Beyond Willpower: Diet Quality and Quantity Matter
It’s no secret that the amount of calories people eat and drink has a direct impact on their weight: Consume the same number of calories that the body burns over time, and weight stays stable. Consume more than the body burns, weight goes up. Less, weight goes down. But what about the type of calories: Does it matter whether they come from specific nutrients—fat, protein, or carbohydrate? Specific foods—whole grains or potato chips? Specific diets—the Mediterranean diet or the “Twinkie” diet? And what about when or where people consume their calories: Does eating breakfast make it easier to control weight? Does eating at fast-food restaurants make it harder?
Conventional wisdom says that since a calorie is a calorie, regardless of its source, the best advice for weight control is simply to eat less and exercise more. Yet emerging research suggests that some foods and eating patterns may make it easier to keep calories in check, while others may make people more likely to overeat.
There’s ample research on foods and diet patterns that protect against heart disease, stroke, diabetes, and other chronic conditions. The good news is that many of the foods that help prevent disease also seem to help with weight control—foods like whole grains, vegetables, fruits, and nuts. And many of the foods that increase disease risk—chief among them, refined grains and sugary drinks—are also factors in weight gain.
This article briefly reviews the research on dietary intake and weight control, highlighting diet strategies that also help prevent chronic disease.
When people eat controlled diets in laboratory studies, the percentage of calories from fat, protein, and carbohydrate do not seem to matter for weight loss. In studies where people can freely choose what they eat, there may be some benefits to a higher protein, lower carbohydrate approach. For chronic disease prevention, though, the quality and food sources of these nutrients matters more than their relative quantity in the diet. And the latest research suggests that the same diet quality message applies for weight control.
Low-fat diets have long been touted as the key to a healthy weight and to good health. But the evidence just isn’t there: Over the past 30 years in the U.S., the percentage of calories from fat in people’s diets has gone down, but obesity rates have skyrocketed. (1,2) Carefully conducted clinical trials have found that following a low-fat diet does not make it any easier to lose weight than following a moderate- or high-fat diet. In fact, study volunteers who follow moderate- or high-fat diets lose just as much weight, and in some studies a bit more, as those who follow low-fat diets. (3,4) And when it comes to disease prevention, low-fat diets don’t appear to offer any special benefits. (5)
Part of the problem with low-fat diets is that they are often high in carbohydrate, especially from rapidly digested sources, such as white bread and white rice. And diets high in such foods increase the risk of weight gain, diabetes, and heart disease. (See Carbohydrates and Weight, below.)
For good health, the type of fat people eat is far more important that the amount (see box), and there’s some evidence that the same may be true for weight control. (6-9) In the Nurses’ Health Study, for example, which followed 42,000 middle-age and older women for eight years, increased consumption of unhealthy fats—trans fats, especially, but also saturated fats—was linked to weight gain, but increased consumption of healthy fats—monounsaturated and polyunsaturated fat—was not. (6)
Higher protein diets seem to have some advantages for weight loss, though more so in short-term trials; in longer term studies, high-protein diets seem to perform equally well as other types of diets. (3,4) High-protein diets tend to be low in carbohydrate and high in fat, so it is difficult to tease apart the benefits of eating lots of protein from those of eating more fat or less carbohydrate. But there are a few reasons why eating a higher percentage of calories from protein may help with weight control:
- More satiety: People tend to feel fuller, on fewer calories, after eating protein than they do after eating carbohydrate or fat. (10)
- Greater thermic effect: It takes more energy to metabolize and store protein than other macronutrients, and this may help people increase the energy they burn each day. (10,11)
- Improved body composition: Protein seems to help people hang on to lean muscle during weight loss, and this, too, can help boost the energy-burned side of the energy balance equation. (11)
Higher protein, lower carbohydrate diets improve blood lipid profiles and other metabolic markers, so they may help prevent heart disease and diabetes. (4,12,13) But some high-protein foods are healthier than others: High intakes of red meat and processed meat are associated with an increased risk of heart disease, diabetes, and colon cancer. (14-16)
Replacing red and processed meat with nuts, beans, fish, or poultry seems to lower the risk of heart disease and diabetes. (14,16) And this diet strategy may help with weight control, too, according to a recent study from the Harvard School of Public Health. Researchers tracked the diet and lifestyle habits of 120,000 men and women for up to 20 years, looking at how small changes contributed to weight gain over time. (9) People who ate more red and processed meat over the course of the study gained more weight—about a pound extra every four years. People who ate more nuts over the course of the study gained less weight—about a half pound less every four years.
Lower carbohydrate, higher protein diets may have some weight loss advantages in the short term. (3,4) Yet when it comes to preventing weight gain and chronic disease, carbohydrate quality is much more important than carbohydrate quantity.
