Low-Glycemic Eating for Mental Clarity

Short answer: In several small studies, low-glycemic breakfasts were followed by better memory or attention two to three hours later than high-glycemic ones. A systematic review found the results inconsistent overall, so the evidence is preliminary. The changes are easy: swap refined carbs for whole ones, pair carbs with protein, fat and fiber, and eat vegetables first.

Low-glycemic eating may help you stay focused through the morning, but the evidence is mixed. Small meal studies lean toward a benefit; a review of 11 studies found the results inconsistent.

Low-glycemic eating does not mean low-carb. It means choosing carbohydrates that release glucose slowly and eating them alongside foods that slow absorption further. For the bigger picture on glucose, insulin resistance and dementia risk, see blood sugar and brain function.

If you have prediabetes, PCOS or metabolic syndrome, low-glycemic eating is worth discussing with your doctor or dietitian. It can improve glucose control alongside, not instead of, any treatment you are prescribed. If you take insulin or a sulfonylurea, ask your doctor before major diet changes, because lower glucose may mean the dose needs adjusting.

Easy low-GI food swaps

Cooking method matters. Al dente pasta has a lower GI than overcooked pasta. Potatoes boiled and then cooled develop resistant starch, which lowers their glycemic impact.

How to build a low-glycemic meal

Pair carbohydrates with protein and fat. Protein and fat slow stomach emptying, so glucose arrives more gradually. White rice eaten alone gives a very different glucose curve from the same rice with grilled salmon and avocado. An apple with a handful of almonds gives a flatter curve than the apple alone. See high-protein diets and the brain.

Eat vegetables and protein first. A small study (Shukla 2015) tested 11 adults with obesity and type 2 diabetes. Eating vegetables and protein before carbohydrates lowered post-meal glucose by 29 percent at 30 minutes and 37 percent at 60 minutes. It also lowered insulin. Start with salad, vegetables or protein, and finish with bread, rice or potatoes.

Eat more fiber. Soluble fiber in oats, barley, legumes, flaxseed and psyllium forms a gel in the gut that slows glucose absorption. A meta-analysis of 15 trials in people with type 2 diabetes (Post 2012) found that more fiber lowered fasting glucose by 0.85 mmol/L and HbA1c by 0.26 percentage points. Most Western adults eat 15 to 18 grams of fiber a day, about half the recommended 25 to 35 grams. Resistant starch, from cooled potatoes, green bananas and legumes, also feeds gut bacteria that make short-chain fatty acids (Koh 2016).

Consider vinegar. In small trials, about a tablespoon of vinegar diluted in water with a meal, or a vinegar dressing, lowered post-meal glucose. In one small study (Johnston 2004), vinegar with a high-carbohydrate meal improved post-meal insulin sensitivity in people with insulin resistance or type 2 diabetes. Always dilute it, because undiluted vinegar can damage tooth enamel and the throat. If you take diabetes medication, ask your doctor first.

A sample day:

• Breakfast: Steel-cut oats with walnuts, ground flaxseed and berries. See our berry walnut smoothie for a quick version.
• Lunch: A large salad with chickpeas, olive oil dressing, grilled chicken or fish, and sourdough bread.
• Dinner: Grilled salmon with roasted sweet potatoes, steamed broccoli and lentils.
• Snack: An apple with almond butter, or a few nuts with a couple of squares of dark chocolate (70 percent cocoa or higher).

Glycemic index versus glycemic load

The glycemic index (GI) ranks carbohydrate foods from 0 to 100 by how fast they raise blood glucose compared with pure glucose (Jenkins 1981). Low-GI is 55 or below, medium is 56 to 69, and high is 70 or above. Lentils (GI 30) raise blood sugar slowly; white bread (GI 75) raises it fast.

GI is measured using a portion with 50 grams of carbohydrate, which can mislead. Watermelon has a GI of about 72, but you would need to eat about 700 grams, over five cups, to get 50 grams of carbohydrate.

The glycemic load (GL) corrects for portion size: GI multiplied by grams of carbohydrate per serving, divided by 100. A GL of 10 or below is low, 11 to 19 medium, and 20 or above high. A typical 120-gram serving of watermelon has about 6 grams of carbohydrate, so its GL is about 4.3, which is low. A bowl of white rice (GI 73, about 45 grams of carbohydrate) has a GL of about 33, which is high. For everyday choices, GL is the more useful number.

What the studies show for focus and memory

The brain uses about 20 percent of the body’s glucose and has almost no fuel reserve (Mergenthaler 2013). The idea is that a fast rise and fall in blood sugar hurts attention and memory, while a gentle, steady rise sustains them.

