Keto’s dark side: The health risks behind rapid weight loss

Few diets have become as alluring in recent years as the ketogenic diet. The principle is simple: eat fewer carbohydrates, consume more fat and protein and, in many cases, watch the weight fall off quickly, at least at first. For many people, it sounds like a straightforward solution in a confusing world of nutrition. But make no mistake: Behind that promise lies an extreme diet that may carry a significant health cost. “In the classic, traditional ketogenic diet, people eat mainly fat and protein and sharply reduce their carbohydrate intake,” explains Dr. Sigal Frishman, head of Nutrition Department at Rabin Medical Center (Beilinson) and chief dietitian of Clalit Health Services’ hospital division. “The body normally uses glucose as its main source of energy. But when people do not consume enough carbohydrates, the body begins breaking down fat, producing ketone bodies in the process. The goal of the keto diet is to maintain a steady level of ketones in the blood so the body uses them for energy instead of glucose.” The ketogenic diet, Frishman notes, did not begin as a weight-loss trend. It was originally developed as a therapeutic tool for children with drug-resistant epilepsy. “Doctors found that in children with epilepsy and seizures, the brain could receive energy from ketone bodies rather than glucose, and in some cases the diet could help reduce seizures or even stop them,” she says. Over time, however, keto moved beyond its original medical use and began attracting people with excess weight, diabetes or elevated triglyceride levels. The appeal is easy to understand: Sharply reducing carbohydrates can, at least in the short term, improve certain measures related to blood sugar and blood fats. “Many people who follow a short-term ketogenic diet lose weight very quickly,” Frishman says. “There is a very sharp decline in appetite, and when there is no appetite, people lose weight.” Beyond its physiological effects, keto also has a powerful psychological appeal because it offers a simple set of rules. “For many people, a strict, black-and-white regimen is easier to follow than broad dietary guidelines,” Frishman says. “It is easier to embrace a rule such as: Eat as much meat, chicken, fish, eggs, butter and full-fat cheese as you like, and everything will work out.” But that simplicity is also part of the problem. When a diet eliminates a major food group and alters how the body produces energy, it is no longer merely a weight-loss method. It becomes a significant nutritional intervention, and that is where the risks begin. The main risk: Higher LDL cholesterol and atherosclerosis The chief concern among experts regarding the ketogenic diet involves a measure that people do not necessarily feel in their bodies or see on the scale: an increase in LDL, commonly known as “bad” cholesterol, which is linked to atherosclerosis and elevated cardiovascular risk. Dr. Chen Gurevitz, head of the preventive cardiology center in Rabin Medical Center says the debate surrounding the ketogenic diet is complex. “What makes this diet so appealing is the rapid weight loss,” she says. “Cutting carbohydrates can also lead to significant improvements in blood sugar levels, diabetes control, triglycerides and fatty liver.” But weight loss and improved blood sugar do not tell the whole story. One of the central questions is what happens simultaneously to the blood vessels, and particularly to LDL levels. “LDL is the main driver of atherosclerosis in the body, and there is a direct connection between LDL and heart attacks, strokes and mortality,” Gurevitz says. “We know from hundreds, if not thousands, of studies that as LDL rises, the risk of cardiovascular disease rises as well.” According to Gurevitz, the concern is not merely theoretical. In some cases, doctors encounter the consequences directly. “We have seen quite a few patients arrive at cardiac catheterization laboratories with a heart attack or requiring an urgent catheterization, and they told us that they had begun a ketogenic diet six months or a year earlier,” she says. “When we examine their LDL levels, we find high and sometimes extreme values. In some cases, these may have been people who already had elevated LDL, then began this diet and sent their LDL soaring. It simply gave another push to the atherosclerosis that blocked their artery.” Not everyone who follows a ketogenic diet experiences such a rise. In some people, LDL levels change very little, and some reports have even found a decline. Gurevitz warns, however, that one group can respond far more dramatically. The phenomenon, which has recently drawn growing attention, is known as “lean mass hyper-responders.” “Some people on a ketogenic diet see little change in their LDL levels, and some reports have even found a decline,” she says. “But there is another group whose prevalence remains unclear. They are typically not overweight and do not begin the diet with obesity, yet their LDL levels can rise dramatically in response to keto.” For some people, the rise in LDL levels can be dramatic, and in such cases, she says, LDL can double or even triple. “We reach truly extraordinary numbers that were once usually seen only in patients with very rare inherited cholesterol disorders,” Gurevitz says. “In these patients, both LDL and ApoB, a protein that reflects the number of atherosclerosis-causing cholesterol particles in the blood, rise sharply. Both are known to be major drivers of atherosclerosis." That is also where the uncertainty in the research becomes important. It remains unclear whether the diet’s potential benefits outweigh its possible risks. “In cardiology, we always follow what is strongly supported by research, and there is not enough research on the ketogenic diet,” Gurevitz says. “There is no evidence that a ketogenic diet is superior to other diets regarded as healthy, such as the Mediterranean diet, which has been shown in numerous studies to be the preferred diet in terms of cardiovascular outcomes.” Keto supporters sometimes argue that higher LDL during ketosis is not necessarily dangerous because it results from a unique change in fat metabolism. Gurevitz treats that claim with considerable caution. “The truth is that we do not have enough information to say that,” she warns. “It is a very problematic statement and may also be irresponsible.” Those who need to be particularly careful, she says, include people who already have high cholesterol, a family predisposition to cholesterol disorders, known atherosclerosis or a tendency to absorb more cholesterol from