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4 reasons to be cautious of smoothie diets

11 April, 2019
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Kale, spinach and pear; blueberry, raspberry and blackberry. At first glance, these two popular meal replacement smoothies are everything the doctor ordered.

This healthy image, combined with the fact that (unlike many weight loss foods) they actually taste good, is a key reason for the modern popularity of smoothie diets.

However, knowing your facts can help you decide whether these dietary drinks are really as healthy as they look. Let’s take a look at the premise behind smoothie diets, and then explore some potential red flags.

What is the smoothie diet?

Smoothie diets typically involve replacing most, or even all, meals with fruit and vegetable based drinks. This allows dieters greater control over the pantomime villain of the healthy eating world: calories.

Calories are a measure of the energy content in food and drink, and are also known as kcals. However they’re referred to, we need them - in fact, according to Healthdirect Australia, the average adult requires roughly 2,080 calories a day.

There are several iterations of the smoothie diet. One of the more common options is drinking them in the place of standard meals as a short-term measure. Due to reductions in, notably, carbohydrates and fat, participants can see calorie intake drop to as little as 1,200 per day - resulting in fast weight loss.

Sounds good, right? Well, there are several side-effects of smoothie diets that could lead to health problems.

Potential problems with the smoothie diet

  1. Nutritional deficiencies:

We know what you’re thinking: “Nutritional deficiencies...with a green smoothie? I can’t win”. Firstly, to be clear, a short-term smoothie diet isn’t going to leave you seriously lacking key nutrients.

However, despite what your Gran may have said, you can’t get everything you need from your five-a-day. Over extended periods, smoothie diets can potentially lead to deficiencies in:

  • B12 and vitamin D: These nutrients are most commonly obtained from meat or dairy products, so over the longer-term, a smoothie diet may not provide the recommended daily intakes. As with the other items in this list, this problem can be encountered by vegans, and sometimes vegetarians.
  • Protein: The majority of protein sources are animal based. Lack of protein on a long term basis has been shown to lead to symptoms such as anaemia.
  • Zinc: While you can get zinc from lentils, beans and seeds, humans access it most commonly through meat and shellfish - not common components of a smoothie diet.
  1. Kidney stones

Kidney stones are painful, and potentially dangerous, solid deposits formed in the kidneys - and as many as 80 per cent are made up of calcium-oxalate compounds.

What does this have to do with smoothies? Well, the ‘oxalate’ part of calcium-oxalate refers to an antinutrient that’s found in high concentrations in favourite smoothie ingredients such as spinach, berries and broccoli.

As a waste product, most oxalates leave the body in stool or urine. However, a diet too high in oxalate combined with other risk factors can cause the waste to bind to calcium in the kidney and form crystalline masses.

  1. Too much acid

While fruit is low in carbs and fat, it still contains significant quantities of naturally-occuring acids.

Among the many problems that too much acid can cause is tooth enamel erosion. Indeed, scientific research has shown that certain fruit smoothies caused significant damage to enamel. The worst culprit in this study was the delicious, but highly acidic, mango and passionfruit smoothie.

  1. Thyroid problems

Glucosinolates, commonly found in green smoothie ingredients such as kale, cabbage and sprouts, have been shown by some studies to hinder the ability of the thyroid to absorb iodine.

Iodine is essential to our thyroid gland’s ability to produce hormones. Among other functions, these hormones control our metabolism, and contribute to brain and bone development in infancy.

So, there you have it. The smoothie diet may for some be an effective way of losing weight in the short term - but these drinks perhaps aren’t as innocent as they first appear. As with any major dietary change, if you’re considering trying a smoothie diet, we recommend talking it over with your GP first.


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