The ketogenic diet, characterized by its low-carbohydrate and high-fat composition, has gained popularity as a weight-loss strategy. While proponents tout its effectiveness, understanding the potential side effects and how it impacts the body is crucial. One common observation among individuals following a keto diet is a decrease in appetite. This article delves into the causes of appetite loss on a keto diet, exploring the physiological mechanisms and potential benefits and drawbacks.
What is the Keto Diet?
The ketogenic diet is a low-carbohydrate, high-fat eating plan used for centuries to treat specific medical conditions. In the 19th century, it helped control diabetes, and in 1920, it was introduced as an effective treatment for epilepsy in children for whom medication was ineffective. Today, it's gaining attention as a potential weight-loss strategy. The premise is that by depriving the body of glucose, its primary energy source, an alternative fuel called ketones is produced from stored fat.
When very little carbohydrate is eaten, the body first pulls stored glucose from the liver and temporarily breaks down muscle to release glucose. If this continues for 3-4 days and stored glucose is fully depleted, blood levels of a hormone called insulin decrease, and the body begins to use fat as its primary fuel. The liver produces ketone bodies from fat, which can be used in the absence of glucose. When ketone bodies accumulate in the blood, this is called ketosis. Healthy individuals naturally experience mild ketosis during periods of fasting (e.g., sleeping overnight) and very strenuous exercise. If the diet is carefully followed, blood levels of ketones should not reach a harmful level (known as “ketoacidosis”) as the brain will use ketones for fuel, and healthy individuals will typically produce enough insulin to prevent excessive ketones from forming. How soon ketosis happens and the number of ketone bodies that accumulate in the blood is variable from person to person and depends on factors such as body fat percentage and resting metabolic rate.
Understanding Ketosis and Ketoacidosis
When your body burns fat instead of glucose, this leads to elevated ketone levels, which is called ketosis. Symptoms can include changes in the way your breath smells, appetite, and energy levels. It is important to understand the difference between ketosis and ketoacidosis. Excessive ketone bodies can produce a dangerously toxic level of acid in the blood, called ketoacidosis. During ketoacidosis, the kidneys begin to excrete ketone bodies along with body water in the urine, causing some fluid-related weight loss. Ketoacidosis most often occurs in individuals with type 1 diabetes because they do not produce insulin, a hormone that prevents the overproduction of ketones. However in a few rare cases, ketoacidosis has been reported to occur in nondiabetic individuals following a prolonged very low carbohydrate diet.
Macronutrient Ratios in a Ketogenic Diet
There is not one “standard” ketogenic diet with a specific ratio of macronutrients (carbohydrates, protein, fat). The ketogenic diet typically reduces total carbohydrate intake to less than 50 grams a day-less than the amount found in a medium plain bagel-and can be as low as 20 grams a day. Generally, popular ketogenic resources suggest an average of 70-80% fat from total daily calories, 5-10% carbohydrate, and 10-20% protein. For a 2000-calorie diet, this translates to about 165 grams fat, 40 grams carbohydrate, and 75 grams protein. The protein amount on the ketogenic diet is kept moderate in comparison with other low-carb high-protein diets, because eating too much protein can prevent ketosis.
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The Role of Ketones in Appetite Suppression
Many people report decreased hunger while following a keto diet. The reasons why this happens are still being investigated. However, it’s been suggested that this hunger reduction may be due to alterations to your body’s hunger hormones, along with an increased intake of proteins. The ketones themselves may also affect your brain, helping to reduce appetite.
Appetite-Regulating Hormones
Diet-induced weight loss leads to changes in energy expenditure and appetite-regulating hormones that facilitate weight regain and the return to initial energy homeostasis. Ghrelin, a peptide produced mainly by the stomach, stimulates ghrelin secretion in the hypophysis and has some neuroendocrine activities. However, its orexigenic properties are the most relevant and ghrelin is the only known peripheral orexigenic hormone. In a study of 39 obese adults placed on a ketogenic very low-calorie diet for 8 weeks found a mean loss of 13% of their starting weight and significant reductions in fat mass, insulin levels, blood pressure, and waist and hip circumferences. Their levels of ghrelin did not increase while they were in ketosis, which contributed to a decreased appetite. However during the 2-week period when they came off the diet, ghrelin levels and urges to eat significantly increased.
Cholecystokinin (CCK) is a peptide produced mainly in the duodenum and jejunum that acts on the vagus nerve and directly on the hypothalamic nuclei. CCK is an anorexigenic factor and it reduces food intake, meal size and duration. Three other related hormones are pancreatic polypeptide (PP), amylin, and peptide YY (PYY). PP is a peptide produced by the endocrine pancreas in relation to the caloric content of meals, and it reduces food intake both in rodents and humans. Amylin is a peptide co-secreted with insulin; its main effect on food control is a reduction of meal sizes and food intake. Peptide YY (PYY) is produced in the gut and is similar to PP. PYY is stored in intestinal cells and released into the circulation as PYY3−36, a truncated form of PYY. The release of PYY3−36 is dependent on a meal's caloric and fat content. The glucagon-like peptide 1 (GLP-1) is produced by the cleavage of pro-glucagon gene in the intestine.
