Tomatoes are a source of carotenoids.BHAKPONG RATTANASAROJ/iStockPhoto / Getty Images
Keep up to date with the weekly Health & Wellness newsletter. Sign up today.
Several studies have reported that eating a diet high in fruits and vegetables, especially ones plentiful in phytochemicals called carotenoids, guards against cognitive decline. Yet, it’s not known how these plant compounds protect the brain.
Now, new research adds to growing evidence that carotenoids help people maintain cognitive heath.
According to a study published this month in the American Journal of Clinical Nutrition, a high intake of carotenoids can substantially cut the risk of developing Alzheimer’s disease. And it may do so by inhibiting the build-up of beta-amyloid, sticky proteins that form plaques and destroy nerve cells.
What are carotenoids?
Carotenoids are naturally occurring compounds found in red, yellow, orange and dark green vegetables and fruits.
Common carotenoids in our diet include beta-carotene (e.g., carrots, sweet potato, mango), lutein/zeaxanthin (e.g., spinach, kale, Swiss chard), lycopene (e.g., tomato sauce, tomato juice, watermelon) and beta-cryptoxanthin (e.g., pumpkin, papaya, tangerines).
The latest study
For the study, researchers followed 927 Chicago adults living in retirement communities, average age 81, for seven years. At the study outset, participants did not have Alzheimer’s disease.
Volunteers completed a comprehensive dietary questionnaire and underwent a battery of cognitive tests annually. During the study, 508 participants died.
Compared to people who had a low intake of total carotenoids, those whose diets contained the most (25 versus 6.7 mg) had a 48 per cent lower risk of Alzheimer’s disease over the study period.
When the researchers looked at individual carotenoids, a significantly lower risk of Alzheimer’s disease was seen for higher intakes of lutein/zeaxanthin and lycopene.
The researchers controlled for other factors related to Alzheimer’s risk, including genetics, physical activity, other antioxidant nutrients, participation in cognitively stimulating activities, alcohol intake, diabetes, hypertension, heart attack and stroke.
Brain autopsies of the 508 participants who died showed less Alzheimer’s disease pathology, including less severe beta-amyloid plaques, among those who consumed the most carotenoids, in particular lutein/zeaxanthin and lycopene.
The brain-protective effect of carotenoids is attributed to their antioxidant and anti-inflammatory properties.
The study’s strengths include its careful control for other potential risk factors for Alzheimer’s disease and repeated measurement of food intake, allowing researchers to capture changes in participants’ diets.
As well, diagnosis of Alzheimer’s disease was based on annual cognitive testing and structured examinations by a clinician.
The main limitation was the observational-study design, which uncovered a strong association, but didn’t prove cause and effect.
How much, which foods?
Study participants who consumed the most carotenoids got 25 mg per day. Major food sources included tomatoes, tomato juice, tomato sauce, carrots, sweet potatoes, squash, kale and spinach.
Outstanding sources of lutein/zeaxanthin are cooked leafy green vegetables such as spinach (15 mg per one-half cup), kale (12.5 mg), Swiss chard (9.5 mg), collards (9 mg) and dandelion greens (5 mg). Other sources of lutein/zeaxanthin include summer and winter squash, pumpkin, green peas, Brussels sprouts and broccoli.
You’ll find plenty of lycopene in tomato juice (22 mg per one cup), tomato soup (26 mg per one cup), tomato sauce (17 mg per one-half cup) and watermelon (7 mg per one cup, diced). Pink grapefruit and guava are other sources.
When it comes to beta-carotene, top picks include carrot juice (11 mg per one-half cup), pumpkin puree (8.5 mg per one-half cup), cooked spinach (7 mg per one-cup), cooked carrots (6.5 mg per one-half cup), cooked collards and kale (each 5.5 mg per one-half cup). Other sources include cantaloupe, papaya, mango and apricots.
Cooking vegetables increases the amount of carotenoids available for the body to absorb. Including some fat in a meal also enhances their absorption (e.g., skip the fat-free salad dressings).
Beyond brain health
There are other reasons to add more carotenoid-rich foods to your diet. A high intake of fruits and vegetables, particularly ones with lots of beta-carotene, is linked to a lower risk of cardiovascular disease.
Eating plenty of foods that contain lutein/zeaxanthin is associated with a lower risk of developing cataracts and macular degeneration. Doing so may also help slow the progression of macular degeneration.
Finally, a higher intake of foods rich in lycopene may help guard against prostate cancer.
Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD