or diabetes, in general their findings were very consistent,” says Spitznagel. “Higher carbohydrate meals are not good for cognition if you have diabetes, which is not surprising,” she says.
The review made Spitznagel consider the potential importance of glucose regulation and fasting glucose levels on post-meal cognition. Previous work had shown that baseline glucoregulation could affect cognition after a meal [9,10]. “There’s a little bit of work showing that people with really excellent glucoregulation do benefit from something that’s a slightly higher carbohydrate meal,” says Spitznagel. “For those who maybe had slightly higher fasting glucose, the hypothesis was that they would need something that was a little bit more balanced,” she says.
In the new study, Spitznagel and her colleagues gave healthy young adults 8 ounces of either 1% milk, apple juice, or water, and had them perform various cognitive tasks 30, 90 and 120 minutes after the meal. They also measured participants’ fasting glucose. “We’re using it as a proxy measure for glucoregulation,” says Spitznagel.
Spitznagel chose to study the effects of milk, apple juice and water because these are common beverages. “I wanted this to be something that would actually apply in the real world,” she says. “We’re kind of looking at the apple juice condition as our very high carbohydrate condition, and milk in terms of its macronutrient profile is more evenly balanced,” she says. “The water control condition was just to keep people hydrated, as there’s been some literature that suggests that when someone’s dehydrated their cognition is not optimized,” says Spitznagel.
The researchers hypothesized that juice might improve cognition in those with lower fasting glucose, while milk might improve cognition in those with higher fasting glucose. “It did pan out the way we expected, so that was exciting,” says Spitznagel. “People who had higher fasting glucose did perform better in terms of working memory or complex concentration tasks after milk compared to juice, and the opposite was the case for people who had lower fasting glucose,” she says.
One of the innovations of the new study is that it considered glucoregulation as a continuous variable. “In the past when glucoregulation has been considered, it’s most typically been in a way where the researcher is splitting people up into two groups, high fasting glucose and low fasting glucose, and sometimes dividing those groups at 100 mg/dL, and sometimes at 90 mg/dL,” says Spitznagel. “Looking at it in a continuous way allows us to more specifically detect what’s going on,” she says.
“I was especially excited that we were able to look at and pinpoint a specific range,” says Spitznagel. “When fasting glucose was above 106 mg/dL, those folks did better after milk in that working memory task, and if fasting glucose was below 77 mg/dL, they performed better with juice,” she says. “What our work is showing is that we see these differences not necessarily at 100 mg/dL, not necessarily at 90 mg/dL, but we’re seeing them at 77 mg/dL and 106 mg/dL,” says Spitznagel.
Spitznagel wants to replicate the findings in other study populations, and is currently working on a follow-up study in children. “I think this is work that should also be done in older adults, as we know that some of the things we’re looking at in terms of glucose tolerance and some of the hormones that are involved may differ across different ages,” she says.
Spitznagel also plans to look at the effects of diet on a greater variety of cognitive tasks. “I think it’s also going to be important for future work to look at other cognitive domains like learning and memory to identify the optimal glucose ranges for different beverages by cognitive domain,” she says.
Future studies could also look at the effects of larger serving sizes. “We used a single serving size, 8 ounces, but especially in this country oftentimes we are not just consuming 8 ounces of beverage but instead consuming 24 or 32 ounces,” says Spitznagel. “I think there’s good potential that the effects could be larger with a larger amount of beverage than we’re looking at, and that potentially might be more reflective of how people are consuming beverages these days,” she says.
The fact that cow’s milk may improve cognition in people with higher fasting glucose may become even more significant in the future, given the increasing prevalence of high fasting glucose levels. “I think increasingly our body mass indices are on the rise, and associated with that we’re having increased incidence of diabetes or impaired fasting glucose, and so I think this is going to become increasingly important,” says Spitznagel.
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Dr. Sandeep Ravindran
Freelance Science Writer