food. “These people need to be cautious about a ketogenic diet, and it would be better for them to choose other diets,” she says. The nutritional cost: Less fiber and less variety Beyond cholesterol and cardiovascular risk, another major concern with the ketogenic diet is what it removes from the plate. Cutting carbohydrates sharply may eliminate not only sugar, sweets and processed foods, but also nutrient-rich foods that play an important role in a balanced diet. “We know how important fiber is,” Frishman says. “It is found in foods such as whole grains, vegetables and fruit. Different fruits and vegetables provide different types of fiber, while their colors and polyphenols supply valuable antioxidants, along with a wide range of vitamins.” In strict versions of keto, she says, many of the foods that provide those nutrients are almost entirely removed. “There are almost no vegetables, no grains and no legumes, which are also an important source of fiber,” Frishman says. “People barely touch these foods.” “The purpose of fiber is first of all to increase the diversity and quantity of the bacteria in our intestines. Those bacteria produce metabolites that protect the body, improve the immune system, maintain a healthy intestine and are also linked to protection from disease.” The effect of reduced fiber intake is not limited to constipation, although that is among the more common and immediate consequences. According to Frishman, a major reduction in fiber may also affect the microbiome, the population of bacteria in the intestines, and impair the gut’s function as a barrier between the digestive system and the bloodstream. “It can lead to a smaller quantity and variety of intestinal bacteria, with potential consequences for the immune system and the body’s defenses,” she says. “Intestinal permeability may also increase. The intestine is a line of defense between the food we eat and the blood. Food must be broken down, and substances that we want to enter the bloodstream need to pass through. But there are also substances we do not want entering the blood because they may activate systems we do not want activated.” “When we do not eat enough fiber, we do not have enough of the bacteria that help keep the intestine from becoming overly permeable.” Low fiber intake has also been linked to an increased risk of intestinal diseases, particularly colorectal cancer. “A classic ketogenic diet sometimes also includes a large amount of saturated fat, which has also been linked to a higher risk of certain diseases,” Frishman says. In other words, the problem is not only what people eat, but also what they stop eating. A diet based mainly on fat and protein may provide fewer fibers, fewer plant-based components and less diversity, all of which are essential to a diet that promotes long-term health. The risks that receive less attention Beyond cholesterol, heart health and nutritional quality, the ketogenic diet has several other weaknesses that are important to recognize, particularly among people who already have underlying health conditions or a predisposition to certain disorders. One is the risk of kidney stones and elevated uric acid. “In principle, a ketogenic diet increases acidity, and failing to drink enough can raise the risk of kidney stones,” Frishman explains. “Some people consume significantly more animal-based foods, and animal proteins contribute to acidity. It is therefore important to drink plenty of water to regulate and reduce that acidity, which naturally lowers the risk.” Another concern involves uric acid, a byproduct of the breakdown of purines, which are found in relatively high amounts in animal-based foods. “When uric acid levels rise in the blood, it can accumulate in the joints and cause gout, which is characterized by pain and sometimes acute joint inflammation,” Frishman says. People with diabetes should be especially cautious when considering a ketogenic diet. “When carbohydrate intake is eliminated, blood sugar levels fall,” Frishman explains. “Because some diabetes medications also lower blood sugar, combining the two can lead to hypoglycemia, a dangerously low blood sugar level. People taking glucose-lowering medication should therefore not begin the diet without closely monitoring their blood sugar.” A strict ketogenic diet is also unsuitable for certain groups without close medical supervision. “Pregnant women should certainly avoid it,” Frishman stresses. “Children should not follow it without medical oversight either.” Even people without underlying health conditions may face a simpler but equally important problem: Keto is extremely difficult to sustain over time. Severely restricting carbohydrates, avoiding everyday foods and living with a constant sense of deprivation can make the diet unmanageable for many people. “That can lead to a significant rebound because most people find it very difficult to live without carbohydrates,” Frishman says. “Most studies show that after a year, many return to eating even less healthfully than before because of the prolonged restriction.” The bottom line: Less extremism, more monitoring Does that mean the ketogenic diet should be ruled out altogether? Not necessarily. Here too, the experts say, the details matter. “I would not dismiss it entirely,” Frishman says. “It can be modified. For some people, it may provide the clear initial framework they need because a black-and-white approach is easier to follow. But I would choose a more Mediterranean-based approach, perhaps one that is less strict than full keto, while monitoring blood tests and the results.” Some people now follow modified versions of keto built around healthier fats such as tahini, olive oil, avocado and olives rather than saturated fat, while also choosing leaner protein sources, including chicken, fish and lower-fat cheeses. “Once they have achieved results, changed their habits and established a healthier routine, the goal should be to gradually transition away from such a restrictive regimen toward a more balanced and sustainable diet,” she says. Even for those who decide to try keto, the cardiological advice is clear: Do not proceed without monitoring. Gurevitz stresses that blood lipid levels should be checked soon after starting the diet, rather than relying solely on weight loss or improved blood sugar readings. “People need to pay very close attention to their blood tests,” she says. “They should conduct careful monitoring and even undergo blood tests several weeks into the diet to see what is happening to their lipid profile, specifically LDL cholesterol, and determine how their body is responding.”