While most Americans eat meals with a mix of healthful nutrients, a small set of eaters are going in the opposite direction: completely separating out their food groups. They’re following a new trend known as food combining, which in its most basic form claims that the three macronutrients in our diets—carbohydrates, fat and protein—need to be eaten separately for optimal digestion. When digestive health and the microbiome are top-of-mind for so many folks, it’s easy to get pulled into a regimen promising a healthy, happy gut. But is food combining just another fad diet, or is it here to stay?

First, a little history

Unlike many other diet trends, food combining has a long history. It is rooted in the ancient Indian practice of Ayurvedic medicine, which is one of the planet’s oldest holistic healing practices. Ayurveda defines every food by its taste, energy (heating or cooling) and post-digestive effect, and food combinations based on these principles are essential for optimal digestion. Although food is one component of Ayurveda, this ancient way of thinking is more spiritually based than later iterations of food combining.

The next historical round of food combining came from a physician named William Howard Hay who viewed and adapted the concept from a medical perspective. Popularized in the 1920s, the Hay Diet drew from Hay’s own personal experience with food and healing and had several strict guidelines to help a person achieve optimal health. Key components of the Hay Diet include:

1. Do not combine starches or sugars with proteins or acid fruits.
2. Eat fruits and vegetables as most of one’s diet.
3. Eat proteins, starches and fats in moderation.
4. Only eat whole, unprocessed grains (e.g., no white flour or refined sugar).
5. Wait at least four to four-and-a-half hours between meals.

Current food combining advocates have taken principles from both Hay and Ayurveda to create appealing food combining charts under the notion that this eating pattern will heal the gut. New food combining rules keep the five Hay principles and add several others, such as that fats should not be eaten alongside proteins unless a green salad is also included. Many also subscribe to the idea that meals need to be either acid or alkaline for ideal digestion, drawing on certain principles from the alkaline diet. According to both food combining and the alkaline diet, acidic foods that should be limited include meat, poultry and eggs; while alkaline foods to increase include fruits, vegetables and nuts.

What does the science say?

While some of these principles offer sound advice—we could all use more fruits and veggies, after all—most of the key components of food combining are not backed up by science. In fact, it’s hard to even define foods in the simplistic ways that food combining advocates try to do. Most foods are not singularly starch (carbohydrate), protein or fat, but often a combination of two or three of these. In fact, the inherent nature of our food supply completely counters food combining principles. Take beans and legumes, for example. They’re a great source of protein, carbohydrate, fiber, vitamins and minerals. They don’t fit into one section of the food combining charts, and our gut can handle digesting them just fine.

In addition, research on food combining is very sparse. Only one human study has been done to assess the health effects of this way of eating. The researchers split participants into groups to receive either a low-calorie “balanced” diet or a low-calorie “dissociated” (food combining) diet. Overall, they found significant decreases in weight, fasting blood sugar, cholesterol and blood pressure in both groups. This means that food combining didn’t make things better or worse—the outcomes were most likely the result of calorie deprivation among all participants rather than the difference in diet composition. Other than this study, there is no research supporting the use of food combining diet principles.

How does our gut actually work?

Food combining proponents claim that it improves gut health. But food combining is not as effective as it claims to be for one key reason: the human gastrointestinal tract is built to digest all macronutrients when eaten in combination.

How about a little review? Following a kick-start from enzymes in our saliva, carbohydrate-rich foods are primarily digested in the stomach. Proteins, however, are partially broken down in the stomach; later, the small intestine does most of the work. And fats are also primarily digested in the small intestine with the help of bile made by the liver. Regardless of the food, whether it’s mostly carbohydrate like fruit or a natural combination of macronutrients, like legumes, the stomach and small intestine are armed and ready to digest different combinations at any time.

The new wave of food combining also recommends separating acidic and alkaline foods for optimal digestion. However, our gut already does a good job self-regulating its acidity. No matter what is eaten, hydrochloric acid will be released in the stomach, creating a very acidic environment. When food leaves the stomach, the small intestine releases bicarbonate to neutralize the partially digested food.

Food combinations that DO work

Despite what food combining advocates think, there is research that encourages the consumption of nutrients and foods in combination with each other. For example, eating a high-carbohydrate food, like fruit, with a high-protein food, like Greek yogurt or almonds, could slow the absorption of glucose into the blood. For individuals with diabetes, this is especially important, as it reduces the risk of an immediate blood sugar spike and subsequent drop right after a meal.

Food combining also leaves out some important combinations that may help with the absorption of vital nutrients. Combining vitamin C–rich foods, like oranges and other acidic fruits, with iron-rich foods, like animal proteins or dark green leafy vegetables, may improve iron absorption. Vitamin C is particularly beneficial for improving the absorption of non-heme iron, which is found in plant sources. Additionally, eating foods like vitamin A–rich sweet potatoes or vitamin K–rich broccoli with a high-fat food can improve vitamin absorption. These fat-soluble vitamins, along with vitamins D and E, require some amount of fat as a vehicle for crossing over the gut.

The final verdict

The food combining diet, in sum, is a fad diet. Its strict rules are not sustainable for most lifestyles, and the research supporting its health benefits are limited at best. Instead, it’s best to eat a balanced combination of nutrient-rich foods at every meal. Fortunately, that means here’s no need to eliminate favorites like peanut butter and jelly sandwiches, tacos, or spaghetti and meatballs. As long as you keep a healthful plate overall, your gut will be pleased.

This blog post was written by Courtney Schupp, MPH, RD, our 2019 Sylvia Rowe Fellow.