Are 'superfoods' really that good for you? From kale to coconut water

I struggle with the term "superfood". The fact is, for all of our perceived health awareness, we simply don't know that much about our food, and we certainly don't know enough to say exactly what makes a true "superfood".

Whole foods have literally hundreds, even thousands, of active compounds, including phenolics, flavonoids, pigments, antioxidants, fatty acids, protein, vitamins, minerals, fibre… the list goes on and on.

As a dietician, I decided to turn a critical eye to all the hype that surrounds so many so-called superfoods. Is there good reason for us to jump off the deep end every time we come across an obscure Himalayan berry? What about common foods that are affordable and accessible? In my book, Unmasking Superfoods, I look at which of these foods we should approach with caution and which have more to offer than their reputation suggests.

Açaí

Açaí is a classic example of the "Dr Oz effect". The US TV medic mentioned the juice of the South American berry on The Oprah Winfrey Show back in 2008, and within the year it reportedly had revenues of $104m (about £66m) in the US. It was claimed that the açaí juice could help prevent cancer and muscle and joint fatigue, as well as promoting weight loss and anti-ageing.

Soon, it was being sold for about $40 (£25.50) a bottle – the same amount of money you could feed a family of four fresh produce for a week on – and it wasn't long before UK consumers bought in to the craze.

The nutrition profile of açaí is unremarkable. It isn't a great source of any major nutrients. A glass of açaí juice will give you six per cent of your daily vitamin A needs, where a carrot will give you a day's worth. It provides 75mg of potassium, the same as you'd get in a bite of banana.

The bottom line: there is nothing wrong with açaí, but it doesn't live up to the hype. There is almost no research on this so-called superfood, and what there is is industry sponsored. While it may be better than a glass of cranberry, orange or apple juice, for antioxidant value you'd be better off with a glass of red wine.

Like açaí, goji berries benefited from being praised by Dr Oz, who described it as "the most potent antioxidant fruit that we know". The only data we have on goji berries comes from those who sell them, who claim that a 28g serving of dried berries contain 140 per cent of our daily vitamin A requirements. This, however, is the same as other orange and red fruits that contain fewer calories, such as carrots and oranges. You'd need to eat 500 calories worth of goji berries to get the vitamin C you'd find in an orange.

The bottom line: like açaí, the benefits of the goji berry are backed by industry-sponsored research and large-scale internet hype.

Coconut water

I'm ambivalent about coconut water. It is lower in sugar and calories than traditional soft drinks and other juices, and has unusually high levels of fibre for a soft drink. Its high levels of potassium, good for blood pressure control, has made it a sports drink sensation – it contains almost as much potassium as a potato or a sweet potato, and more than a banana.

The bottom line: there is some evidence to support coconut water as a sports drink, but for those not doing sport, it's little more than a sugar drink with extra calories.

Quinoa

Quinoa is a real sweetheart in the health-food industry. If oats and broccoli were the original superfoods, quinoa is their love child. Nutritionally, it seems to have much to offer: 200g of cooked quinoa provides 222 calories, 5g of fibre, as much protein as a cup of milk (8g), and more than 10 per cent of your daily vitamin B needs, as well as unusually high iron and zinc levels for a plant food.

It is an impressive grain but there is very little evidence to support the wider claims that it can prevent or manage medical conditions such as diabetes, heart disease or coeliac disease.

The bottom line: quinoa is richly nutritious and poorly understood, but its popularity could come with social and ecological consequences: the vast increase in demand has raised concerns for Bolivian farmers.

Almonds

We have the evidence to show that almonds, which have been on the list of superfoods for as long as the term has been used, can improve heart health and diabetes management.

All nuts are high-calorie foods, but 28g of almonds, 23 kernels, has fewer calories than was once thought – 129 instead of 162 calories. This is thought to be due to the incomplete digestion of some nuts and seeds, which end up literally flushed down the toilet.

Without changing anything else in their diets, people who eat almonds tend to lose weight, which is pretty exciting for a high-calorie and high-fat food, and they are one of the richest sources of vitamin E, which is one of the most difficult nutrients to obtain in your diet.

The bottom line: almonds are at the top of the superfood chain. As well as the good evidence that they can help control cholesterol, blood sugar and reduce inflammation, they do this without causing unwanted weight gain.

Avocados

Avocados are so insanely nutritious, I almost think it's unfair to other foods. Three quarters of an avocado, 150g, provides 40 per cent of the daily fibre needs for an average female, 25 per cent of your vitamin C, 16 per cent of the hard-to-get antioxidant vitamin E and 39 per cent of vitamin K requirements, as well as 10 per cent of B vitamins and 30 per cent of folic acid, making it get a great choice for women who want to conceive. It also contains magnesium, copper and potassium.

Avocados are full of Mufas – monounsaturated "good" fats, the foundation of the healthy Mediterranean diet. So they're good for controlling cholesterol and helping with diabetes, and can even act as a natural sunscreen, though more research is needed in this area.

The bottom line: when it comes to superfoods, avocados seem to be the real deal, and they won't cause your waistline to expand if you eat them sensibly.

Kale

There has been a huge amount of hype around kale and its nutrition profile is almost absurd. A hundred grams of uncooked kale contains just 33 calories, but 200 per cent of daily vitamin A requirements, 134 per cent of vitamin C and an incredible 700 per cent of vitamin K. We need vitamin K for bone health and blood clotting (although it may counteract some blood thinning medication) and if you eat it with some fat, such as olive oil, avocado, nuts or seeds, your body will absorb more of the vitamin K.

The bottom line: kale is so nutrient-dense, it's almost like a medication, which means eating it a few times a week is enough. It's hard to argue that kale doesn't deserve a spot at, or near, the top of any superfood list.

Oysters and mussels

Salmon is so popular, and I don't want to put it down, but it's worth looking at other seafoods for their nutritional value. Oysters may be seen as a delicacy, but they have high levels of "good fats" – low compared with salmon, but then eight oysters contain 67 calories compared with the 250 in a salmon fillet. They provide 250 per cent of your daily zinc needs, important for testosterone and therefore libido. Oysters are also the food richest in vitamin B12, important for the development of healthy red blood cells, and which can be difficult to get as we age.

Mussels aren't as high in zinc but contain impressive amounts of selenium (important for cognitive function); 80g provides 109 per cent of your daily requirements. They are also a source of almost every major and minor nutrient the human body needs.

Diets rich in shellfish were thought to have negative effects on heart health because of their relatively high cholesterol content, but studies now show that shellfish actually seem to have heart health benefits.

The bottom line: both fish and shellfish are wildly nutritious, but the depletion of fish stocks is a serious ecological issue the world over, so it's reassuring that oysters and mussels can be good for us and for the planet.

'Unmasking Superfoods: The Truth and Hype About Açaí, Quinoa, Chia, Blueberries and More' by Jennifer Sygo (Harper Collins, £12.99)