Given the current situation, many of us are more interested than ever in how food can (and can’t) support our health. To help you sort out fact from fiction, BBC Future is bringing back some of our most popular nutrition stories.
Our colleagues at BBC Good Food are also focusing on practical solutions for ingredient swaps, nutritious storecupboard recipes and all aspects of cooking and eating during lockdown.
It’s hard to imagine now, but there was a time when humans only had access to sugar for a few months a year when fruit was in season. Some 80,000 years ago, hunter-gatherers ate fruit sporadically and infrequently, since they were competing with birds.
Now, our sugar hits come all year round, often with less nutritional value and far more easily – by simply opening a soft drink or cereal box. It doesn’t take an expert to see that our modern sugar intake is less healthy than it was in our foraging days. Today, sugar has become public health enemy number one: governments are taxing it, schools and hospitals are removing it from vending machines and experts are advising that we remove it completely from our diets. It has even been blamed for possibly increasing the risk of contracting infections because it allegedly suppresses the immune system, although in reality the impact it has on our ability to fight off diseases is a great deal more complicated than that.
And so far, scientists have had a difficult time proving how it affects our health, independent of a diet too high in calories. A review of research conducted over the last five years summarised that a diet of more than 150g of fructose per day reduces insulin sensitivity – and therefore increases the risk of developing health problems like high blood pressure and cholesterol levels. But the researchers also concluded that this occurs most often when high sugar intake is combined with excess calories, and that the effects on health are "more likely" due to sugar intake increasing the chance of excess calories, not the impact of sugar alone.
Meanwhile, there is also a growing argument that demonising a single food is dangerous – and causes confusion that risks us cutting out vital foods.
Sugar, otherwise known as "added sugar", includes table sugar, sweeteners, honey and fruit juices, and is extracted, refined and added to food and drink to improve taste.
The term ‘sugar’ includes a broad array of sweeteners, including the fructose naturally found in fruit (Credit: Getty)
But both complex and simple carbohydrates are made up of sugar molecules, which are broken down by digestion into glucose and used by every cell in the body to generate energy and fuel the brain. Complex carbohydrates include wholegrains and vegetables. Simple carbohydrates are more easily digested and quickly release sugar into the bloodstream. They include sugars found naturally in the foods we eat, such as fructose, lactose, sucrose and glucose and others, like high fructose corn syrup, which are manmade.
Before the 16th Century only the rich could afford sugar. But it became more available with colonial trade.
Then, in the 1960s, the development of large-scale conversion of glucose into fructose led to the creation of high fructose corn syrup, a concentrate of glucose and fructose.
This potent combination, above any other single type of sugar, is the one many public health advocates consider the most lethal – and it is the one that many people think of when they think of "sugar".
Consumption of high fructose corn syrup in the US increased tenfold between 1970 and 1990, more than any other food group. Researchers have pointed out that this mirrors the increase in obesity across the country.
Some researchers believe that once high fructose corn syrup became so common, an obesity crisis was only a matter of time (Credit: Getty)
Meanwhile, sugary drinks, which usually use high fructose corn syrup, have been central to research examining the effects of sugar on our health. One meta-analysis of 88 studies found a link between sugary drinks consumption and body weight. In other words, people don’t fully compensate for getting energy from soft drinks by consuming less of other foods – possibly because these drinks increase hunger or decrease satiety.
But the researchers concluded that while the intake of soft drinks and added sugars has increased alongside obesity in the US, the data only represents broad correlations.
And not everyone agrees that high fructose corn syrup is the driving factor in the obesity crisis. Some experts point out that consumption of the sugar has been declining for the past 10 years in countries including the US, even while obesity levels have been rising. There also are epidemics of obesity and diabetes in areas where there is little or no high fructose corn syrup available, such as Australia and Europe.
High fructose corn syrup isn’t the only kind of sugar seen as problematic. Added sugar, particularly fructose, is blamed for a variety of problems.
People who consumed 25% or more of calories as added sugar were more than twice as likely to die from heart disease
For one, it’s said to cause heart disease. When liver cells break down fructose, one of the end products is triglyceride – a form of fat – which can build up in liver cells over time. When it is released into the bloodstream, it can contribute to the growth of fat-filled plaque inside artery walls.
Fructose, which naturally occurs in fruit and is a key component of high-fructose corn syrup, can contribute to the growth of plaque in arteries (Credit: Getty)
One 15-year study seemed to back this up: it found that people who consumed 25% or more of their daily calories as added sugar were more than twice as likely to die from heart disease than those who consumed less than 10%. Type 2 diabetes also is attributed to added sugar intake. Two large studies in the 1990s found that women who consumed more than one soft drink or fruit juice per day were twice as likely to develop diabetes as those who rarely did so.
But again, it’s unclear if that means sugar actually causes heart disease or diabetes. Luc Tappy, professor of physiology at the University of Lausanne, is one of many scientists who argue that the main cause of diabetes, obesity and high blood pressure is excess calorie intake, and that sugar is simply one component of this.
Many scientists believe that sugar alone doesn’t cause obesity – but that, instead, sugar often is part of a diet with too high of a calorie intake in general (Credit: Getty)
“More energy intake than energy expenditure will, in the long term, lead to fat deposition, insulin resistance and a fatty liver, whatever the diet composition,” he says. “In people with a high energy output and a matched energy intake, even a high fructose/sugar diet will be well tolerated.”
Overall, evidence that added sugar directly causes type 2 diabetes, heart disease, obesity or cancer is thin
Tappy points out that athletes, for example, often have higher sugar consumption but lower rates of cardiovascular disease: high fructose intake can be metabolised during exercise to increase performance.
