Does aspartame cause cancer? You’ve probably heard that it might. And PepsiCo removing the artificial sweetener from Diet Pepsi suggests there’s something iffy about it. New Diet Pepsi cans boast that the beverage is “now aspartame free,” a statement probably meant to placate consumers who cite aspartame as the reason they're cutting back on the beverage.
Whether or not PepsiCo's move lifts sagging diet soda sales, it's a decision rooted in marketing rather than science. Decades of investigation have turned up no credible evidence that aspartame is harmful to humans in the amounts diet soda drinkers are likely to consume. Someone would have to drink about 20 cans of diet soda a day to reach the U.S. Food and Drug Administration's recommended daily limit for aspartame, and even that amount is just 1 percent of the level that raised health concerns in lab rats, according to the American Cancer Society. That group, along with the U.S. National Cancer Institute, the European Food Safety Authority, and Britain’s Food Standards Agency, all agree that aspartame is safe to consume.
Some people will believe what they want to regardless of what the evidence says. Others want to make good choices about what they eat, but the information needed to make those decisions is a cacophony of often conflicting claims from marketers, academics, activists, and the media.
Consumers can exhaust themselves trying to assess the risks of GMOs, BPA, meat from animals raised with antibiotics, or whether goji berries will help them live longer. The evidence can range from outright bunk to solid consensus—and many degrees in between. But it's possible, with a basic understanding of nutrition science and a bit of effort, to make rational decisions based on science rather than speculation, marketing, or propaganda. Here's a guide for reasonable people.
You have to be especially skeptical of food claims. Medications are tested in double-blind, placebo-controlled trials to measure the effects of the drug isolated from other factors that might bias results. That’s basically impossible in nutrition. “To test this sort of thing, you would have to put people in a locked metabolic ward for decades,” says Marion Nestle, a New York University nutrition professor and food writer.
Most nutrition science is observational, which means researchers track people over time. Studies can try to control for other factors, like whether people who eat more vegetables are also more likely to exercise. But evidence from this kind of research is inherently weaker than randomized control trials.
For example, a study earlier this year linked diet sodas to widening waistlines in senior citizens tracked over nearly a decade. But the association doesn't tell you whether diet soda caused people to gain weight. Maybe people who were already putting on pounds decided to drink more diet soda.
Look for scientific consensus
Any one article can mislead. Small studies or short duration experiments are especially prone to mistaking random variation for meaning. That’s how one provocateur, using valid study results, fooled some reporters into writing that chocolate aids weight loss. “If it sounds fantastic, it probably is,” Nestle says. “That immediately eliminates breakthroughs."
Science is a process that accumulates evidence over time. “Sometimes people will take a study in rats or even in test tube data and extrapolate it to humans,” says Don Hensrud, director of the Mayo Clinic Healthy Living program. Look for consensus statements from scientific authorities that have evaluated all the research. The FDA has pages of information on ingredients and additives. So does the European regulator. If you have a specific health condition, look for guidance from medical foundations such as the American Diabetes Association. Or search for studies that incorporate lots of research, like systematic reviews or meta-analysis.
Consider your entire diet
People tend to latch on to findings about particular ingredients they think are especially harmful or beneficial. Kristin Kirkpatrick, manager of nutrition services at the Cleveland Clinic’s wellness institute, recalls a diabetic patient who drank three cups of cranberry juice every night, because he had read that cranberries can help prevent bladder infections. He'd had one bladder infection five years ago. Meanwhile, the cranberry juice was driving up his blood sugar.
"I will have patients whose diet is really in trouble, really, really in trouble, and they’ll come back and say, ‘It’s not that bad because everything I eat is organic,’” Kirkpatrick says. "An organic chocolate chip cookie is still a chocolate chip cookie." Focusing narrowly on certain foods can obscure bigger nutritional problems.
Think about where food comes from
Just because aspartame doesn’t cause cancer doesn’t mean Diet Pepsi is a healthy choice. The evidence suggests it’s probably better than cola with added sugar. But if you really want a healthy diet, soda shouldn’t be a big part of it. "Mainstream dietary advice tells you to eat lots of fruits and vegetables, eat foods from animals in smaller amounts, and minimize junk food,” says Nestle. "All of the business about artificial colors and flavors, those are markers for junk food."
Supermarkets don't slap labels on apples or eggplants boasting of their health benefits. “Do I really need to do a full-on Internet search and look for studies to determine if blueberries are good for me?” Kirkpatrick says. “Typically the best foods that are out there for our health, for our weight, for everything, need no claims."