You know the old saying, “You are what you eat.” To that, we can add another one, “You smell like what you eat.” My father-in-law learned that lesson when he served in the Persian Gulf Command during World War II. He heard that the local people in Iran complained that the American soldiers smelled like sour milk. He laughed it off at the time; but when he got back to the United States after the war, he noticed the same thing. People who had been eating the standard American diet did smell like sour milk! People who had been eating Persian food did not. The reason was simple. The Americans were eating far more meat and milk and eggs and fish than the Persian people were.
Eating a lot of animal protein makes you stink. It can give you bad breath, bad body odor, and terribly stinky gas. Part of the problem is the sheer overload of protein that you get from animal-based foods. The other part of the problem is that animal protein tends to be particularly rich in the sulfur-containing amino acids. When you eat more of these sulfur-containing amino acids than your body needs, your body will simply break them down to burn them for energy. In the process, it can produce some very stinky byproducts.
Protein is made up of strings of amino acids. To make your own body’s proteins, your cells need an adequate supply of up to 20 different amino acids. You need to get eight of them ready-made in your food. It’s easy to get enough protein, including enough of all eight essential amino acids, from any practical plant-based diet as long as you are eating enough food to get enough calories. So you don’t really need to worry about protein deficiency.
If you eat too much fat, your body can store the excess fat in your fat cells. But eating too much protein does NOT cause your body to build bigger muscles. Instead, your liver will convert the surplus amino acids to sugar, which will then be burned for energy. Unfortunately, when your liver converts amino acids to sugar, it releases some nasty byproducts. In the long run, an overload of these byproducts can damage your liver, kidneys, and bones. In the short run, they can make you stink.
Animal proteins tend to be particularly rich in sulfur-containing amino acids. Besides cysteine and methionine, which your body can use for making protein, animal foods also contain taurine and homocysteine. When these substances are broken down, they release sulfur compounds. The methyl mercaptan (CH3-SH) that is produced from the breakdown of methionine is a major contributor to bad breath. These smelly compounds can also be excreted in sweat. Even an excess of some of the non-sulfur amino acids can produce stinky byproducts. For example, the breakdown of tryptophan by bacteria in the intestine produces skatole, which is largely responsible for the smell of feces.
Dental problems can cause bad breath because the bacteria in the mouth can release some smelly compounds, including methyl mercaptan. Thus, brushing and flossing are important. However, some of the odors in breath come from elsewhere in the body. The smelly chemicals can be carried to the lungs by the bloodstream and exhaled with the breath. The classic examples are the ketones that are produced when someone is fasting or has untreated type 1 diabetes. Low-carb diets tend to cause horrible breath for two reasons: ketones plus the overload of stinky substances from the high protein intake.
Bad breath and body odor should be viewed as a warning sign. Eating too much methionine can make you stink, but it can also promote the growth of cancer. A change in diet can make a big difference in how you look and how you smell. A friend of mine was pleasantly surprised by those benefits when she went on a plant-based diet for weight loss. First, she noticed that her acne cleared up. Then, she noticed that she no longer had a problem with body odor.
Update: For more information about why low-carbohydrate diets make people smell bad, see my book Thin Diabetes, Fat Diabetes: Prevent Type 1, Cure Type 2.