Nuts and berries are widely recognized as integral components of a healthy eating pattern. These foods boast a complex nutrient profile, encompassing essential vitamins, minerals, fiber, polyunsaturated fatty acids, and phenolics, all of which contribute to improved physiological outcomes. The health benefits associated with nut and berry consumption span a wide spectrum, including cardiovascular, gut microbiome, and cognitive functions. Recent insights into the bioactive compounds present in both nuts and berries have further solidified their role in precision nutrition strategies.
The Value of Nuts and Berries
Tree nuts and berries stand out due to their unique composition, bioactivity, and numerous health-promoting qualities.
Regular consumption of tree nuts and berries is linked to a reduced risk of cardiovascular disease and positive effects on brain and gut health. Tree nuts offer protein, fiber, monounsaturated and polyunsaturated fatty acids, vitamins, minerals, and bioactive compounds like carotenoids, phytosterols, phenolics, flavonoids, lignans, and tannins. Berries are rich in fiber, vitamin C, bioactive carotenoids, and phenolics, including proanthocyanins, ellagitannins, and anthocyanins, which give berries their vibrant colors.
Economic Significance
Tree nuts and berries are significant commodities in the United States. In California alone, the total value of tree nuts grown in 2021 was estimated at $8.961 billion, with almonds accounting for $5.028 billion, pistachios for $2.911 billion, and walnuts for $1.022 billion. The total value of berries grown in California in 2021 was approximately $3.667 billion, with strawberries contributing $3.023 billion, raspberries $420.7 million, and blueberries $223.5 million.
Navigating Research Challenges
Despite promising research results, the precise mechanisms of action, particularly for vascular and gut health, remain to be fully elucidated. There is a need for more data that can be generalized to diverse population groups, as well as for the development of precision nutrition recommendations.
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Many different study designs have been employed to assess the effects of nuts and berries on cardiometabolic health.
Study Design Considerations
Several factors can affect the generalizability of outcomes from clinical studies, including inconsistency in study designs, homogeneity of test populations, variability in test products and control foods, and assessing realistic portion sizes.
One common study design for whole foods involves replacing the test food with a nutritionally matched, isocaloric substitute. However, matching nutritional content can be difficult because food processing, such as blending berries and roasting nuts, can disrupt the nutrient matrix, potentially altering the bioavailability of key nutrients. For nuts, controls often involve the complete omission of the nut of interest.
For berry research, alternatives to consuming the whole food include using freeze-dried berry powders as the test product, controlled with an isocaloric powder either lower or devoid of potential bioactives. While attempts have been made to mask the control powders, issues such as product color, texture, scent, and mouth feel are challenging to completely match, creating both performance and detection bias. Freeze-dried berry powders can also have a different food matrix compared to the whole food, which could influence outcome measures and limit generalizability to the whole fruit.
Another approach for berry research is the encapsulation of test and control powders, which can aid in participant masking. However, the total amount of test product provided can be limiting, and large intakes of control gelatin capsules have resulted in adverse effects. A third option involves examining two or more intake levels, with or without a true control group. Finally, the use of macro- and micronutrient matched gummies with similar amounts of calories, sugars, and fiber, but devoid of other bioactives, is a novel option for use as a comparative control. In all of these approaches, the potential bioactivity of the control itself must be considered.
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Cultivar Variability
Multiple cultivars of berries exist, some of which have differences in the content of bioactive ingredients, thus limiting comparison and extrapolation of results. For nuts, walnuts contain a variety of phenolic acids, catechins, and flavonoids, most of which have been reported to possess bioactivity. Significant differences in the concentration of 16 phenolic compounds were identified when comparing black and English walnuts. More than 50 cultivars of strawberries exist in the United States.
To help reduce the potential experimental variability created with the use of different cultivars, the California Strawberry Commission has produced a freeze-dried test material that utilizes a composite of genotypes to produce a powder that is characterized for its macro- and micronutrients and bioactive components. The US Highbush Blueberry Council also provides a powder that is a 50/50 mixture of two cultivars. A limitation of this approach is that the standardized mixture may contain varieties with reduced or low bioactivity.