Athletes often consume more sugar than other people, but because they metabolise it during exercise, they still have lower rates of cardiovascular disease (Credit: Getty)
Overall, evidence that added sugar directly causes type 2 diabetes, heart disease, obesity or cancer is thin. Yes, higher intakes are associated with these conditions. But clinical trials have yet to establish that it causes them.
Sugar also has been associated with addiction… but this finding, too, may not be what it seems. A review published in the British Journal of Sports Medicine in 2017 cited findings that mice can experience sugar withdrawal and argued that sugar produces similar effects to cocaine, such as craving. But the paper was widely accused of misinterpreting the evidence. One key criticism was that the animals were restricted to having sugar for two hours a day: if you allow them to have it whenever they want it, which reflects how we consume it, they don’t show addiction-like behaviours.
Still, studies have demonstrated other ways in which sugar affects our brains. Matthew Pase, research fellow at Swinburne’s Centre for Human Psychopharmacology in Australia, examined the association between self-reported sugary beverage consumption and markers of brain health determined by MRI scans. Those who drank soft drinks and fruit juices more frequently displayed smaller average brain volumes and poorer memory function. Consuming two sugary drinks per day aged the brain two years compared to those who didn’t drink any at all. But Pase explains that since he only measured fruit juice intake, he can’t be sure that sugar alone is what affects brain health.
One study found that consuming two juice drinks per day aged the brain by two years, compared to not drinking any (Credit: Getty)
“People who drink more fruit juice or soft drinks may share other dietary or lifestyle habits that relate to brain health. For example, they may also exercise less,” Pase says.
One recent study found that consuming sugar can make older people more motivated to perform difficult tasks
One recent study found that sugar may even help improve memory and performance in older adults. Researchers gave participants a drink containing a small amount of glucose and asked them to perform various memory tasks. Other participants were given a drink containing artificial sweetener as a control. They measured the participants' levels of engagement, their memory score, and their own perception of how much effort they’d applied.
The results suggested that consuming sugar can make older people more motivated to perform difficult tasks at full capacity – without them feeling as if they tried harder. Increased blood sugar levels also made them feel happier during the task.
In older adults, consuming a sugary drink may make them more motivated and even happier during a task (Credit: Getty)
Younger adults showed increased energy after consuming the glucose drink, but it didn’t affect their mood or memory.
Teaspoon of sugar
While current guidelines advise that added sugars shouldn't make up more than 5% of our daily calorie intake, dietitian Renee McGregor says it’s important to understand that a healthy, balanced diet is different for everyone.
“I work with athletes who need to take on more sugar when doing a hard session because it’s easily digestible. But they worry they’re going over the guidelines,” she says.
For most of us non-athletes, it’s true that added sugar isn’t crucial for a healthy diet. But some experts warn we shouldn’t single it out as toxic.
McGregor, whose clients include those with orthorexia, a fixation with eating healthily, says that it isn’t healthy to label foods as "good" or "bad". And turning sugar into a taboo may only make it more tempting. “As soon as you say you can’t have something, you want it,” she says. “That’s why I never say anything is off-limits. I’ll say a food has no nutritional value. But sometimes foods have other values.”
Even when sweet foods have little or no nutritional value, they have other values (Credit: Getty)
Associate professor at James Madison University Alan Levinovitz studies the relationship between religion and science. He says there’s a simple reason we look at sugar as evil: throughout history, we’ve demonised the things we find hardest to resist (think of sexual pleasure in the Victorian times).
Today, we do this with sugar to gain control over cravings.
Sugar is intensely pleasurable, so we have to see it as a cardinal sin – Alan Levinovitz
“Sugar is intensely pleasurable, so we have to see it as a cardinal sin. When we see things in simple good and evil binaries, it becomes unthinkable that this evil thing can exist in moderation. This is happening with sugar,” he says.
He argues that that seeing food in such extremes can make us anxious about what we’re eating – and add a moral judgment onto something as necessary, and as everyday, as deciding what to eat.
We demonise the things we find difficult to resist – including sugar (Credit: Getty)
Taking sugar out of our diets can even be counterproductive: it can mean replacing it with something potentially more calorific, such as if you substitute a fat for a sugar in a recipe.
And amid the rising debate around sugar, we risk confusing those foods and drinks with added sugar that lack other essential nutrients, like soft drinks, with healthy foods that have sugars, like fruit.
One person who struggled with this distinction is 28-year-old Tina Grundin of Sweden, who says she used to think all sugars were unhealthy. She pursued a high-protein, high-fat vegan diet, which she says led to an undiagnosed eating disorder.
“When I started throwing up after eating, I knew I couldn’t go on much longer. I’d grown up fearing sugar in all forms,” she says. “Then I realised there was a difference between added sugar and sugar as a carbohydrate and I adopted a high-fructose, high-starch diet with natural sugars found in fruit, vegetables, starches and legumes.
“From the first day, it was like the fog lifted and I could see clearly. I finally gave my cells fuel, found in glucose, from carbohydrates, from sugars.”
While there’s disagreement around how different types of sugars affect our health, the irony is we might be better off thinking about it less.
“We’ve really overcomplicated nutrition because, fundamentally, what everyone is searching for is a need to feel complete, to feel perfect and successful,” says McGregor. “But that doesn’t exist.”
Correction: An earlier version of this story referred to glucose and sucrose as being manmade. This has been changed. An earlier version of the story also said that high fructose corn syrup doesn't lead to a rise in leptin; however, as most related research has been done on fructose, not high fructose corn syrup, this has been changed. Sugar's effect on insulin and its relation also has been clarified.
If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Culture, Capital, and Travel, delivered to your inbox every Friday.