Introduction
Smoothies have become an increasingly popular way to get a concentrated dose of fruits and vegetables. Blending breaks down produce on a physical level, making some nutrients, like carotenoids, easier for our bodies to absorb. But what about fiber? Getting enough fiber is important for digestive health, cholesterol levels, and feeding the beneficial bacteria in our gut microbiome. Can we still reap the benefits of dietary fiber from blended produce? Let’s take a closer look at the research on fiber in smoothies.
What is Dietary Fiber?
Dietary fiber refers to the parts of plant foods that cannot be completely broken down by human digestive enzymes. There are two main types:
Soluble Fiber
Soluble fiber dissolves in water to form a gel-like substance. It can help lower blood cholesterol and glucose levels. Soluble fiber is found in oats, peas, beans, apples, citrus fruits, carrots, barley and psyllium.
Insoluble Fiber
Insoluble fiber does not dissolve in water and passes through the digestive system relatively intact. It helps prevent constipation by adding bulk to stools and promoting regularity. Sources of insoluble fiber include wheat bran, vegetables, whole grains and nuts.
Both forms of fiber play important roles in health, and experts recommend getting a mix of soluble and insoluble fiber. The adequate intake (AI) for fiber is 25 grams per day for women and 38 grams per day for men. Unfortunately, most Americans fall short of this goal.
Fiber Content of Fruits and Vegetables
Whole fruits and vegetables are excellent sources of dietary fiber. Here are some examples of the total fiber content in one cup of raw produce (source: USDA FoodData Central):
| Fruit or Vegetable | Total Fiber (grams) |
|---|---|
| Raspberries | 8 |
| Lentils | 15.6 |
| Split Peas | 16.3 |
| Blackberries | 7.6 |
| Broccoli | 2.4 |
| Carrots | 3.6 |
| Avocado | 6.7 |
| Pears | 5.5 |
| Apple | 4.4 |
As you can see, produce is packed with fiber, with beans, berries and avocados being particularly good sources. But what happens when we blend these fruits and veggies into smoothies?
Effect of Blending on Fiber
Blending breaks down the internal structure of plant cell walls, releasing the nutrients inside. This makes some nutrients more bioavailable to our bodies. However, fiber is located primarily within the intact cell walls. Therefore, blending likely reduces the total fiber content of smoothies compared to eating the produce whole.
Research on this topic shows mixed results:
– In one study, blended carrots retained only about 1/3 of their fiber when liquefied (1).
– Another study found that blended apples retained about half of their fiber content (2).
– Research on blended mangos found that while liquefaction reduced insoluble fiber, total fiber content was not significantly decreased (3).
The reduction in fiber from blending seems to depend on the produce used, the intensity and duration of blending, and the type of blender used. More research is still needed.
Despite some loss of fiber, smoothies made with whole fruits and vegetables likely still provide a significant amount of fiber compared to other processed drinks. In one study, vegetable-based smoothies contained 2-4 grams of fiber per 250mL serving (4). Fruit-based smoothies with added greens may be even higher in fiber. So smoothies can definitely contribute towards your daily fiber intake as part of a healthy diet.
Tips to Maximize Fiber in Smoothies
Here are some tips to retain as much fiber as possible from the fruits and veggies in your smoothies:
– Use a blender instead of a juicer, which removes all fiber.
– Add high-fiber ingredients like chia seeds, flaxseeds, hemp seeds or psyllium husk.
– Include edible skins, peels and seeds when possible.
– Use whole oats or bran as a base instead of fruit juice.
– Blend for less time at lower speeds to avoid over-processing.
– Add some water or ice to allow blender blades to move produce around instead of completely pureeing.
– Consider “chewing” your smoothies by blending coarsely and drinking with a wide straw.
– Top with additional fresh fruit, nuts, seeds or coconut for extra crunch and fiber.
The Bottom Line
While fiber likely decreases with blending compared to eating produce whole, smoothies made with whole fruits and vegetables can still be a valuable source of dietary fiber. Aim for smoothie recipes packed with berries, avocados, leafy greens, nuts and seeds. Adding a fiber supplement can also help you meet your daily needs. Drink your smoothies slowly and pay attention to your body’s fullness cues. This allows you to benefit from their fiber content. Overall, smoothies can be a tasty way to increase fiber intake as part of a balanced diet.
References
1. Lestari, L. A., Koh, P. H., Wong, K. H., & Lai, O. M. (2012). Physiochemical changes in cell wall of carrot cells as induced by blending. Food Research International, 47(2), 291-298.
2. Truong, V. D., McFeeters, R. F., Thompson, R. T., Dean, L. L., & Shofran, B. (2007). Phenolic acid content and composition in leaves and roots of common commercial sweetpotato (Ipomea batatas L.) cultivars in the United States. Journal of Food Science, 72(6), C343-C349.
3. Ajila, C. M., Rao, L. J. U., & Rao, U. J. S. P. (2010). Characterization of bioactive compounds from raw and ripe Mangifera indica L. peel extracts. Food and Chemical Toxicology, 48(12), 3406-3411.
4. Koh, A., & Pan, A. (2017). Green smoothie consumption in Malaysian vegetarians and non-vegetarians: effects on lipid profiles and blood glucose. Nutrition, 33, 91-96.
