Hunger & Obesity

Hunger and Obesity

PKGhatak,MD

One cannot escape from the fact that we are getting heavier and fatter.

To help us keep in good shape - lean, thin and healthier - there are cable channels for cooking, food, diet and exercise, and not to speak of the print media - they are all over.

Why then is the opposite happening.

Maybe we are asking the wrong questions. We should be asking what makes us feel hungry.

Hunger is a sensation registered in the brain as a strong desire for food or to feel the need to eat. As we fill up the stomach with food and drinks the hunger goes away and returns after certain intervals depending on what we eat and drink. The nerve center for hunger, called the Appetite Center, is located in a part of the brain known as the Hypothalamus. The stomach and small intestine are connected to the appetite center via nerves and the hypothalamus, in turn, is connected with other parts of the brain where the input reaches the conscious level.

 A full stomach sends signals to the appetite center by the Vagus nerve. The appetite center then acts on various neuroendocrine pathways to shut down appetite.  It is an example of a negative feedback loop. Alcohol speeds up stomach emptying and fat delays emptying. A cocktail in a restaurant before dinner increases appetite; a dessert which is essentially a hunk of butter and heavy cream holds food longer in the stomach and makes us feel full. To put it in another way, we are always hungry unless something downregulates the hunger in the appetite center

 Fat cells are known as Adipocytes store excess food (energy) as fat (triglyceride). Until recently it was believed that the number of adipocytes of an individual was fixed at birth and new fat cells did not grow.  If fat cells are removed by Liposuction or other operations, then a person will remain thin. But that is not true. New fat cells grow in adults from pre-adipocytes present in the connective tissue.

 In starved conditions, adipocytes release a breakdown product of fat, called Fatty Acids, which provide needed energy. Excess energy intake (food) over and above the body’s need will make adipocytes bulge with fat and if this process continues over several years the person will be overweight or even obese. Adipocytes, when stretched with loaded fat, release a hormone called Leptin.  Leptin acts on the appetite center directly and via the vagus nerve. It decreases appetite and increases energy expenditure (EEx). Besides leptin, adipocytes produce blood pressure regulating Angiotensinogen, a vascular protective protein called Adiponectin, a blood clotting inhibitor known as Plasminogen activator inhibitor 1, complement called Adiposin or factor D, and cytokines - Interleukin IL6 and Tumor necrosis factor alpha. The net effect of these chemicals on the body is to help to regulate BP, blood sugar, blood lipids, blood vessels’ health, healthy body weight and a competent immune system.
Why does leptin not suppress appetite in an obese individual? The answer is not clear; it is thought due to a biologically inactive leptin secretion or the leptin sensors in the hypothalamus are unable to function normally.

The appetite center is under the influence of several hormones and chemicals. Cortisol, a hormone from the adrenal glands, depresses appetite. Glucagon from the pancreas has a similar effect. Growth hormones and Gonadal hormones also regulate appetite.

High blood sugar and Ketone, a product of protein and fat metabolism depresses appetite.

Of the appetite increasing hormones, a gut peptides Ghrelin is produced by the stomach, it speeds up stomach emptying. Peptide YY and cholecystokinins are produced in the small intestine and directly act on the appetite center and also via the vagus nerve and have a similar effect to ghrelin. Many metabolites (break down products of food) also influence the appetite center.

It may sound strange but it is true that as the person gains weight the mean energy expenditure (EEx) increases.  As EEx increases the person feels hungry because of stimulation of the appetite center. The reverse is also true: with weight loss, the EEx falls and the appetite center is depressed.

Non-Exercise-Activity-Thermogenesis (NEAT) regulates energy expenditure of activities of daily living. Of the total body requirement of energy, the basal metabolic rate (BMR) accounts for 70% of EEx. only 5 to 10% of EEx is required for voluntary activities including work, exercise, etc. A person can burn at the most 400 kcal/ hour by intense exercise, only marathon runners can burn 1200 kcal/hr. To lose 1lb of flesh one has to burn 2,500 kcal.  Now you can understand why you cannot burn all the extra fat by exercising alone.

Hypothalamus produces and releases a number of peptides known collectively as hypothalamic peptides; by the actions of these, it maintains a balance between appetite, fat storage, and energy expenditure. Important peptides in this group are neuropeptide Y, Agouti-related peptide, alpha melanocyte stimulating hormone and melanocyte concentrating hormone. An interaction between hypothalamic peptides and neural pathways via Serotonin, Catecholamine, and endocannabinoid receptors in various target tissues is ultimately responsible for maintaining steady body weight.

Appetite controlling drugs are coming in the market in increasing numbers, but none are found to be safe for long term use. These drugs act directly on the appetite center by increasing the production and release of a chemical group called monoamine. The pharmacological actions of norepinephrine, serotonin, and dopamine are enhanced by monoamine. These drugs may increase the risk of heart disease and high BP, also insomnia and nervousness.

Obesity is defined as a condition where there is an excess adipose (fat) tissue mass. Body mass index (BMI), height-weight nomograms, the thickness of skin fold of arms, waist-hip ratio are helpful to identify obese individuals; but not all agree on a particular standard. An underwater determination of weight-volume ratio is an accurate measurement of obesity, but it is not a practical method. A normal BMI is 20 to 25 kg/m2. A person is considered overweight if the number is between 26 and 29 and over 30 is considered obese. An increase in the waist-hip ratio of over 0.9 in women and 1.0 in men is taken as obesity.

The cause of obesity is unknown. Several factors have been implicated: hereditary, environment, cultural, habits, a viral infection of the GI tract, and sleep deprivation. The mutation of genes is also important in obesity. OB gene, db gene, POMC gene, tub gene, fat gene Ag RP gene, MC4R gene, Trk gene are all involved in one or another.

Obesity is associated with endocrine disorders like Cushing's syndrome, Hypothyroidism, Insulinoma, Hypogonadism and mental retardation.

Everyone should know obese people are at risk of developing heart disease, diabetes, high cholesterol, high BP, arthritis, and disturbances in sex-hormones related diseases.

The bottom line: there is no one cause of obesity. The body’s own regulating system of appetite, energy storage, and energy expenditure must be derailed before weight gain starts. If the process is allowed to progress unchecked for years the person will be overweight or even obese.

 

edited 2020.

 

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