Can a smoothie diet help you lose weight, or are you missing out on valuable nutrition? We explore the facts surrounding this trendy diet.
Eating a variety of fruit and vegetables every day has long been advised by medical professionals, but fitting the contents of a small garden into our daily diets can be a struggle to say the least.
One way to get around this is to simply shove a variety of fruit and vegetables into a blender, give it a whizz and then drink the resulting concoction.
The theory goes that this won't only help you meet your daily quota of fruit and veg, but can also prevent you from over-eating or snacking in-between meals by keeping you full.
But can a smoothie diet really help you lose weight and keep it off in the long term? In this guide, we'll find out.
Does It Work?
Smoothies seem like an obvious solution to the never-ending battle to get enough nutrition and they fit perfectly with the hectic pace of modern life. However, the evidence on these health shakes is far from clear cut.
Pro: Smoothies release more nutrients and make it easier to digest these
It's thought that blending is even better than chewing when it comes to releasing the nutrients contained in the cell walls of fruits and vegetables, as well as enhancing the efficiency of digestion. This claim is backed up by evidence, which does suggest blending can produce smaller particles and more readily-available nutrients.
However, in some cases, this boost is marginal, while nutrients like vitamin C don't seem to be affected by pre-chopping processes at all. Cooking vegetables can also break down the cell walls and enhance the absorption of nutrients, although even this pales in comparison to blending.
Con: You might be starving the bacteria in your gut
Your digestive tract, and particularly your intestines, are home to a diverse community of microorganisms (microbiota) that help you digest food â€“ producing nutrients as they feast on the things you've eaten.
Chewing in a normal way leaves plenty of intact bits of food for your microbiota to gobble up, but evidence suggests breaking down fruits and vegetables through processes like blending leaves a lot less for your friendly gut bacteria.
This could have potentially serious consequences, since your microbiota plays a wide range of roles in maintaining many aspects of your health.
Pro: Smoothie diets bring all the advantages of fruit and veg
Smoothies give you access to all the benefits of a diet high in fruit and veg, without all the inconvenience of actually eating them.
Studies have shown they help improve athletic performance and recovery, boost the amounts of antioxidants in your bloodstream, improve arterial health and much, much more.
Con: Easier digestion is a double-edged sword
While the smaller particles produced by blending can be easier for your body to deal with, this could have an adverse effect when it comes to your blood sugar.
The danger is that by making the contents of foods so easy to absorb, blending will cause blood sugar levels and the corresponding insulin response to spike dangerously.
Having too much sugar in your blood for an extended period can cause a range of problems, especially if you've got a pre-existing condition like diabetes, which makes you even more vulnerable to adverse effects.
Similarly, having high insulin and blood sugar levels can lead the body to store the fat it would usually burn as â€˜fuel' â€“ potentially leading to weight gain.
Pro: Blending the right fruits can actually improve your blood sugar levels
In fact, berries were found to be so potent at helping your body to control blood sugar levels they could largely negate the effects of a glass of sugar water.
Con: Smoothies aren't (always) as filling as solids
The effects of solid versus liquid calories have long been a subject of contention. One famous study compared the impact of soft drinks versus jelly beans and found the group on solids ate less throughout the day.
While this isn't strictly a like-for-like comparison, the definitive study on the subject supported its findings. It measured two groups â€“ one ate a solid fruit salad while the other ingested the meal in liquid form.
Researchers discovered that participants in the smoothie group reported feeling significantly less full than those eating their meal as normal. Other studies suggest this could be due to the act of chewing, which may act as a signalling mechanism to let your brain know you're in the process of eating.
But it's worth noting that participants in a third group, who used a spoon to â€˜eat' the liquid like a soup, reported being nearly exactly as full as those in the normal eating group.
This somewhat confusing finding seems to suggest that the time we take to ingest the food, as well as our perception of it definitely seems to play a role in how full it makes us feel.
Pro: Smoothies make it easier to ingest nutritious parts of fruit and veg you might otherwise discard
Smoothies, especially of the green variety, make it much easier to cram in nutrients found in leafy vegetables and fruits. However, they also let you take advantage of the really nutritious parts of fruit and vegetables that might usually be discarded.
This includes stuff like lemon seeds and peel, which have shown to suppress the growth of cells associated with both breast and colon cancer.
Con: Smoothies are bad for your teeth
Smoothies tend to be highly acidic, which can lead to the erosion of the enamel in your teeth. One study found that the acid erosion in a famous brand of smoothies was comparable to that of Diet Coke.
However, you can potentially minimise exposure by drinking them through a straw, rather than straight from the glass.
Con: A typical â€˜green' smoothie has lots of sugar
In his post for the Centre for Nutrition Studies, on the topic Dr Thomas Campbell expressed concerns about the amount of sugar contained within a typical â€˜green' smoothie, containing spinach, orange juice, strawberries, blueberries and banana:
â€œBy consuming this in the liquid form you may be less satisfied, less full, and subsequently eat more calories during the day than you otherwise would have if you ate those ingredients as solid foods,â€ he said.
â€œAlso consider that by taking those foods as liquids instead of solids you may be significantly changing the immediate blood sugar spike and subsequent blood sugar fall you experience (in a bad way).â€
Dr Campbell went on to recommend just having the occasional smoothie and using your mouth and teeth â€œthe way nature intendedâ€ - by including plenty of healthy greens in your diet.
The Bottom Line
Unlike other diets we've explored, smoothie-based plans can take a range of forms â€“ from replacing a single meal, to entirely supplementing your regular diet. They can also involve a range of ingredients, from mainly fruit-based concoctions to smoothies involving green, leafy vegetables and even seeds.
Because of this, it's hard to evaluate them accurately, however, smoothies can definitely help to supplement your nutrition (provided you're using the right kind of ingredients). If used in the right way, they may also be able to help you lose weight, but unless you're prepared to swap out your regular meals for a cold blend of fruits and vegetables â€“ results aren't likely to be dramatic.
And if you do go the whole hog on a smoothie diet â€“ you shouldn't underestimate how hard it'll be to keep to such a restrictive meal plan and the dangers of â€˜rebound' weight gain once you switch back to regular food.
Sadly, it seems there's only one guaranteed way to lose weight and keep it off â€“ and that's by making a sustained effort to eat healthy, combined with regular exercise.
What's your verdict on smoothie diets â€“ fad or rad? Let us know in the comments, or get in touch on Twitter or Facebook, we always love to hear from you.
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