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Under nature's original plan, the fatter you are, the better your survival.
This is because fats are an important source of energy that your cells need for function, growth and development. In ancient times, food was not always available. So being fat meant that our ancestors had an extra store of fuel to see them through times of scarcity, such as during winter or during periods of famine.
The body derives energy from burning three macronutrients:
- Glucose from starchy carbohydrates;
- Fatty acids from fats;
- Amino acids from proteins..
The ancient diet appears to contain more fat and protein than carbohydrates. This is because many of the high-carbohydrate foods that we eat today - including grains, beans and many types of root and leafy vegetables - are toxic unless well cooked. Yet cooking became widespread only about 50,000 to 100,000 years ago. Before that, our early ancestors would not have been able to eat those foods.
So our ancestors were likely to have been "fat burners", using fat as their primary fuel. Fat is a superior fuel especially for the brain, although it is a slower energy-producing fuel than glucose.
Survival of the fattest
Glucose is like jet fuel that gives an instant energy boost. It is an ideal fuel in emergencies like the "fight-or-flight response" and acute mental stress. Glucose also gives us immediate relief from hunger and mental fatigue. That's why when you eat a meal containing the three macronutrients, glucose is the preferred fuel for energy production while fatty acids and amino acids are diverted to storage. If you eat a mainly glucose-rich diet, the excess glucose is converted to glycogen and fat for storage.
As an emergency fuel, glucose is critical for survival. Thus, our genetic design incorporates certain mechanisms to source glucose and to convert the surplus into fat reserves for future use. This is achieved through the work of "thrifty genes", which act to accumulate and conserve fat reserves. Their goal is "the survival of the fattest".
From Fat burners to Glucose Burners
The Agricultural Revolution made glucose cheap and readily available. Since then humans began to use glucose as the primary fuel and most of us today have become glucose burners. In addition, the addictive nature of glucose has made us carbohydrate addicts. Fat doesn't have this addictive effect.
Glucose, however, is toxic in large amounts. In the bloodstream, a safe level of glucose is below 85mg/dl. To keep glucose levels safe, the pancreas releases insulin to perform two roles:
- Escort excess glucose into the cells to burn off as energy;
- Covert surplus glucose into safer products such as glycogen and fat for storage.
So the more glucose you consume, the fatter you become. Thus, glucose should not be our primary fuel. We need very little blood glucose for emergency energy. Most of us today are eating just too much glucose to our detriment. We are now in deep trouble, suffering from a host of modern lifestyle diseases linked to excess glucose. The potential problems of glucose will be discussed more thoroughly in Book 2 Chapter 1, Fuels for Life and Book 2 Chapter 2, Staying Lean.
Earlier, I mentioned that thrifty genes help accumulate fat reserves for periods of food scarcity. Such genes are evident in animals too. Hibernating animals like bears eat excessively during autumn when food is abundant. They double their weight and become obese and diabetic. However, this fat reserve is used up during winter. By spring, the animal is lean again.
Significance of Thrifty genes and pregnancy
A similar situation occurs in a woman during pregnancy. Special hormones make her eat more and accumulate fat during pregnancy, so that she can nourish her growing foetus and later breastfeed her infant. During pregnancy, she can gain considerable weight and develop diabetes (commonly known as gestational diabetes). After childbirth, the woman clears her fat reserves through breastfeeding. If she does not breastfeed, or does so only briefly, she will continue to carry her fat reserves. Total breastfeeding for at least six months will return a woman to her pre-pregnancy weight.
Abundance of food and chronic illness
Apart from pregnant women, the rest of us no longer need to accumulate fat reserves because we now have a constant abundance of food. Because of this, ancient thrifty genes are likely to cause widespread modern-day chronic diseases like obesity and related health problems like type 2 diabetes.
Thrifty genes promote weight gain in two main ways –
- they make you eat more to create more fat reserve and
- they make it difficult for you to spend the fat reserve.
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.