Lactose intolerance is not an allergy or a real intolerance at all, but a deficiency. But it, too, results from a failure to break down certain nutrients completely into their component parts. Lactose is found in milk. It is derived from two sugar molecules that are linked together by chemical bonds. The body requires a digestive enzyme to break that bond, but, unlike other enzymes, this one does not come from the papilla. The cells of the small intestine secrete it themselves on the tips of their tiny little villi. Lactose breaks down when it comes into contact with the enzyme on the gut wall, and the resulting single sugars can then be absorbed. If the enzyme is missing, similar problems arise to those caused by gluten intolerance or gluten sensitivity, including bellyache, diarrhea, and flatulence. Unlike in celiac disease, however, no undigested lactose particles pass through the gut wall. They simply move on down the line, into the large intestine, where they become food for the gas-producing bacteria there. Consider the resulting flatulence and other unpleasant symptoms as votes of thanks from extremely satisfied, overfed microbes. Although the results can be unpleasant, lactose intolerance is far less harmful to health than undiagnosed celiac disease.
Every human being has the genes needed to digest lactose. In extremely rare cases, problems with lactose digestion can occur from birth. Such newborns are unable to digest their mother’s milk, and drinking it causes severe diarrhea. In 75 percent of the world’s population, the gene for digesting lactose slowly begins to switch off as they get older. This is not surprising, as by then we are no longer reliant on our mothers’ milk, or formula milk, to nourish us. Outside of Western Europe, Australia, and the United States, adults who are tolerant to dairy products are a rarity. Even in our parts of the world, supermarket shelves are increasingly full of lactose-free products. Recent estimates say about 25 percent of people in the United States lose their ability to break down lactose after weaning. The older a person, the greater the probability that she will be unable to break down lactose—although very few sixty-year-olds would dream of blaming their daily glass of milk or that delicious dollop of cream for a bloated stomach or a little bit of diarrhea.
However, lactose intolerance does not mean you must cut out milk products altogether. Most people have enough lactose-splitting enzymes in their gut. It is just that their activity is somewhat reduced—down to about 10 to 15 percent of their initial level, let’s say. So if you notice your stomach feels better when you don’t drink that glass of milk, you can simply use trial and error to find out just how much your body can deal with, and how much dairy produce it takes to make the problems come back. A nibble of cheese or a splash of milk in your tea or coffee will usually be fine, as will the occasional milk pudding or cream filling in your cake.
A common children’s counting rhyme in Germany, comparable to the “eeny, meeny, miny, moe” of the English-speaking world, translates as “Ate cherries, drank water, got tummy ache, went to hospital!” The most common food intolerance in the Western hemisphere is trouble digesting the fruit sugar fructose, with about 40 percent of the population affected. Fructose intolerance can be the result of a severe hereditary inability to metabolize fruit sugar, which causes patients’ digestive systems to react to even the tiniest amounts of the substance. But most people affected by fructose intolerance actually have a condition more accurately described as fructose malabsorption, and they experience problems only when they are exposed to large amounts of the sugar. When fructose is described on food packages as “fruit sugar,” consumers often assume it is a healthier, more natural option. This explains why food manufacturers choose to sweeten their products with pure fructose, and consequently why our digestive system is exposed to more of this type of sugar than ever before.
An apple a day would not present a problem to most people who are fructose intolerant if it weren’t for the fact that the ketchup on their fries, the sweetening agent in their breakfast yogurt, and the can of soup they heated up for lunch, all contain added fructose. Some types of tomato are specially bred to contain large amounts of this sugar. Furthermore, globalization and air transport mean that we are now exposed to a previously unheard-of overabundance of fruit. Pineapples from the tropics nestle on our supermarket shelves in the middle of winter, next to fresh strawberries from Mexico, and some dried figs from Morocco. So, what we label a food intolerance may in fact be nothing more than the reaction of a healthy body as it tries to adapt within a single generation to a food situation that was completely unknown during the millions of years of our evolution.
The mechanism behind fructose intolerance is different once again from that involved in the digestion of gluten or lactose. The cells of people with hereditary fructose intolerance contain fewer fructose-processing enzymes. That means fructose may gather in their cells, where it can interfere with other processes. Fructose intolerance that appears later in life is thought to be caused by a reduced ability of the gut to absorb fruit sugars. Such patients often have fewer transporters (called GLUT5 transporters) in their gut wall. When they ingest even a small amount of fruit sugar—for example, by eating a pear—their limited transporters are overwhelmed and, as with lactose intolerance, the sugar from the pear ends up feeding the flora of the large intestine. However, some researchers question whether a lack of sufficient transporters really is the cause of this problem, since even those without the condition pass some of the fructose they eat into the large intestine undigested (especially after eating very large amounts of it). The problems experienced by such people may be due to an imbalance in their gut flora. When they eat a pear, the extra sugar is gobbled up by the gut bacteria squad, which then produces rather unpleasant symptoms. Of course, the more ketchup, canned soup, or sweetened yogurt they have eaten, the worse their troubles will be.
