Food combining is a philosophy of eating that has ancient roots, but has become extremely popular in the recent past.
Proponents of food-combining diets believe that improper food combinations can lead to disease, toxin buildup and digestive distress.
They also believe that proper combinations can relieve these problems.
But is there any truth to these claims?
Food combining is the term for the idea that certain foods pair well together, while others do not.
The belief is that combining foods improperly — for example, eating steak with potatoes — can lead to negative health and digestive effects.
Food combining principles first appeared in the Ayurvedic medicine of ancient India, but they became more widely popularized in the mid-1800s under the term trophology, or "the science of food combining."
The principles of food combining were revived in the early 1900s by the Hay diet. Since then, they've become a foundation for many modern diets.
Generally, food-combining diets assign foods to different groups.
These are usually broken down into carbs and starches, fruits (including sweet fruits, acidic fruits and melons), vegetables, proteins and fats.
Alternatively, some plans classify foods as either acidic, alkaline or neutral.
Food-combining diets specify how you should combine these groups in a meal.
Example Rules of Food Combining
The laws of food combining can vary somewhat depending on the source, but the most common rules include the following:
- Only eat fruit on an empty stomach, especially melons.
- Don't combine starches and proteins.
- Don't combine starches with acidic foods.
- Don't combine different types of protein.
- Only consume dairy products on an empty stomach, especially milk.
Other rules include that protein should not be mixed with fat, sugar should only be eaten alone, and fruits and vegetables should be eaten separately.
Two Beliefs Behind Food Combining
The rules of food combining are mostly based on two beliefs.
The first is that, because different foods are digested at different speeds, combining a fast-digesting food with a slow-digesting food causes a "traffic jam" in your digestive tract, leading to negative digestive and health consequences.
The second belief is that different foods require different enzymes to be broken down and that these enzymes work at different pH levels — levels of acidity — in your gut.
The idea is that if two foods require different pH levels, the body cannot properly digest both at the same time.
Proponents of food-combining diets believe that these principles are essential to proper health and digestion.
It is also believed that the improper combination of foods leads to negative health consequences such as digestive distress, the production of toxins and disease.
Bottom Line: Food combining refers to a way of eating in which certain types of foods are not eaten together. Proponents of food-combining diets believe improper combinations lead to disease and digestive distress.
So far, only one study has examined the principles of food combining. It tested whether a diet based on food combining had an effect on weight loss.
Participants were split into two groups and given either a balanced diet or a diet based on the principles of food combining.
On both diets, they were only allowed to eat 1,100 calories per day.
After six weeks, participants in both groups had lost an average of about 13–18 lbs (6–8 kg), but the food-combining diet offered no benefit over the balanced diet ().
In fact, there is no evidence to support most of the supposedly scientific principles of food combining.
Many of the original food-combining diets were developed more than 100 years ago, when much less was known about human nutrition and digestion.
But what is now known about basic biochemistry and nutritional science directly contradicts most of the principles of food combining.
Here's a closer look at the science behind the claims.
On Avoiding Mixed Meals
The term "mixed meals" refers to meals that contain a combination of fat, carbs and protein.
The rules of food combining are largely based on the idea that the body is not equipped to digest mixed meals.
However, this is simply not the case. The human body evolved on a diet of whole foods, which almost always contain some combination of carbs, protein and fat.
For example, vegetables and grains are typically considered to be carb-containing foods. But they all also contain several grams of protein per serving. And meat is considered to be a protein food, but even lean meat contains some fat.
Therefore — because many foods contain a combination of carbs, fat and protein — your digestive tract is always prepared to digest a mixed meal.
When food enters your stomach, gastric acid is released. The enzymes pepsin and lipase are also released, which help start protein and fat digestion.
Evidence shows that pepsin and lipase are released even if there is no protein or fat present in your food (, ).
Next, food moves into the small intestine. There, the gastric acid from the stomach is neutralized and the intestine is flooded with enzymes that work to break down proteins, fats and carbs (, , ).
Therefore, there's no need to worry that your body will have to choose between digesting protein and fat or starches and proteins.
In fact, it's specifically prepared for this type of multitasking.
On Food Altering the pH of the Digestive Tract
Another theory behind food combining is that eating the wrong foods together can hinder digestion by creating the wrong pH for certain enzymes to function.
First, a quick refresher on pH. It's a scale that measures how acidic or alkaline a solution is. The scale ranges from 0–14, where 0 is the most acidic, 7 is neutral and 14 is the most alkaline.
It is true that enzymes need a specific pH range in order to function properly and that not all enzymes in the digestive tract require the same pH.
However, eating foods that are more alkaline or acidic does not significantly change the pH of your digestive tract. Your body has several ways of keeping the pH of each part of your digestive tract in the correct range.
For example, the stomach is usually very acidic with a low pH of 1–2.5, but when you eat a meal, it may initially rise as high as 5. However, more gastric acid is quickly released until the pH is brought back down again ().
It is important to maintain this low pH because it helps start the digestion of proteins and activates the enzymes produced in the stomach. It also helps kill any bacteria in your food.
In fact, the pH inside your stomach is so acidic that the only reason the stomach lining isn't destroyed is because it's protected by a layer of mucus.