The Brain's Hunger/Satiety Centers
Hunger and satiety are two important mechanisms involved in body weight regulation. Even though humans can regulate food intake by will, there are systems within the central nervous system (CNS) that regulate food intake and energy expenditure. This complex network, whose control center is spread over different brain areas, receives information from adipose tissue, the gastrointestinal tract (GIT), and from blood and peripheral sensory receptors. The actions of the brain's hunger/satiety centers are influenced by nutrients, hormones and other signaling molecules. The hypothalamus is the brain's main center responsible for hunger/satiety (H/S) control.
Mayer suggested that depletion of carbohydrate availability leads to hunger, and the hypothalamic centers with receptors sensitive to glucose levels might be involved in the short-term regulation of energy intake. The “feeding center” in the lateral hypothalamic area (LHA), according to the glucostatic theory, reacts to the between-meal fall of blood glucose and stimulates food intake. The LHA contains glucose-inhibited neurons that are stimulated by hypoglycemia, a process crucial to mediating the hyperphagia normally induced by hypoglycemia. Nutrient-sensitive neurons reacting to glucose but also to fatty acids (FAs) concentrations are present at many sites throughout the brain and may play a key role in the neural control of energy and glucose homoeostasis. More recently, other hypothalamic appetite control regions have been identified, including those in the arcuate nucleus (ARC), the periventricular nucleus (PVN) and the dorsomedial hypothalamic nucleus (DMH). These are sites of convergence and integration of many central and peripheral signals, not just macronutrients, that are involved in food intake and energy expenditure mechanisms, e.g., a group of neurons in the ARC stimulating food intake via neuropeptide Y (NPY) and agouti gene-related protein (AGRP). These neurons interact with those producing the anorexigenic pro-opiomelanocortin (POMC) and the cocaine/amphetamine-regulated transcript (CART).
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Gut Microbiome
The gut-brain link is important not only for the hormones produced by the gut, but also for the long-term body weight regulation. Studies in mice indicate that the gut microbiome influences both sides of the energy balance by contributing to nutrient absorption and regulating host genes that affect adiposity. However, it remains uncertain just how important gut microbiota are for nutrient absorption in humans. A cohort study has demonstrated that the nutrient load is a key variable that can influence the gut/fecal bacterial content over short time frames.
Potential Benefits of Appetite Suppression
The reduced appetite experienced on a keto diet can be a beneficial factor for weight loss. A systematic review of 26 short-term intervention trials evaluated the appetites of overweight and obese individuals on either a very low calorie (~800 calories daily) or ketogenic diet (no calorie restriction but ≤50 gm carbohydrate daily) using a standardized and validated appetite scale. None of the studies compared the two diets with each other; rather, the participants’ appetites were compared at baseline before starting the diet and at the end. Despite losing a significant amount of weight on both diets, participants reported less hunger and a reduced desire to eat compared with baseline measures. The authors noted the lack of increased hunger despite extreme restrictions of both diets, which they theorized were due to changes in appetite hormones such as ghrelin and leptin, ketone bodies, and increased fat and protein intakes.
Preventing Increased Appetite
The clinical benefit of a ketogenic diet is in preventing an increase in appetite, despite weight loss, although individuals may indeed feel slightly less hungry (or more full or satisfied).
Potential Drawbacks of Appetite Suppression
While appetite suppression can aid weight loss, it's essential to ensure adequate nutrient intake. Some negative side effects of a long-term ketogenic diet have been suggested, including increased risk of kidney stones and osteoporosis, and increased blood levels of uric acid (a risk factor for gout). Possible nutrient deficiencies may arise if a variety of recommended foods on the ketogenic diet are not included.
Nutrient Deficiencies
It is important to not solely focus on eating high-fat foods, but to include a daily variety of the allowed meats, fish, vegetables, fruits, nuts, and seeds to ensure adequate intakes of fiber, B vitamins, and minerals (iron, magnesium, zinc)-nutrients typically found in foods like whole grains that are restricted from the diet.
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Long-Term Sustainability
Following a very high-fat diet may be challenging to maintain. Eliminating several food groups and the potential for unpleasant symptoms may make compliance difficult.
Practical Considerations
Monitoring Ketone Levels
Several key signs and symptoms can help you identify whether you’re in ketosis. Ultimately, if you are following the guidelines of a keto diet and staying consistent, you should be in some form of ketosis. If you want a more accurate assessment, monitor ketone levels in your blood or breath every week. The most reliable and accurate method of measuring ketosis is to measure your blood ketone levels using a specialized meter. The meter measures your ketone levels by calculating the amount of beta-hydroxybutyrate (BHB) in your blood. This is one of the primary ketones present in the bloodstream. According to some experts on the keto diet, nutritional ketosis is defined as blood ketones equal to or above 0.5 millimolar (mM). Another way to measure blood ketone levels is a breath analyzer, which monitors acetone, one of the three main ketones in your blood during ketosis.
Consulting with Professionals
The exact ratio of fat, carbohydrate, and protein that is needed to achieve health benefits will vary among individuals due to their genetic makeup and body composition. Therefore, if one chooses to start a ketogenic diet, it is recommended to consult with one’s physician and a dietitian to closely monitor any biochemical changes after starting the regimen, and to create a meal plan that is tailored to one’s existing health conditions and to prevent nutritional deficiencies or other health complications.