In addition to cultivar differences, factors such as climate and seasonal differences due to heat, sunlight, and rainfall can contribute additional variability. Given the above, the characterization of bioactives within these foods is critical. New analytical equipment and techniques have increased the precision of food composition compared to analyses performed decades ago.
Population and Dietary Considerations
Free-living populations have differences in background diets that can influence their responses to the intake of test foods, potentially creating significant variation in baseline measurements. This variability presents a challenge when elucidating clinically relevant effects, especially if unknown a priori, where statistical significance can be masked by combining and analyzing groups together. Interindividual variability may be mitigated by increasing sample size as well as using a crossover design, but challenges in recruitment, retention, and budget constraints exist.
One way to help minimize experimental variability is through a run-in period to identify participants who may be differentially metabolizing bioactive phenolics or with the goal of minimizing or removing potentially confounding metabolites from circulation prior to the intervention. However, study designs that employ highly controlled settings, strict inclusion and exclusion criteria, extended washout periods that alter background diets, and ask participants to follow an atypical consumption pattern does not reflect “normal” life and may have limited applicability to the general population.
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Another useful model that also has limitations is the provision of nuts or berries in amounts and duration that are greater than normally consumed. Feeding relatively high amounts of nuts or berries for a limited period of time has been employed to demonstrate proof-of-concept and provide a basis for further exploration for changes in physiology, cognitive performance, and gut microbiome profiles. Subsequent study designs must be realistic, guided by the USDA FoodCentral database for portion size. These trial designs should also use a duration that is realistically achievable by consumers, whose food purchasing behavior can be influenced by cost, access, and seasonal availability of the food.
Studies using average daily portion sizes typically require intervention periods of months, which present challenges regarding participant compliance and retention and cost of the study. In a review of 231 reports on berries and health, approximately 70% of studies used interventions of less than 3 months or contained less than 50 participants.
The health and functional levels of participants are other factors that influence study designs and outcomes. For example, studies on cognitive performance with both nuts and berries have assessed effects among those both with and without cognitive impairments. In such studies, short-term interventions may show little or no response after the addition of nuts or berries to the diet. Although the net change may not be statistically significant, this model does not address the ability of the food to prevent decline, which would require long-term testing. Further, an individual with cognitive impairments might demonstrate favorable responses compared to baseline measures following nut or berry intake but may still not reach the level of performance of a healthy individual.
Dietary interventions require the incorporation of foods into an individual’s eating pattern, which may present a number of challenges. One is the creation of boredom with eating the same food on a regular basis. Second is that the caloric load of the test nut or berry may displace the intake of other nutrient-dense foods. These factors may make compliance for the entire study duration an issue, particularly if the intervention is weeks or months in duration. A third challenge involves compliance. In berry research studies, compliance is often not reported, or the reported range of intake is so variable that it is hard to discern the significance of the results.
In addition to compliance, dietary patterns are an important consideration needed for the interpretation of results because individuals do not eat a single food in the absence of other foods. Background or habitual intake is often not addressed in nutritional trials. The potential variability in habitual dietary intake of participants is often a confounding factor in nutrition research. Dietary assessment methods, with 24-h recalls, 3-d food records, and food frequency questionnaires, all have limitations. These subjective measures may also not accurately capture the potential for nutrient-nutrient interactions that may alter polyphenolic or other bioactive components attributed to nut and berry consumption. Further complicating this issue is the observation that study designs utilizing longer-term interventions or that require the intake of a large amount of the test food are more likely to result in overreporting food intake due to fear that participants may be dismissed from the intervention. Innovations in dietary assessment methodology using “smart” eyeglasses or other image-based technologies have been proposed to address this issue.
Assessing the relationship between the intake of nutrients and bioactives from a whole food product to physiologic responses is difficult, as a multitude of processes are affected, including regulation of vascular function, provision of oxidant defense, and changes in gut microbiome profiles and subsequent output of secondary metabolites. Additionally, bioactives from nuts and berries can interact with each other as well as other dietary components to alter bioavailability and health-promoting properties. Results could also be confounded by dietary changes made by participants in addition to incorporation of the test nut or berry.