• Benton 2003: Young adults ate breakfasts with different glycemic properties. The low-GI breakfast was followed by better verbal memory, especially 150 and 210 minutes after eating. A parallel rat experiment agreed. The authors noted that performance was only remotely related to blood glucose, so the mechanism is unclear.
• Ingwersen 2007: In a crossover study of 64 children aged 6 to 11, performance fell across the morning after both cereals. But attention accuracy and memory fell clearly less after the low-GI cereal.
• Benton 1994: In young adults, a glucose drink improved word recall compared with placebo. This shows glucose supply matters in the short term, but not that low-GI meals are better.
• Philippou 2014: A systematic review of 11 studies found mixed results: some favored the low-GI meal, some the high-GI meal, and some found no difference. The authors concluded a low-GI meal may favor cognition in adults, but the evidence is inconclusive.

Most earlier studies used glucose drinks or pure-carbohydrate meals rather than ordinary food (Benton and Nabb 2003). For students and others who need a sharp morning, the Benton studies suggest breakfast may influence memory for the next three to four hours. Our page on breakfast and cognition covers this further.

What about the “crash”? After a fast spike, insulin overshoots in some people and blood sugar dips below where it started, two to three hours later. True hypoglycemia impairs attention and reaction time. Whether the smaller dips after a typical high-GI meal affect thinking in healthy people is not well established.

Longer-term effects

A diet high in refined carbohydrates may contribute to insulin resistance over years. The brain has its own insulin receptors, concentrated in memory areas (Fernandez 2012).

• Kerti 2013: In 141 healthy older adults without diabetes, higher HbA1c and glucose were linked to poorer memory, partly through differences in the hippocampus.
• Ceriello 2008: Raising blood glucose to 10–15 mmol/L with an intravenous clamp impaired blood-vessel function and raised oxidative stress, more so when glucose swung up and down. Whether ordinary post-meal spikes do this in the brain has not been shown.
• Willette 2015: In 150 cognitively normal late-middle-aged adults, higher insulin resistance was linked to lower glucose use across large parts of the brain, especially the memory regions of the temporal lobe. This was a snapshot study, so it can’t show which came first.
• Chatterjee 2016: In a pooled analysis of 2.3 million people, type 2 diabetes was linked to about a 60 percent higher risk of dementia. It was also linked to about a 50 percent higher risk of non-vascular dementia (which includes Alzheimer’s). Suzanne de la Monte called Alzheimer’s “type 3 diabetes”, a form of brain insulin resistance (de la Monte 2008).

Not all high-glycemic eating leads to diabetes, and whether a low-glycemic diet lowers dementia risk has not been tested in trials.

Who might benefit most

• People with prediabetes or metabolic syndrome. This means fasting glucose of 5.6 to 6.9 mmol/L or HbA1c of 5.7 to 6.4 percent. In 266 healthy adults aged 60 to 64 without diabetes (Cherbuin 2012), fasting glucose in the high-normal range was linked to more shrinkage of memory regions over four years.
• Women with PCOS. Polycystic ovary syndrome involves insulin resistance, and brain fog is a common complaint. In a trial of 96 overweight women with PCOS (Marsh 2010), about half dropped out. Among those who finished, a low-GI diet improved insulin sensitivity and regular cycles more than a conventional healthy diet. Reproductive hormones did not differ. Cognition was not measured.
• Older adults. Insulin sensitivity falls with age, so the same meal causes bigger glucose swings. Low-GI eating is an easy change, though it has not been tested for preventing decline. See the over-50 brain diet.

Frequently Asked Questions

Are GI tables reliable for me personally?

They are rough guides. A food’s GI changes with ripeness, cooking and what you eat with it. In a large study (Zeevi 2015), people responded very differently to identical foods: some spiked after bananas but not cookies, and others the reverse. A two-week trial with a continuous glucose monitor can show your own patterns.

Can low-glycemic eating fix brain fog?

If your fog comes from glucose swings or insulin resistance, as may be the case with prediabetes, PCOS or metabolic syndrome, it may help. This has not been tested in trials. Brain fog also has many other causes, including poor sleep, nutrient deficiencies, thyroid problems and depression.

How does this relate to the Mediterranean or MIND diet?

They overlap a lot. The Mediterranean diet is naturally moderate in glycemic load because it centers on legumes, whole grains, vegetables, olive oil and fish. Low-glycemic eating may explain part of why such diets are linked to better brain health. You can follow a Mediterranean diet with attention to glycemic load.

