The Diet Review: 39 popular nutrition and weight-loss plans and the science (or lack of science) behind them

These days, there are so many diet plans, it’s almost impossible to keep them all straight. Any diet book that becomes a blockbuster inevitably spawns variations, as publishers seek to capitalize on a trend—until the next big idea comes along, and throngs rush to embrace yet another new approach. Each diet is different, yet it seems to be accompanied by a raft of testimonials and purported science, showing why it is the ultimate diet for weight loss or health—or both.

Adding to the confusion are media reports of research studies that claim to “upend everything we thought we knew about nutrition.” Claims like these have launched countless diet books. But when you see such dramatic claims, remember that the science of nutrition doesn’t turn on a dime, and massive paradigm shifts don’t happen overnight. Rather, new evidence gets added to existing knowledge, and the overall consensus about optimal nutrition—based on many, many studies—evolves.

You might wonder, if so many nutrition “experts” disagree about how to eat, who’s right and who’s wrong? The truth is that there’s no one right way to eat. There are actually many ways to eat for health, but not every diet out there is one of those ways. This Special Health Report will help you sort through more than three dozen diet plans, so you can make the decision that’s right for you. You’ll learn about the common denominators of all healthy diets, and you’ll see plenty of examples of which diet patterns get it right and which ones miss the mark. Along with that, you’ll learn why the quality of the foods you eat matters more than choosing the “right” ratio of carbohydrates, protein, and fat.

For each diet we cover, we provide specific information—including any research that’s been done on the diet, how it meshes with nutrition research in general, whether it provides a good balance of nutrients, and whether it’s affordable and easy to follow. What works for one person will not necessarily work for another.

How will you know when you’ve found the right diet for you? It should provide balanced nutrition, appeal to your tastes, and be compatible with your cooking ability and schedule. Your diet should make your life healthier, not more complicated. Although making dietary changes can take time, effort, thought, and planning—as does any new healthful habit—a diet plan that works for you will gradually feel normal, and some of your healthy behaviors will even come to feel effortless. It’s the job of this report to help make sure that whatever diet you choose, it’s one that’s good for you.

Five principles of a healthy diet

While details may vary from diet to diet, all healthy eating plans have these five principles in common:

1. Lots of plants. Plant foods—vegetables, fruits, legumes, whole grains, nuts, and seeds—offer a wealth of vitamins and minerals, as well as fiber and healthful compounds called phytochemicals (literally “plant chemicals,” natural substances in plants that offer humans a range of health benefits, including antioxidant, anti-inflammatory, and even anti-cancer activity). At the same time, while many plant foods are high in nutrients, they are relatively low in calories. The combination of high nutrient content and low calories—a quality known as nutrient density—means that a plant-heavy diet can be good for both health and weight loss. Because people often underestimate how large their portions of fruits and vegetables should be, Harvard nutritionists devised the Harvard Healthy Eating Plate (see Figure 1) to provide a graphic representation of a healthy dinner. Fully half the plate contains produce.

2. Adequate protein. Abundant research shows it’s important to eat enough protein, but there are many ways to get that protein, and some are more healthful than others. People who limit how much meat they eat tend to have lower risks for chronic diseases. Plant protein sources (beans, lentils, soy foods, nuts, seeds) and seafood offer the most health benefits. Getting enough protein, along with physical activity, is important for staying strong, healthy, and independent.

3. Minimally processed foods. A 2019 National Institutes of Health study definitively showed that eating a diet high in ultra-processed foods causes weight gain and unhealthy shifts in blood sugar and blood cholesterol. For the healthiest diet, rely as much as possible on whole foods (that is, unprocessed foods, such as broccoli, apples, and almonds) and minimally processed foods (such as plain yogurt, canned tuna, and natural peanut butter). Processing tends to strip away nutrients while adding extra fats, sugars, and sodium, not to mention other additives and preservatives.

4. Limited saturated fats, added sugars, and sodium. The U.S. government’s Dietary Guidelines for Americans recommend limiting saturated fat intake to less than 10% of daily calories. The same goes for added sugars (sugars added during processing). If you have a 2,000-calorie-a-day diet, that means that no more than 200 calories a day should come from added sugars. As for sodium, keep it below 2,300 milligrams per day. The average American consumes more than 3,400 milligrams per day.

5. Balance. To meet nutrient needs, it’s important to choose a variety of nutrient-dense foods across and within all food groups (see “What the food groups do for you”). Choosing nutrient-dense foods helps you get the nutrients you need without taking in too many calories.

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