Habitual dietary intake is often measured through food frequency questionnaires or repeated 24-h dietary recalls. However, these subjective measures may not accurately capture the potential for nutrient-nutrient interactions that may alter polyphenolic or other bioactive components attributed to nut and berry consumption.
Expanding the scope of populations to be studied is another key area for future research. Most clinical trials using nuts and berries have been conducted in middle-aged or older Caucasian adults with one or more cardiometabolic risk factors.
Specific Health Benefits of Berries
Berries are not only delicious and nutritious but also offer a wide array of impressive health benefits.
Antioxidant Powerhouse
Berries are packed with antioxidants, which help keep free radicals under control. Free radicals are unstable molecules that, in excess, can damage cells and cause oxidative stress. Berries are a great source of antioxidants like anthocyanins, ellagic acid, and resveratrol, which may reduce disease risk. Studies have shown that blueberries, blackberries, and raspberries have some of the highest antioxidant activity among commonly consumed fruits, second only to pomegranates. These antioxidants may help reduce oxidative stress.
Blood Sugar and Insulin Regulation
Berries may improve blood sugar and insulin levels. Research suggests they can protect cells from high blood sugar levels, increase insulin sensitivity, and reduce blood sugar and insulin response to high-carb meals. These effects have been observed in both healthy individuals and those with insulin resistance.
High in Fiber
Berries are a good source of fiber, including soluble fiber. Soluble fiber slows down the movement of food through the digestive tract, leading to reduced hunger and increased feelings of fullness, which can aid in weight management. Fiber also helps reduce the number of calories absorbed from mixed meals. The high fiber content of berries also means they are low in digestible or net carbs.
Nutrient-Rich
Berries are low in calories and extremely nutritious, containing several vitamins and minerals in addition to antioxidants. Strawberries, in particular, are high in vitamin C, with just 1 cup providing 150% of the Reference Daily Intake (RDI).
Anti-Inflammatory Properties
The antioxidants in berries may help lower inflammatory markers, potentially reducing the risk of conditions like diabetes, heart disease, and obesity, which are associated with chronic inflammation.
Cholesterol Management
Berries can be a heart-healthy addition to your diet. Black raspberries and strawberries have been shown to help lower cholesterol in people who are obese or have metabolic syndrome. Berries may also help prevent LDL cholesterol from becoming oxidized or damaged, a major risk factor for heart disease.
Skin Health
Berries may help reduce skin wrinkling, as their antioxidants help control free radicals, one of the leading causes of skin damage that contributes to aging. Ellagic acid, in particular, may protect skin by blocking the production of enzymes that break down collagen in sun-damaged skin.
Potential Cancer Protection
Several antioxidants in berries, including anthocyanins, ellagic acid, and resveratrol, may reduce cancer risk. Studies suggest that berries may protect against cancer of the esophagus, mouth, breast, and colon.
Diet-Friendly
Berries can be incorporated into many types of diets, including low-carb, ketogenic, paleo, Mediterranean, vegetarian, and vegan diets. Their low calorie count makes them ideal for those looking to lose weight.
Arterial Health
In addition to lowering cholesterol, berries provide other benefits for heart health, including improving the function of arteries. They have been found to improve endothelial function in healthy adults, individuals with metabolic syndrome, and people who smoke.
Culinary Versatility
Berries are delicious on their own or in healthy recipes. They can be enjoyed as a snack or dessert, added to breakfast dishes, or incorporated into salads.
Cognitive Benefits of Nuts and Berries
Eating more berries and nuts has been linked to reduced cognitive decline in the elderly. Observational studies, such as the Harvard Nurses’ Health Study, have shown that berry intake may delay cognitive aging by up to two and a half years, while nut intake may delay brain aging by two years.
Interventional studies have also demonstrated the cognitive benefits of nuts and berries. For example, a study on the effects of walnut consumption on cognitive performance found that college students who ate walnuts for two months showed a significant improvement in inference capacity, the ability to accurately draw conclusions from a set of facts. Another study found that people who consumed a smoothie made with various berries performed better on short-term memory tests. Similarly, individuals who consumed Concord grape juice showed improved performance on everyday tasks, such as quicker response times in driving tests.