Milled, refined grains and the foods made with them—white rice, white bread, white pasta, processed breakfast cereals, and the like—are rich in rapidly digested carbohydrate. So are potatoes and sugary drinks. The scientific term for this is that they have a high glycemic index and glycemic load. Such foods cause fast and furious increases in blood sugar and insulin that, in the short term, can cause hunger to spike and can lead to overeating—and over the long term, increase the risk of weight gain, diabetes, and heart disease. (17-19)
For example, in the diet and lifestyle change study, people who increased their consumption of French fries, potatoes and potato chips, sugary drinks, and refined grains gained more weight over time—an extra 3.4, 1.3, 1.0, and 0.6 pounds every four years, respectively. (9) People who decreased their intake of these foods gained less weight.
There’s growing evidence that specific food choices may help with weight control. The good news is that many of the foods that are beneficial for weight control also help prevent heart disease, diabetes, and other chronic diseases. Conversely, foods and drinks that contribute to weight gain—chief among them, refined grains and sugary drinks—also contribute to chronic disease.
Whole grains—whole wheat, brown rice, barley, and the like, especially in their less-processed forms—are digested more slowly than refined grains. So they have a gentler effect on blood sugar and insulin, which may help keep hunger at bay. The same is true for most vegetables and fruits. These “slow carb” foods have bountiful benefits for disease prevention, and there’s also evidence that they can help prevent weight gain.
The weight control evidence is stronger for whole grains than it is for fruits and vegetables. (20-22) The most recent support comes from the Harvard School of Public Health diet and lifestyle change study: People who increased their intake of whole grains, whole fruits (not fruit juice), and vegetables over the course of the 20-year study gained less weight—0.4, 0.5, and 0.2 pounds less every four years, respectively. (9)
Of course, the calories from whole grains, whole fruits, and vegetables don’t disappear. What’s likely happening is that when people increase their intake of these foods, they cut back on calories from other foods. Fiber may be responsible for these foods’ weight control benefits, since fiber slows digestion, helping to curb hunger. Fruits and vegetables are also high in water, which may help people feel fuller on fewer calories.
Nuts pack a lot of calories into a small package and are high in fat, so they were once considered taboo for dieters. As it turns out, studies find that eating nuts does not lead to weight gain and may instead help with weight control, perhaps because nuts are rich in protein and fiber, both of which may help people feel fuller and less hungry. (9,23-25) People who regularly eat nuts are less likely to have heart attacks or die from heart disease than those who rarely eat them, which is another reason to include nuts in a healthy diet. (19)
The U.S. dairy industry has aggressively promoted the weight-loss benefits of milk and other dairy products, based largely on findings from short-term studies it has funded. (26,27) But a recent review of nearly 50 randomized trials finds little evidence that high dairy or calcium intakes help with weight loss. (28) Similarly, most long-term follow-up studies have not found that dairy or calcium protect against weight gain, (29-32) and one study in adolescents found high milk intakes to be associated with increased body mass index. (33)
One exception is the recent dietary and lifestyle change study from the Harvard School of Public Health, which found that people who increased their yogurt intake gained less weight; increases in milk and cheese intake, however, did not appear to promote weight loss or gain. (9) It’s possible that the beneficial bacteria in yogurt may influence weight control, but more research is needed.
There’s convincing evidence that sugary drinks increase the risk of weight gain, obesity, and diabetes: (34-36) A systematic review and meta-analysis of 88 studies found “clear associations of soft drink intake with increased caloric intake and body weight.” (34) In children and adolescents, a more recent meta analysis estimates that for every additional 12-ounce serving of sugary beverage consumed each day, body mass index increases by 0.08 units. (35) Another meta analysis finds that adults who regularly drink sugared beverages have a 26 percent higher risk of developing type 2 diabetes than people who rarely drink sugared beverages. (36) Emerging evidence also suggests that high sugary beverage intake increases the risk of heart disease. (37)
Like refined grains and potatoes, sugary beverages are high in rapidly-digested carbohydrate. (See Carbohydrates and Weight, above.) Research suggests that when that carbohydrate is delivered in liquid form, rather than solid form, it is not as satiating, and people don’t eat less to compensate for the extra calories. (38)
These findings on sugary drinks are alarming, given that children and adults are drinking ever-larger quantities of them: In the U.S., sugared beverages made up about 4 percent of daily calorie intake in the 1970s, but by 2001, represented about 9 percent of calories. (36) The most recent data find that on any given day, half of Americans consume some type of sugared beverage, 25 percent consume at least 200 calories from sugared drinks, and 5 percent of consume at least 567 calories—the equivalent of four cans of sugary soda. (39)
The good news is that studies in children and adults have also shown that cutting back on sugary drinks can lead to weight loss. (40,41) Sugary drinks have become an important target for obesity prevention efforts, prompting discussions of policy initiatives such as taxing soda. (42)
It’s important to note that fruit juices are not a better option for weight control than sugar-sweetened beverages. Ounce for ounce, fruit juices—even those that are 100 percent fruit juice, with no added sugar— are as high in sugar and calories as sugary sodas. So it’s no surprise that a recent Harvard School of Public Health study, which tracked the diet and lifestyle habits of 120,000 men and women for up to 20 years, found that people who increased their intake of fruit juice gained more weight over time than people who did not. (9) Pediatricians and public health advocates recommend that children and adults limit fruit juice to just a small glass a day, if they consume it at all.