The small intestine, on the other hand, is not equipped to handle such an acidic pH.
Your small intestine adds bicarbonate to the mix as soon as the contents of your stomach enter it. Bicarbonate is your body's natural buffering system. It's very alkaline, so it neutralizes the gastric acid, keeping the pH between 5.5 and 7.8 (, ).
This is the pH at which the enzymes in the small intestine function best.
In this way, the different levels of acidity in your digestive tract are well controlled by the body's own sensors.
If you eat a very acidic or alkaline meal, your body will simply add more or less digestive juices in order to achieve the necessary pH level.
On Food Fermenting in the Stomach
Lastly, one of the most common claimed effects of improper food combining is that food ferments or putrefies in the stomach.
Supposedly, when a fast-digesting food is combined with a slow-digesting food, the fast-digesting food stays in the stomach so long that it begins to ferment.
This simply does not happen.
Fermentation and rotting occur when microorganisms start to digest your food. But, as mentioned earlier, the stomach maintains such an acidic pH that your food is essentially sterilized and almost no bacteria can survive ().
However, there is one place in your digestive tract where bacteria thrive and fermentation does occur. This is in your large intestine, also known as your colon, where trillions of beneficial bacteria live ().
The bacteria in your large intestine ferment any undigested carbs, such as fiber, that were not broken down in your small intestine. They release gas and beneficial short-chain fatty acids as waste products ().
In this case, fermentation is actually a good thing. The fatty acids the bacteria produce have been linked to health benefits such as reduced inflammation, improved blood sugar control and a lower risk of colon cancer (, ).
This also means that the gas you experience after a meal is not necessarily a bad thing. It can just be a sign that your friendly bacteria are well fed.
Bottom Line: There is no evidence that the practice of food combining offers any benefits. In fact, modern science directly contradicts many of its principles.
The principles of food combining diets are not backed by science, but that doesn't mean that the way you combine foods is always irrelevant.
For instance, there are many evidence-based food combinations that can significantly improve or reduce the digestion and absorption of certain foods.
Here are just a few examples.
Citrus Fruits and Iron
Iron comes in two forms in the diet: heme iron, which comes from meat, and non-heme iron, which comes from plant sources.
Heme iron is well absorbed, but the absorption of non-heme iron is very low — between 1–10%. Luckily, there are several things you can do to increase the absorption of this kind of iron ().
Adding vitamin C is one of the most effective things you can do.
It works in two ways. First, it makes non-heme iron more easily absorbable. Second, it decreases the ability of phytic acid to block iron absorption ().
This means that combining foods rich in vitamin C (such as citrus fruits or bell peppers) with plant-based sources of iron (such as spinach, beans or fortified cereals) is an excellent choice.
Unfortunately, studies have not shown that this combination actually increases iron levels in the body. However, this could simply be because the studies to date have been too small ().
Carrots and Fat
Certain nutrients, such as fat-soluble vitamins and carotenoids, need fat in order to be absorbed by the body.
Carotenoids are compounds found in red, orange and dark green vegetables. You can get them from veggies like carrots, tomatoes, red bell peppers, spinach and broccoli.
They have been linked with benefits such as a decreased risk of certain cancers, heart disease and vision problems ().
However, research has shown that if you consume these vegetables without any fat — eating plain carrot sticks or salad with fat-free dressing, for instance — you may be missing out on the benefits.
One study examined the absorption of carotenoids with fat-free, reduced-fat and full-fat dressing. It found that salad had to be consumed with a fat-containing dressing in order for any carotenoids to be absorbed ().
Your best bet to avoid missing out on these important nutrients is to consume a minimum of 5–6 grams of fat with carotenoid-containing vegetables (, ).
Try adding some cheese or olive oil to your salad, or top your steamed broccoli with a little bit of butter.
Spinach and Dairy Products
Foods such as spinach, chocolate and tea contain oxalate, an antinutrient that can bind with calcium to form an insoluble compound (, ).
This can be good or bad for you, depending on the circumstances.
For people who are prone to certain types of kidney stones, consuming calcium sources such as dairy products with oxalate-containing foods can actually decrease the risk of developing kidney stones (, ).
On the other hand, combining oxalates and calcium decreases the absorption of calcium. For most people, this is no problem in the context of a balanced diet.
But for people who don't eat much calcium in the first place or who eat a diet very high in oxalates, this interaction might cause a problem.
If you are concerned about getting enough calcium from your diet, avoid combining dairy products and other calcium-rich foods with foods that are high in oxalates.
Foods that are high in oxalates include spinach, nuts, chocolate, tea, beets, rhubarb and strawberries, among others ().
Bottom Line: The principles of most food-combining diets are not evidence-based. However, there are a few food combinations that have been scientifically shown to affect the digestion and absorption of nutrients.
The principles of food combining are not based on science. The claim that improper food combining is responsible for disease and toxins in the body is unfounded.
If you feel that the rules of food combining work for you, then you should certainly continue with it. If your diet isn't broken, then there's no need to fix it.
However, food combining diets may be overwhelming and unmanageable for a lot of people because of the many complicated rules they entail.
Plus, there is no evidence that they offer any unique benefits.