The Role of Vegetables
In addition to fruits and nuts, vegetables also play a role in cognitive health. Consumers of cruciferous vegetables (cabbage, cauliflower, broccoli, and Brussels sprouts) performed better in several cognitive tests than non-users. Women who consumed the most leafy greens experienced slower brain aging.
As we age, our cerebral blood flow decreases, which may be due to an age-related decrease in the production of nitric oxide (NO). Nitrate-rich vegetables, such as leafy greens and beets, can improve physiological performance and cognitive performance by boosting nitric oxide production and improving blood flow to the brain.
The Benefits of Nuts
Nuts are nutrient-dense foods with complex matrices rich in unsaturated fatty acids and other bioactive compounds, such as L-arginine, fiber, minerals, tocopherols, phytosterols, and polyphenols.
Epidemiologic studies have associated nut consumption with a reduced incidence of coronary heart disease in both genders and diabetes in women. Limited evidence also suggests beneficial effects on hypertension and inflammation. Interventional studies consistently show that nut intake has a cholesterol-lowering effect, and there is emerging evidence of beneficial effects on oxidative stress, inflammation, and vascular reactivity. Blood pressure, visceral adiposity, and glycemic control also appear to be positively influenced by frequent nut consumption without evidence of undue weight gain.
Nutritional Value of Nuts and Seeds
Nuts and seeds are high in protein, fiber, and monounsaturated and polyunsaturated fats (including omega-3 and omega-6 fatty acids). They are also rich in vitamins and minerals and a range of phytochemicals, such as polyphenolic compounds (e.g., phenolic acids, flavonoids), phytosterols, and carotenoids. The amounts and types of nutrients vary between different types of nuts and seeds.
Including a variety of nuts and seeds in your diet can be an excellent way to boost your plant-based protein intake. Nuts and seeds are also high in dietary fiber, which gets fermented by our gut microbiota, producing short-chain fatty acids that protect our health by improving insulin sensitivity and ensuring lower levels of glucose and lipids in our bloodstream. Many nuts and seeds are good sources of iron, although the type of iron found in plants and plant-based foods is mainly non-haem iron, which our bodies can absorb less effectively than animal-based iron. Some nuts and seeds are high in essential omega-3 fatty acids, including walnuts, flaxseeds, chia seeds, hemp seeds, and rapeseeds.
Health Benefits of Nuts and Seeds
There is growing evidence that eating nuts and seeds regularly as part of a healthy diet and lifestyle may help to reduce the risk of many common diseases. A low intake of nuts and seeds (below 21 g/day) is one of the major leading dietary risk factors for the number of years lost due to ill-health, disability, or early death in Europe.
Nuts and seeds reduce several CVD risk factors such as blood lipids (e.g., total cholesterol, LDL cholesterol, triglycerides, and Apolipoprotein B), blood pressure, inflammation, and markers of glycemic control. They may also be beneficial for weight management. The European Food Safety Authority (EFSA) has also authorized a health claim for walnuts, stating that they contribute to the improvement of the elasticity of blood vessels. Diets rich in fiber, found in foods such as nuts and seeds, help to lower the risk of developing bowel cancer. Diets high in nuts and seeds (~28 g/d) are also associated with a lower cancer-related mortality.
Nuts and seeds have been found to slightly reduce body weight, body mass index (BMI), and waist circumference. Weight gain is the most important risk factor for type 2 diabetes. Analyses that did not consider BMI found that eating nuts and seeds was associated with a lower risk of diabetes. This implies that the positive effect of eating nuts on diabetes risk might be partially explained by its potential role in reducing body weight.
Recommendations for Nut and Seed Consumption
Almost all European food-based dietary guidelines (FBDGs) recommend including nuts and seeds in your diet, ranging from a daily intake to eating nuts and seeds one to two times per week. Most recommend a daily intake of approximately 30 g/day, which is equal to a small handful. When choosing nuts and seeds, opt for unsalted and unsweetened varieties. Nut/seed butter (e.g., peanut butter, almond butter, cashew butter, etc.) also counts towards your daily consumption of nuts and seeds as long as they contain no added salt, sugar, or oils.