Even though most alcoholic beverages have more calories per ounce than sugar-sweetened beverages, there’s no clear-cut evidence that moderate drinking contributes to weight gain. While the recent diet and lifestyle change study found that people who increased their alcohol intake gained more weight over time, the findings varied by type of alcohol. (9) In most previous prospective studies, there was no difference in weight gain over time between light-to-moderate drinkers and nondrinkers, or the light-to-moderate drinkers gained less weight than nondrinkers. (43-47)
People don’t eat nutrients or foods in isolation. They eat meals that fall into an overall eating pattern, and researchers have begun exploring whether particular diet or meal patterns help with weight control or contribute to weight gain. Portion sizes have also increased dramatically over the past three decades, as has consumption of fast food—U.S. children, for example, consume a greater percentage of calories from fast food than they do from school food (48)—and these trends are also thought to be contributors to the obesity epidemic.
So-called “prudent” dietary patterns—diets that feature whole grains, vegetables, and fruits—seem to protect against weight gain, whereas “Western-style” dietary patterns—with more red meat or processed meat, sugared drinks, sweets, refined carbohydrates, or potatoes—have been linked to obesity. (49-52) The Western-style dietary pattern is also linked to increased risk of heart disease, diabetes, and other chronic conditions.
Following a Mediterranean-style diet, well-documented to protect against chronic disease, (53) appears to be promising for weight control, too. The traditional Mediterranean-style diet is higher in fat (about 40 percent of calories) than the typical American diet (34 percent of calories (54)), but most of the fat comes from olive oil and other plant sources. The diet is also rich in fruits, vegetables, nuts, beans, and fish. A 2008 systematic review found that in most (but not all) studies, people who followed a Mediterranean-style diet had lower rates of obesity or more weight loss. (55) There is no single “Mediterranean” diet, however, and studies often use different definitions, so more research is needed.
There is some evidence that skipping breakfast increases the risk of weight gain and obesity, though the evidence is stronger in children, especially teens, than it is in adults. (56) Meal frequency and snacking have increased over the past 30 years in the U.S. (57)—on average, children get 27 percent of their daily calories from snacks, primarily from desserts and sugary drinks, and increasingly from salty snacks and candy. But there have been conflicting findings on the relationship between meal frequency, snacking, and weight control, and more research is needed. (56)
Since the 1970s, portion sizes have increased both for food eaten at home and for food eaten away from home, in adults and children. (58,59) Short-term studies clearly demonstrate that when people are served larger portions, they eat more. One study, for example, gave moviegoers containers of stale popcorn in either large or medium-sized buckets; people reported that they did not like the taste of the popcorn—and even so, those who received large containers ate about 30 percent more popcorn than those who received medium-sized containers. (60) Another study showed that people given larger beverages tended to drink significantly more, but did not decrease their subsequent food consumption . (67) An additional study provided evidence that when provided with larger portion sizes, people tended to eat more, with no decrease in later food intake. (68) There is an intuitive appeal to the idea that portion sizes increase obesity, but long-term prospective studies would help to strengthen this hypothesis.
Fast food is known for its large portions, low prices, high palatability, and high sugar content, and there’s evidence from studies in teens and adults that frequent fast-food consumption contributes to overeating and weight gain. (61-66) The CARDIA study, for example, followed 3,000 young adults for 13 years. People who had higher fast-food–intake levels at the start of the study weighed an average of about 13 pounds more than people who had the lowest fast-food–intake levels. They also had larger waist circumferences and greater increases in triglycercides, and double the odds of developing metabolic syndrome. (62) More research is needed to tease apart the effect of eating fast food itself from the effect of the neighborhood people live in, or other individual traits that may make people more likely to eat fast food.
Weight gain in adulthood is often gradual, about a pound a year (9)—too slow of a gain for most people to notice, but one that can add up, over time, to a weighty personal and public health problem. There’s increasing evidence that the same healthful food choices and diet patterns that help prevent heart disease, diabetes, and other chronic conditions may also help to prevent weight gain:
- Choose minimally processed, whole foods—whole grains, vegetables, fruits, nuts, healthful sources of protein (fish, poultry, beans), and plant oils.
- Limit sugared beverages, refined grains, potatoes, red and processed meats, and other highly processed foods, such as fast food.
Though the contribution of any one diet change to weight control may be small, together, the changes could add up to a considerable effect, over time and across the whole society. (9) Since people’s food choices are shaped by their surroundings, it’s imperative for governments to promote policy and environmental changes that make healthy foods more accessible and decrease the availability and marketing of unhealthful foods.
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The aim of the Harvard School of Public Health Obesity Prevention Source Web site is to provide timely information about obesity’s global causes, consequences, prevention, and control, for the public, health and public health practitioners, business and community leaders, and policymakers. The contents of this Web site are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Web site. The Web site’s obesity prevention policy recommendations are based primarily on a review of U.S. expert guidance, unless otherwise indicated; in other countries, different policy approaches may be needed to achieve improvements in food and physical activity environments, so that healthy choices are easy choices, for all.