Several widely believed ideas about food and nutrition have little or no scientific support. Sugar making kids hyperactive, the need for eight glasses of water a day, and the belief that eating late at night automatically causes weight gain are all common food myths that persist despite decades of research showing otherwise. Here’s what the evidence actually says about the most popular ones.
Sugar Does Not Make Kids Hyperactive
This is arguably the most persistent food myth of all. A meta-analysis of 23 controlled studies found that sugar does not affect the behavior or cognitive performance of children. These weren’t casual surveys. Each study gave children a known quantity of sugar or a placebo sweetener, and neither the kids, parents, nor researchers knew who got what. Across 14 different measures of behavior and cognition, none showed a statistically significant effect from sugar.
So why does every parent at a birthday party swear their child went wild after cake? The researchers pointed to expectancy bias. Parents who believe sugar causes hyperactivity interpret normal excited behavior as proof. When you put kids together at a party with games and presents, they act like kids at a party, regardless of what they eat.
You Don’t Need Eight Glasses of Water a Day
The “8×8” rule (eight glasses of eight ounces each) has no scientific basis. Kidney specialist Heinz Valtin spent years searching for evidence and found none supporting the recommendation. The myth likely traces back to a 1945 guideline from the U.S. Food and Nutrition Board suggesting people consume about two and a half liters of fluid per day. The critical detail that got lost: “Most of this quantity is contained in prepared foods.” The water in your coffee, soup, fruit, and vegetables all counts toward your daily intake.
Your actual hydration needs depend on your body size, activity level, climate, and overall diet. Thirst is a reliable signal for most healthy adults. Your urine color is another practical gauge: pale yellow means you’re well hydrated, dark yellow means you need more fluids.
Eating Late at Night Doesn’t Magically Add Pounds
The idea that calories consumed after a certain hour “turn to fat” is a simplification that misses the real picture. A study from the CALERIE trial, one of the most rigorous calorie-restriction studies in healthy adults, found that what mattered most for weight loss was consistency. Eating at regular times, keeping a shorter eating window, and consuming more of your calories earlier in the day were all associated with greater calorie restriction and slightly more weight loss, but the effects were modest. The time of your last meal explained only about 1% of the variation in weight loss.
The real problem with late-night eating isn’t the clock. It’s that late-night snacking tends to involve extra calories on top of what you already ate during the day, often from less nutritious foods. If your total calorie intake stays the same, eating dinner at 9 p.m. instead of 6 p.m. won’t meaningfully change your weight.
Frozen Produce Is Not Inferior to Fresh
Many people assume fresh fruits and vegetables are always more nutritious than frozen. In reality, frozen produce is typically processed within hours of harvest. The blanching step (a brief dip in hot water before freezing) does cause small losses of certain heat-sensitive vitamins, but the nutritional density of frozen fruits and vegetables is almost comparable to eating them freshly harvested, according to researchers at UCLA Health.
Meanwhile, “fresh” produce at your grocery store may have spent days or weeks in transit and on shelves, losing nutrients the entire time. Vitamin C, for example, degrades with exposure to light and air. A bag of frozen broccoli that was flash-frozen the day it was picked can easily contain more vitamins than a “fresh” head that’s been sitting under store lights for a week.
Gluten-Free Isn’t Healthier for Most People
Celiac disease affects roughly 1 in 100 Americans, and for those individuals, avoiding gluten is essential. But the popularity of gluten-free diets has far outpaced the number of people who actually need them. A large study involving participants tracked over decades found no association between gluten intake and coronary heart disease risk in people without celiac disease. Those who ate the least gluten had the same rate of heart disease as those who ate the most.
Going gluten-free without a medical reason can actually backfire. Many gluten-free products are lower in fiber and whole grains, which have a well-established protective effect against heart disease. Whole grains also provide B vitamins and minerals that are harder to get from refined gluten-free alternatives.
Detox Diets Don’t Do What Your Body Already Does
Juice cleanses and detox teas claim to flush toxins from your body. Your liver and kidneys already do this around the clock. Your liver contains enzyme systems that oxidize, reduce, and break down both internal waste products and external substances. These enzymes then attach chemical tags to the byproducts, making them water-soluble so your kidneys can filter them out. This process runs continuously in the liver, intestines, kidneys, and even the brain.
No commercial detox product has been shown to enhance this system in healthy people. Certain foods (cruciferous vegetables, for instance) do contain compounds that can modulate detoxification enzymes, but that’s an argument for eating a varied diet, not for spending a week drinking only celery juice.
Cutting Fat or Carbs Matters Less Than Total Calories
The debate between low-fat and low-carb diets has raged for decades, but a tightly controlled NIH study where participants lived in a research facility for four weeks found that both approaches led to weight loss. The low-fat diet led participants to eat 550 to 700 fewer calories per day spontaneously compared to the low-carb diet, and it produced more body fat loss. The low-carb diet, on the other hand, resulted in lower and steadier blood sugar and insulin levels.
The takeaway isn’t that one diet wins. It’s that the macronutrient ratio matters far less than most people think. Total calorie intake, food quality, and whether you can sustain the eating pattern long-term are what drive lasting results. Any diet that creates a calorie deficit will cause weight loss, regardless of whether those calories come primarily from fat or carbohydrates.
Vitamin C Won’t Prevent Your Cold
Taking vitamin C once you feel a cold coming on does very little. However, regular daily supplementation before getting sick does shorten colds slightly. In adults taking 1 gram per day, the median reduction in cold duration was about 6%, roughly half a day less of symptoms. Children taking 2 grams daily saw a larger benefit, with a 26% reduction in duration.
That’s a real but modest effect, and it only applies to people who were already taking vitamin C consistently before they caught the cold. Mega-dosing at the first sniffle, which is what most people do, has not shown the same benefit. You’re better off getting consistent vitamin C through citrus fruits, bell peppers, and strawberries than relying on emergency supplement packets.