Fructose intolerance can also affect our mood. Sugar helps the body absorb many other nutrients into the bloodstream. The amino acid tryptophan likes to latch on to fructose during digestion, for example. When there is so much fructose in our gut that most of it cannot be absorbed into the blood and we lose that sugar, we also lose the tryptophan attached to it. Tryptophan, for its part, is needed by the body to produce serotonin—a neurotransmitter that gained fame as the happiness hormone after it was discovered that a lack of it can cause depression. Thus, a long-unrecognized fructose intolerance can lead to depressive disorders. General practitioners and family doctors are only now beginning to include this knowledge in their diagnostic toolkit.
This raises the question whether a diet that includes too much fructose can also affect our mood, even in the absence of an intolerance. For more than 50 percent of people, eating 2 ounces (50 grams) of fructose or more per day (equivalent to five pears, eight bananas, or about six apples) will overtax their natural transporters. Eating more than that can lead to health problems such as diarrhea, stomachaches, and flatulence, and, over longer periods, to depressive disorders. The fructose intake of the average American is currently close to 3 ounces (80 grams) a day. Our parents’ generation, consuming just honey on their toast, far fewer processed foods, and a normal amount of fruit, took in no more than ½ to 1 ounce (only around 16 to 24 grams) a day.
Serotonin not only puts us in a good mood, it is also responsible for making us feel pleasantly full after a meal. Snack attacks or constant grazing on snacks may be a side effect of fructose intolerance if they are accompanied by other symptoms, such as stomachaches. This is also an interesting hint for all diet-conscious salad eaters, since many salad dressings found on our supermarket shelves or at fast-food outlets now contain fructose-glucose syrup (often known as high-fructose corn syrup). Studies have shown that this syrup can suppress leptin, the hormone that makes us feel full, even in people who are not fructose intolerant. A salad containing the same amount of calories but with a homemade vinaigrette or yogurt dressing will keep you feeling full for longer.
Like everything else in the world, food production is constantly changing. Sometimes those changes are good for us and sometimes they are bad. Curing was once the state-of-the-art way of ensuring people were not poisoned by eating rotten meat. For centuries, it was common practice to cure meats and sausages with large quantities of nitrite salts. This gives them a pinkish red, fresh color, and explains why products such as ham, salami, tinned pork, or gammon do not turn the same brown-gray color in the frying pan as an unprocessed chop or steak.
Use of nitrites for food preservation has been highly regulated since the 1980s, due to concerns about possible negative effects on human health. In the United States, sausage and cold meat products must contain no more than 156 parts per million of nitrite salt, approximately one-fifth of the level allowed twenty-five years ago. Rates of stomach cancer have fallen considerably since these regulations were introduced. This shows that what had once been a very sensible meat-preserving technique was in drastic need of correction. Today, canny butchers mix large amounts of vitamin C with small amounts of nitrite to cure their meats safely.
A similar modern reassessment of ancient practices may be in order in the case of wheat, milk, and fructose. It is good to include these foodstuffs in our diet, since they contain valuable nutrients, but it may be time to reassess the quantities we consume. While our hunter-gatherer ancestors ate up to five hundred different local roots, herbs, and other plants in a year, a typical modern diet includes seventeen different agricultural plant crops at most. It is not surprising that our gut has a few problems with a dietary change of that scale.
Digestive issues divide society into two groups: those who worry about their health and pay great attention to their nutrition, and those who get annoyed by the fact that they can no longer invite a group of friends round for a meal without having to go shopping at the pharmacy. Both groups are right. Many people err on the side of caution after hearing from their doctor about a food intolerance and then noticing that they do feel better if they avoid certain foods. They might decide to cut out fruit, wheat, or dairy products, and then often act as if they were poisonous. In fact, most people react to excessive amounts of these foodstuffs without being genetically intolerant to them. Most have enough enzymes to process a small portion of creamy sauce, the occasional pretzel, or a fruit pudding.
However, this does not mean that real intolerances should be ignored. We do not need to swallow every new development in our food culture blindly. Wheat products for breakfast, lunch, and dinner, fructose in practically all processed foods, or milk products long after weaning—it is not surprising that our bodies sometimes rebel. Symptoms like regular stomachaches, repeated bouts of diarrhea, or severe fatigue do not occur for no reason, and nobody should be expected to just accept them as “one of those things.” Even if your doctor has ruled out celiac disease or hereditary fructose intolerance, nobody can deny you the right to avoid certain foods if you notice that doing so improves your general well-being.
Apart from this general overconsumption, antibiotics, high stress levels, or gastrointestinal infections, for example, can also trigger temporary sensitivities to certain foods. When the body has returned to a healthy equilibrium, even a sensitive gut can usually sort itself out. Then there is no need to impose a lifelong ban on certain products, but simply to make sure you consume them in quantities your system can easily cope with.