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Thrifty genes make us eat more, particularly more carbohydrates, in the following ways:
Thrifty genes give us a strong preference for the sweet taste, to make us eat foods that contain glucose and fructose, such as ripened fruits, starchy root vegetables and grains. In modern societies, we also develop a liking for cakes, pastries and other products made with flour (glucose) and sugar (glucose and fructose).
Fructose contributes even more to weight gain compared to glucose. The combination of fructose and glucose is especially fattening. It stops glucose from being used up as energy and sends it straight to the liver for fat production.
Although starchy carbohydrates like root vegetables and grains are not sweet to entice your sweet desire. Nature has designed another mechanism called addiction for glucose to make you crave for them. The digestion of these starchy carbohydrates produces glucose which releases dopamine - the anti-stress hormone that makes us feel good. This makes us crave glucose when we feel depressed. We would eat glucose as a "comfort food" in order to feel good again. This makes glucose addictive. It also makes us eat glucose even when we are not hungry or depressed. The way out of this glucose addiction is to eat a low glycemic diet. This will be discussed in greater detail in Book 2 Chapter 2 on Staying Lean.
Insulin and insulin resistance
Insulin sends glucose to the cells for energy production. In doing so, it keeps the blood glucose level in the safe zone. Unfortunately, the action of insulin is not perfect and it often results in blood glucose levels falling too low. When this happens, it causes strong hunger pangs and makes us feel an urgent need to eat more glucose. This drop in blood glucose usually follows a blood glucose surge after we eat too much glucose, or when we eat refined carbohydrates that are easily and quickly digested.
Over time, excessive glucose consumption leads to insulin resistance. This prevents glucose from going into cells for energy production. Instead, it drives excess glucose immediately into the liver for conversion into fat. It also blocks the body from releasing its fat reserves. Insulin and insulin resistance are designed to accumulate fat reserves.
Leptin/ leptin resistance
As fat reserves accumulate, fat cells release leptin to stimulate the feeling of satiety and block off the desire for sweet foods. Leptin is the hormone that regulates energy reserve. When your appetite centre is no longer sensitive to leptin, you develop leptin resistance. And your appetite becomes unbridled.
Leptin resistance appears when you eat incessantly, not listening to your satiety-hunger signals. You accumulate a massive fat reserve and become obese. This comes about because we no longer eat because we are hungry. Rather, we eat because "it's time". For those on the move, due to work, travel or other reasons, eating before we feel hungry is not unusual. Just don't make it a habit, as such habits promote weight gain.
Excessive fructose consumption leads to leptin resistance. Since fructose is often taken with glucose, such as in fruits, sugar and high fructose corn syrup, leptin resistance develops when we eat excessive amounts of such foods - and continue to eat even after we are full.
Satiety needs a bit more explanation. It is not just a feeling of fullness, but also a feeling of satisfaction. Fullness is regulated by ghrelin, a hunger hormone released by the stomach when it is empty. When your stomach starts filling up, ghrelin is no longer released and your feeling of hunger ceases. This effect takes time. If you eat until you are nearly full, you may feel totally full about 10 minutes later. So if you feel like you want to eat more after a meal, it's a good idea to wait a while before deciding if you really want to do so.
Satisfaction is regulated by the balance of nutrients. Nowadays, many people feel dissatisfied even when they are full. Such urges are not always due to greed or "bad habits”. More likely, they indicate that your meal was imbalanced or lacking in certain nutrients. So your body continues to crave foods in order to acquire the desired nutrients.
For example, a craving for sweets might mean that your meal was too salty. Or if your meal was too bland, you might yearn for some salty or oily food. I know of former vegetarians who experienced strong cravings for meat - and they felt better after giving in to those cravings. They probably did not balance their vegetarian diets properly.
Satiety is, in fact, a good indication that whatever diet plan you follow is balanced and nutritious. If you have constant cravings even after a full meal, you need to seriously examine the foods you eat. You also need to respond appropriately. If you eat foods without the desired nutrients, you will remain dissatisfied and you continue to crave more food.
Today, many people ignore their hunger-satiety signals. They eat not because they feel hungry but because it has become a routine or habit. Or, despite feeling hungry, they may delay eating because of work and other commitments. After that, they feel too hungry and they overeat. If you are not in sync with your body signals, your chances of growing fat - and becoming ill - are that much greater. You have developed insulin and leptin resistance.
This content is adapted, with permission, from Book 1 of 2 : The Wonders of Nutrition by Dr Ang Poon Liat. MBBS, M.MED (PAED), MRCP (UK PAED), FAMS, MD.