December 19, 2019

Metabolic syndrome, driven by chronically elevated insulin, has been called “the major health hazard of [the] modern world.” This cluster of factors affects more than 34 percent of US adults, and over 30 million people in the US have type 2 diabetes. With those statistics in mind, there’s no question that a significant portion of the population has difficulty keeping blood glucose and insulin levels within a healthy range. Ketogenic diets have been shown to be effective for improving this, and research published earlier this year adds to the body of literature showing that restriction of dietary carbohydrate is effective not only for improving various cardiometabolic parameters but also for promoting cognitive function.

With Alzheimer’s disease frequently called “type 3 diabetes” and the possibility that this form of cognitive decline may have its roots in metabolic syndrome (MetSyn), it’s not surprising that improvements in glycemic control and insulin sensitivity are associated with improved cognitive function. A randomized crossover trial published recently showed that a low-carb Paleo-style diet improved cognitive performance in subjects with MetSyn and that adding in high-intensity exercise strengthened this effect even further.   

The study was relatively small—just twelve subjects—but the results are worth looking at, especially when factored in as additional supportive evidence for the role of carbohydrate restriction and exercise in improving cognition. Subjects were adults with MetSyn (mean age 40.9 ± 20.2) who were relatively sedentary at baseline. The dietary intervention was an ad libitum low-carb Paleo diet: total carbohydrate less than 50 grams per day and allowing for unlimited consumption of lean meat, eggs, fish, cruciferous and leafy vegetables, root vegetables, fruit, and nuts, moderate amounts of dried fruit, starchy roots, and wine, and exclusion of cereal grains, dairy, beans/legumes, beer, and added salt and sugar. Dietary compliance was reported as good, with carbohydrate restriction verified by slightly elevated blood ketone levels. 

The exercise protocol was performed three days per week and called for ten 60-second ergometer cycling intervals interspersed with 60 seconds of active recovery (plus 3 minutes each for warmup and cooldown). Subjects’ heart rates were monitored to ensure they were engaging in an all-out effort, evidenced by their attaining approximately 90% of their maximal heart rate.

The crossover aspect of the study involved four weeks of the low-carb Paleo diet alone or the low-carb Paleo diet with exercise in randomized order separated by a four-week washout period. Various biomarkers were measured along with serum levels of brain-derived neurotrophic factor (BDNF) as well as cognitive performance on the Stroop test and self-reported assessments of cognition.

Results showed that the diet alone had a powerful effect on improving BDNF, cognitive speed and flexibility. However, these improvements were even larger when subjects added in the exercise. During the sedentary portion, BDNF increased 20 percent; an increase of 38 percent was observed after the exercise intervention. Clinically assessed cognitive speed and flexibility were also increased more by the diet plus exercise than by the diet alone, as was self-reported cognitive function. Serum BDNF level was found to inversely correlate with fasting glucose, HOMA-IR, triglycerides, and body fat percentage: improved markers of cardiometabolic health were associated with higher levels of BDNF.

This study provides at least two things to be heartened by. First, it shows that dietary changes alone can have a significant impact on cognitive function, and second, it shows that combining these dietary changes with high-intensity exercise has an additive effect that can lead to even greater improvements. The first point is worth noting because individuals who are unable to exercise due to physical disability or other limitations may experience substantial benefits from a low-carb Paleo diet even without exercise. The second point may encourage those who incorporate exercise into their lives to do so—and based on this study, the exercise need not take more than a half-hour three times per week. Even people who claim to have no time for exercise can probably squeeze in that much.

On the other hand, exercise alone—in the absence of any dietary change—has long been recognized for increasing BDNF. So exercise can be a way for individuals who prefer to consume a higher carb diet to get a BDNF boost. The boost is temporary, however. A long-lasting increase in BDNF from acute bouts of exercise has not been demonstrated, so exercise would have to be performed regularly to get this brain enhancing and potentially fat-burning benefit. But we already knew that; of course, exercise needs to be done on a regular basis to sustain its positive effects. (If only you could do it once and be done forever! Like any healthy habit—eating nutritious food, meditating, getting adequate sleep—it has to be a regular, ongoing part of life if it’s going to have the intended impact.)

BDNF is most often regarded for its role in promoting neuroplasticity and cognitive function, but this interesting molecule may also be involved in the browning of white adipose tissue—giving white fat some of the positive aspects of brown fat. Brown fat is more metabolically active than white fat, and interventions that target this are being explored as treatments for obesity. So for individuals with metabolic syndrome—which often, but not always, correlates with being overweight—a diet lower in carbohydrates combined with regular exercise may be beneficial for improving numerous cardiometabolic parameters as well as cognitive function.