If you want to wage a war, you have to have an enemy. By almost any measure—scientific reports, documentaries, government announcements, and nearly thirty million Google results—we have been waging a War on Obesity for many years. It is not going well. The most comprehensive recent data, from the 2012 National Health and Nutrition Examination Survey, found that sixty-eight million Americans qualified as obese, and sixty-five million more were overweight. Smaller but more recent studies show no easing of these numbers, which clearly amount to a public-health crisis.

In most wars, the enemy stays the enemy. In this one, however, our allies and demons keep swapping places. When I was young, fat was the principal nemesis, and any other form of calorie was considered acceptable. My mother, keenly interested in the health of her children, insisted that we put margarine on our Wonder Bread. I am not sure I knew what butter was until I read about it in a Hardy Boys book.

These days, though, our culinary enemies have shifted again, and sugar has increasingly come to be seen as the principal poison in our diets (except, of course, for those who are certain that the devil is gluten). Sugar is now often cited as the cause of various chronic illnesses, from diabetes to Alzheimer’s disease. One study published last year in the Journal of American Medicine found that a sugar-laden diet can increase the risk of death—even for people who are not overweight.

The danger sugar poses for us all is the essential premise of “That Sugar Film,’’ a documentary directed by the Australian actor Damon Gameau, which was released in the United States this summer. In it, Gameau makes himself the subject of a contrived—but, at times, compelling—experiment to demonstrate exactly what our sugar addiction does to our bodies.

Gameau describes himself as a man who lived on cigarettes and pizza until, under the influence of his girlfriend, he cleaned up his diet and gave up refined sugar altogether. For the movie, though, he spent a two-month period consuming forty teaspoons per day—the average daily sugar consumption for young Australian men. (That figure is about the same for men in the U.S. Over all, Americans consume an average of twenty-three teaspoons a day. The American Heart Association recommends that men consume no more than nine teaspoons of added sugars daily, and women no more than six. The World Health Organization suggests that all adults and children consume less than five teaspoons of sugar (excluding fruit and milk), “if possible.”)*

The food horror movie has become its own genre of sorts, and “That Sugar Film” follows the exact template of “Super Size Me,” an earlier, and better, representative of the form. In that 2004 documentary, the independent director Morgan Spurlock ate nothing but food from McDonald’s for a month, and consumed the caloric equivalent of nine Big Macs every day. The movie documents his physical and psychological deterioration, which is credibly linked to the horrific diet he has created for himself. Gameau seeks to do the same thing with sugar. He adds it to water and to baked chicken. He gurgles Mountain Dew. In one particularly frightening scene, he downs a smoothie from an airport Jamba Juice which contains a hundred and thirty-nine grams of sugar—that’s about thirty-four teaspoons, or about five times the recommended daily intake.

“What a great jet-lag cure,’’ Gameau says, only half in jest. He then explains, with the help of a nifty graphic, that to consume an equivalent amount of sugar in “actual” food he would have had to eat four peaches, nine limes, thirty lemons, and thirty strawberries. The point, of course, is that nobody eats real food that way.

Real food, unlike sugar, is its own best regulatory system. Four apples contain sixteen teaspoons of sugar—way more than the recommended daily limit for added sugars. But few people want to eat that many apples, because the fruit contains fibre and other nutrients that help tell our bodies when we are full. Turn those four apples into juice, however, and you can circumvent the body’s signals, basically mainlining the excess sugar directly into your bloodstream. You get the bad stuff without the good stuff. (This is the American way. White bread is also the bad stuff without the good stuff: it’s made from wheat that has been stripped of the portion of the grain that contains fibre and other nutrients.)

Gameau does a solid job making the point, often overlooked, that sugar by any other name is still sugar: agave, honey, castor sugar, and even the much reviled high-fructose corn syrup all have similar effects. Eat too much of any one of them and we overtax our insulin, the hormone in our bodies that breaks down sugar, metabolizes it, and stores what we don’t need. That can lead to diabetes, liver disease, and other ailments.

Gameau argues that if we just cut out the sugar, we will all be fine. To illustrate his point, he travels to an aboriginal town whose residents, before 1973, consumed no processed or store-bought food. Then Coca-Cola and other purveyors of sugar-laden, highly processed products came to the area. The people had no nutritional education; not surprisingly, they began to develop diabetes and other chronic illnesses associated with poor Western diets.

It’s almost impossible to dispute that sugar plays a prominent and very damaging role in cases like that one, and in the health of most Western nations. What’s less certain is how directly, or to what extent, it is to blame. There are two main camps in the sugar wars: those who think that sugar is directly responsible for the majority of obesity, diabetes, and other chronic diseases, and those who think it’s merely one part in the greater problem of a nation eating too many calories, most of which are nutritionally empty. (There is a third group who, ignoring both staggering amounts of data and the anecdotal evidence you can gather at any mall, contends that sugar causes little harm.)

In just two months, Gameau’s sugary diet caused him to gain huge amounts of weight, add many inches to his gut, and flood his liver with fat. The medical team he hired to track his progress (or its opposite) found that, at the end of the experiment, Gameau was well on his way to developing the metabolic conditions that almost always lead to diabetes. After reverting to a healthy diet, though, his health improved and so did his blood tests.

Was it the sugar or the calories? The film makes the good point that a calorie is not simply a calorie—your body processes different kinds in different ways, and added sugar rarely provides benefits. Yet, like the anti-fat credo of my youth, “That Sugar Film” places too much certainty in the belief that a single food can be the problem. If only it were that simple. Many nutritionists believe it’s time to stop demonizing individual components of our diet—whether it’s fat, carbohydrates, cholesterol, or even sugar. It’s time to acknowledge that the special diets that obsess us—whether they consist of fruit or eliminate fruit, rely on meat or banish it—don’t work.

We eat too much processed food, and our diets are disgraceful. Live solely on M&M’s and sugary drinks and you are going to get sick. For sugar to exist as a normal, safe part of our food, all we need to do is maintain balanced diets. The problem, of course, is that we don’t.

*This article has been revised to correct the type and amount of sugar intake recommended by various health authorities.