At the most fundamental level, our food choices build our bodies and drive our health. But scientists may never be able to unravel all of the ways that diet affects our well-being. It’s simply not practical to randomize large groups of people and expect them to change their food habits over years or a lifetime, the kind of study it would take, for example, to precisely calculate how certain food patterns raise or lower the risk of cancer or heart disease. Since many nutritional questions will forever remain out of reach of classic experimental science, there will always be room for debate. Low-carb, low-fat, low-glycemic, vegetarian, vegan — people follow many paths to healthy eating, and some diets are bound to be more realistic and effective than others.
In 2014, David Katz, a physician at Yale University and the founding director of the Yale University Prevention Research Center, cut through the clutter with “Can We Say What Diet is Best for Health?”, an article he coauthored with Stephanie Meller in the Annual Review of Public Health. By examining the pros and cons of popular diets, Katz made the case that many dieters and nutritionists have misplaced priorities. His message: Instead of counting carbs or grams of fat, we should embrace the basic framework of healthy eating.
We spoke with Katz — who is now at work on The Truth About Food, a book due out next year — about his current thinking on healthy habits that last, the hallmarks of a healthy diet and his personal dietary choices.
Your 2014 review examined findings from more than 160 sources. And research marches on. Have any of your major conclusions changed over time?
When it comes to nutrition, the weight of evidence shifts very slowly. New research adds refinement, but it rarely changes anything fundamental. The basic truths of nutrition have been stable not just since 2014, but for my entire lifetime.
What do the healthiest diets have in common?
Some details may vary, but the healthiest diets rely on real, minimally processed foods, mostly plants, with few refined carbs or added sugars.
Is there any difference between the basic framework of a diet that’s optimal for overall health and a diet that’s optimal for weight loss?
There are lots of ways to lose weight fast that don’t promote long-term health. For healthy weight loss, people should stick to the basic dietary principles. Real, wholesome foods, mostly plants.
Which diet is easier to sustain, low-fat or low-carb?
In the real world, low-fat is more sustainable than low-carb. Some low-fat diets stand the test of time, but it’s very hard to find free-living populations anywhere who stick to a low-carb diet. My preferred answer is: Who cares? Focusing on macronutrients is a mistake.
It’s not important to pay attention to fat?
The dietary patterns associated with the best health outcomes can range from very low in fat to very high in fat. The fat amount doesn’t seem to make a difference. What really matters: Are the foods wholesome, are they arranged sensibly and is there a balance? You can achieve all of that with high or low fat intakes.
Shouldn’t most people cut carbs?
Carbs shouldn’t be the focus. All plant foods contain carbohydrates. When you’re talking about a low-carb diet, you’re talking about a diet that’s low in vegetables, fruits, whole grains, beans, lentils, nuts and seeds. And if you ask me to list the foods most likely to promote health, I’d say vegetables, fruits, whole grains, beans, lentils, nuts and seeds. While it makes sense to avoid refined starches and added sugars, a strictly low-carb diet cannot be a good diet.
Can a low-glycemic-index diet help people manage their blood sugar, lose weight and avoid disease?
There’s an active debate about the long-term effects of a low-glycemic-index diet on the incidence of chronic disease. A Cochrane Review article published in July found no evidence that low-glycemic-index diets prevent cardiovascular disease. That could be because such diets can discourage people from eating fruits and vegetables that have a high glycemic index.
Popular books warn people away from grapes, berries and carrots because of the glycemic index. But if you show me a person who can blame their diabetes or obesity on grapes, berries or carrots, I’ll quit my day job. It’s ridiculous.
Fruits are a concentrated source of fruit sugar, but they are also a concentrated source of soluble fiber. The net effect of that fiber is to help stabilize blood sugar, reduce requirements for blood insulin, lower blood lipids and possibly lower blood pressure. There’s a very decisive net metabolic benefit.
What about fruit juices?
Juice has two problems. You’re stripping away the fiber and other nutrients that come with whole fruit. And you’re dumping all of that sugar rapidly into the bloodstream, so the insulin response is much greater. Your pancreas is happier responding to a small, steady delivery of sugar to the bloodstream than sudden spikes.
Is there a place for red meat in a healthy diet?
We can get by on a wide array of dietary choices. The human body is remarkably forgiving.
Can you eat hamburgers your whole life and live to a ripe old age? Yeah, maybe with a stent in your coronary arteries, but that’s how we roll in America.
But the kind of red meat people eat in the modern world is nothing like we’re adapted for eating. If you grab a bow and arrow and kill an antelope, you’ll be fine. Only 7 percent of calories from that antelope come from fat, almost none of that is saturated, and quite a lot of unsaturated fat is omega 3, the type that prevents inflammation. For a typical cut of red meat from a grain-fed cow, fat provides 35 percent of calories, much of it is saturated, and of the polyunsaturates, it’s almost all omega 6, the type that promotes inflammation. It’s night and day.
Also, let’s assume a hypothetical person who eats a stable number of calories each day. When those calories are taken up by meat, there’s something else that they’re not eating. Harvard researchers did a study in 2010 looking at dietary sources of protein and the risk of cardiovascular disease in women. The biggest benefit was for those who substituted a daily serving of beans or nuts for a serving of red meat.
Are healthy diets anti-inflammatory?
Modern living is inflammatory. It’s true of our politics, and it’s true of our immune systems. Concentrated doses of sugar and refined carbohydrates induce the insulin responses and endocrine imbalance that cause dysfunction in the immune system, including excess inflammation. Too much inflammation in the immune system causes allergies and autoimmune disease. But you also want your immune system to be strong enough to fight bacteria, viruses and cancer cells. A wholesome, plant-based diet clearly moves you toward the right balance.
How important is it to eat with your microbiome in mind?
The microbiome is fascinating, but it can also be an excuse for procrastination. We can’t raise the red flag and say that we can’t do anything about diet until we know how it will affect every one of the quadrillions of bacteria in our body. We should study the effect of the microbiome on the disease state, but we don’t need bacterial colony counts to know that healthy people also have healthy microbiomes. It’s already perfectly clear what diets and habits lead to good health.
What is your diet like?
My wife and I eat some poultry and fish, but we’re moving more toward a plant-based diet. I haven’t eaten a mammal for 35 years, for ethical and ecological reasons. The direct health effects of meat are tricky to measure. It depends on what you eat and how much. But the effects of meat on the planet are a slam dunk. Everyone needs to eat a whole lot less or our goose is cooked. There are no healthy people on an unhealthy planet.