It's a mantra nutritionists might consider abandoning: Eat everything in moderation. According to a new Harvard study, the notion that there are no "good" or "bad" foods is a myth that needs to be debunked.
The study, published last week in the New England Journal of Medicine, evaluated how changes in a number of lifestyle factors - diet, exercise, sleep, television watching - affected long-term weight gain.
The researchers tracked the weight and lifestyle habits of 120,877 men and women in three large, ongoing medical studies every four years, over two decades. All were healthy and none were obese at the start of the study.
On average, participants gained 3.35 pounds during each four-year period. That added up to a weight gain of almost 17 pounds over the 20-year period.
When lifestyle changes associated with weight gain were evaluated, the findings were similar in all three studies.
People who regularly ate French fries, potato chips, mashed potatoes, processed meat, meat, sugary drinks, sweets and refined grains were more likely to gain weight.
The food that contributed to weight gain the most: French fries. One daily serving led to a 3.3-pound gain over four years. For potato chips, one daily serving led to an additional 1.7 pounds over four years. That's compared to sugary drinks, which added one pound every four years.
Other offenders included processed meat (0.93 lbs), unprocessed meat (0.95 lbs), boiled, baked or mashed potatoes (0.57 lbs), desserts (0.41 lbs), refined grains (0.39 lbs) and 100 per cent fruit juice (0.31 lbs).
Folks who increased their intake of vegetables, fruit, nuts, whole grains, and yogurt during the study were far less likely to gain weight. In fact, they were more likely to lose a little weight over time.
Differences in weight gain related to specific foods could be due to portion sizes, insulin levels, effect on feeling of fullness, or the displacement of other foods or beverages from the diet.
For instance, eating refined grains is less satiating and increases hunger signals and total calorie intake compared to eating the same number of calories from less processed, fibre-rich foods that also contain protein and fat.
Eating more fruit, vegetables, nuts and whole grains can push higher calorie foods out of the diet, such as potato chips and desserts, thereby reducing one's overall calorie intake.
Yogurt was also associated with less weight gain in all three studies. Evidence suggests that probiotic bacteria in yogurt may alter the composition of gut bacteria in a way that influences weight gain.
While diet was the most important contributor to weight gain over time, other lifestyle factors also played a role. Watching an hour of television each day added 0.4 pounds to the scale every four years. So did consuming one alcoholic drink every day.
People who slept six to eight hours a night put on fewer pounds than those who slept less than six or more than eight hours nightly. Individuals who increased their physical activity during the study also gained less weight every four years.
You might be wondering what's so surprising here. After all, don't most of us know that French fries and potato chips deliver a hefty dose of calories? Did we really need a big expensive study to tell us that?
I think we did. In my opinion, these findings are important. Sixty per cent of Canadian adults are overweight or obese; so are 26 per cent of children.
The pounds accumulate gradually and most people don't know what specifically causes them to struggle with weight gain. (The average adult gains one pound every year.) Knowing which precise behaviours impact body weight over the long term could help prevent obesity.
In the battle of the bulge, telling people to eat less and exercise more is not good enough. It's overly simplistic. Instead, it's your overall food choices - which foods you eat and which ones you don't - that predict how much weight you will or will not gain over time. It seems there are some foods that simply can't be eaten regularly if you want to stay lean four or 20 years from now.
When it comes to preventing weight gain, it's time to focus on the quality of our diets, rather than single nutrients like fat or carbohydrates. The results of this study show that diet quality influences diet quantity (e.g. total calories).
There's another message here. Small differences add up over time. If you pay attention to your diet, a handful of small changes - adding an extra serving of vegetables at dinner, snacking on a yogurt, switching to whole grain bread - can make a big difference to your weight. Perhaps not today. But most certainly down the road.
Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Her website is lesliebeck.com .