Sources

• Benton, D., Owens, D. S., & Parker, P. Y. (1994). Blood glucose influences memory and attention in young adults. Neuropsychologia, 32(5), 595–607. doi:10.1016/0028-3932(94)90147-3
• Benton, D., Ruffin, M. P., Lassel, T., et al. (2003). The delivery rate of dietary carbohydrates affects cognitive performance in both rats and humans. Psychopharmacology, 166(1), 86–90. doi:10.1007/s00213-002-1334-5
• Benton, D., & Nabb, S. (2003). Carbohydrate, memory, and mood. Nutrition Reviews, 61(5 Pt 2), S61–S67. doi:10.1301/nr.2003.may.S61-S67
• Ceriello, A., Esposito, K., Piconi, L., et al. (2008). Oscillating glucose is more deleterious to endothelial function and oxidative stress than mean glucose in normal and type 2 diabetic patients. Diabetes, 57(5), 1349–1354. doi:10.2337/db08-0063
• Chatterjee, S., Peters, S. A. E., Woodward, M., et al. (2016). Type 2 diabetes as a risk factor for dementia in women compared with men: a pooled analysis of 2.3 million people comprising more than 100,000 cases of dementia. Diabetes Care, 39(2), 300–307. doi:10.2337/dc15-1588
• Cherbuin, N., Sachdev, P., & Anstey, K. J. (2012). Higher normal fasting plasma glucose is associated with hippocampal atrophy: the PATH Study. Neurology, 79(10), 1019–1026. doi:10.1212/WNL.0b013e31826846de
• de la Monte, S. M., & Wands, J. R. (2008). Alzheimer’s disease is type 3 diabetes — evidence reviewed. Journal of Diabetes Science and Technology, 2(6), 1101–1113. doi:10.1177/193229680800200619
• Fernandez, A. M., & Torres-Alemán, I. (2012). The many faces of insulin-like peptide signalling in the brain. Nature Reviews Neuroscience, 13(4), 225–239. doi:10.1038/nrn3209
• Ingwersen, J., Defeyter, M. A., Kennedy, D. O., Wesnes, K. A., & Scholey, A. B. (2007). A low glycaemic index breakfast cereal preferentially prevents children’s cognitive performance from declining throughout the morning. Appetite, 49(1), 240–244. doi:10.1016/j.appet.2006.06.009
• Jenkins, D. J. A., Wolever, T. M. S., Taylor, R. H., et al. (1981). Glycemic index of foods: a physiological basis for carbohydrate exchange. The American Journal of Clinical Nutrition, 34(3), 362–366. doi:10.1093/ajcn/34.3.362
• Johnston, C. S., Kim, C. M., & Buller, A. J. (2004). Vinegar improves insulin sensitivity to a high-carbohydrate meal in subjects with insulin resistance or type 2 diabetes. Diabetes Care, 27(1), 281–282. doi:10.2337/diacare.27.1.281
• Kerti, L., Witte, A. V., Winkler, A., et al. (2013). Higher glucose levels associated with lower memory and reduced hippocampal microstructure. Neurology, 81(20), 1746–1752. doi:10.1212/01.wnl.0000435561.00234.ee
• Koh, A., De Vadder, F., Kovatcheva-Datchary, P., & Bäckhed, F. (2016). From dietary fiber to host physiology: short-chain fatty acids as key bacterial metabolites. Cell, 165(6), 1332–1345. doi:10.1016/j.cell.2016.05.041
• Marsh, K. A., Steinbeck, K. S., Atkinson, F. S., Petocz, P., & Brand-Miller, J. C. (2010). Effect of a low glycemic index compared with a conventional healthy diet on polycystic ovary syndrome. The American Journal of Clinical Nutrition, 92(1), 83–92. doi:10.3945/ajcn.2010.29261
• Mergenthaler, P., Lindauer, U., Dienel, G. A., & Meisel, A. (2013). Sugar for the brain: the role of glucose in physiological and pathological brain function. Trends in Neurosciences, 36(10), 587–597. doi:10.1016/j.tins.2013.07.001
• Philippou, E., & Constantinou, M. (2014). The influence of glycemic index on cognitive functioning: a systematic review of the evidence. Advances in Nutrition, 5(2), 119–130. doi:10.3945/an.113.004960
• Post, R. E., Mainous, A. G., King, D. E., & Simpson, K. N. (2012). Dietary fiber for the treatment of type 2 diabetes mellitus: a meta-analysis. The Journal of the American Board of Family Medicine, 25(1), 16–23. doi:10.3122/jabfm.2012.01.110148
• Shukla, A. P., Iliescu, R. G., Thomas, C. E., & Aronne, L. J. (2015). Food order has a significant impact on postprandial glucose and insulin levels. Diabetes Care, 38(7), e98–e99. doi:10.2337/dc15-0429
• Willette, A. A., Bendlin, B. B., Starks, E. J., et al. (2015). Association of insulin resistance with cerebral glucose uptake in late middle-aged adults at risk for Alzheimer disease. JAMA Neurology, 72(9), 1013–1020. doi:10.1001/jamaneurol.2015.0613
• Zeevi, D., Korem, T., Zmora, N., et al. (2015). Personalized nutrition by prediction of glycemic responses. Cell, 163(5), 1079–1094. doi:10.1016/j.cell.2015.11.001