Cellular respiration and dieting

In my younger years I had maintained a body mass index (BMI) in the category of “normal weight” (in the lower range) without being conscious of my diet and without any help from so-called diet or weight loss products and programs. As I reached the golden age, my body catabolism (breaking-down metabolism) began to slow down and was exceeded by anabolism (building-up metabolism). So my BMI is now in the category that approaches “overweight” despite being mindful of my diet most of the time (i.e., except during special events or when I entertain guests in the house and cannot resist gustatory cravings).

Role of nutrients in cellular energy production

The metabolism in our body includes the cells’ biochemical processes that transform food nutrients to energy. The foods that we eat contribute to the ability of the cells to do biochemical reactions. Proper nutrition helps the cells produce the needed energy though respiration. The biochemical pathways include the citric acid cycle (also known as tricarboxylic acid cycle or Krebs cycle) and the respiratory chain (also called electron transport chain), which are involved in transforming carbohydrates, fats and proteins into carbon dioxide, water and energy. The citric acid cycle is a series of chemical reactions that make up one phase of cellular respiration, before the metabolites enter the respiratory chain that uses oxygen to achieve energy production within the cells. Thus, cellular respiration is a necessary process for the maintenance of life.

The carbohydrates, fats and proteins from the diet can propel metabolism by contributing to the citric acid cycle. Carbohydrates are broken down to glucose, and through the glycolytic pathway the cells process glucose to form pyruvate that can then be converted to acetyl coenzyme A. Fatty acids which are derived from fats are converted to acetyl coenzyme A. Amino acids, the building blocks of proteins, contribute by forming pyruvate, acetyl coenzyme A, oxaloacetate or other metabolic intermediates of the citric acid cycle. The citric acid cycle is initiated by key metabolites, such as acetyl coenzyme A and oxaloacetate.

Carbohydrates, fats and proteins help our body cells harness chemical energy. A proper diet with carbohydrates, fats and proteins provides the body with nutrients that serve as sources of energy for the cells, as well as maintain protein-rich tissues, such as muscles. A balanced diet rich in micronutrients also affects the citric acid cycle. The conversions of citric acid cycle metabolites are controlled by enzymes that involve vitamin-derived coenzymes. Also, the conversion of pyruvate to acetyl coenzyme A requires coenzymes derived from thiamine (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), and lipoic acid. When these nutrients are deficient, the ability of the cells to do the chemical reactions in the citric acid cycle is inhibited, eventually diminishing energy production.

We are what we eat

There is a saying that goes “You are what you eat,” which could mean that you attain good health if you eat just enough good food. In fact, many people are now mindful of their health and looks so that they spend a lot of resources to stay fit.

With a proper diet, under normal conditions, all the reactions in the citric acid cycle occur efficiently and energy is produced without any undesirable byproduct. Excessive accumulation of acetyl coenzyme A, particularly from a diet high in carbohydrates and fats, can lead to excessive production of storage fats in the form of triglycerides that may show up as bulges in the body. Acetyl coenzyme A is also a material for the biosynthesis of cholesterol which when left unchecked may lead to a high blood cholesterol level and can result in the thickening of arteries that is a predisposing factor in cardiovascular disease.

We gain weight and tend to grow fat when we eat more than what is needed for the energy requirement of the body. As we grow old, we tend to have less strenuous activities and so less energy requirement. High food intake can therefore lead to excessive stored fats.

Nutritional supplement for ‘weight loss’

Many nutritional supplements are available in the market, and are claimed to have beneficial effects in weight loss. Some young people I know have taken L-carnitine as a drink supplement. Let me clarify this based on studies on L-carnitine.

L-carnitine is synthesized in our body from the amino acids L-lysine and L-methionine. In healthy individuals, enough L-carnitine that meets the body needs is produced. In older or sick individuals, the biosynthesis of L-carnitine may be reduced due to a decline in cellular function. L-carnitine can be obtained from meat, fish and dairy products, among other foods.

Carnitine occurs in two isomers: L-carnitine is the biologically active form, whereas its enantiomer (mirror image isomer) D-carnitine is not biologically active. L-carnitine is needed for the transport of fatty acid from the cytosol to the mitochondrion of the cell. The transport process occurs prior to the oxidation of fatty acid to acetyl coenzyme A needed for the production of energy through cellular respiration.

Many studies including the potential of L-carnitine as a weight loss product and as a drug for several disorders, for e.g., heart disease, as well as fatigue and weight loss due to advanced diseases, have been done. Regarding the use of L-carnitine as weight loss product, R.G. Villani and colleagues at the Royal Melbourne Institute of Technology, Australia found out: “L-carnitine supplementation combined with aerobic training does not promote weight loss in moderately obese women” (International Journal of Sport Nutrition and Exercise Metabolism, 2000, vol. 10, pp. 199-207). In relation to the use of L-carnitine as drug, S.M. Marcovina and colleagues at the University of Washington, WA, USA noted: “L-carnitine and its derivatives show promise in the treatment of chronic conditions and diseases associated with mitochondrial dysfunction but further translational studies are needed to fully explore their potential” (Translational Research, 2013, vol. 161, pp. 73-84). In their review of substantial scientific literature, J. Pekala and colleagues at Wroclaw University of Technology, Poland concluded: “Further research is warranted in order to evaluate the biochemical, pharmacological, and physiological determinants of the response to carnitine supplementation, as well as to determine the potential benefits of carnitine supplements in selected categories of individuals who do not have fatty acid oxidation defects” (Current Drug Metabolism, 2011, vol. 12, pp. 667-678).

Thus far, there is no well-founded and far-reaching scientific evidence to show that L-carnitine works for losing weight or dieting. There is no sound scientific basis for the use of L-carnitine by individuals who do not have disease conditions that affect fatty acid oxidation and utilization.

Back to basic biochemistry

We can benefit from nutritional strategies that optimize metabolism and energy production, and remain in shape. Proper diet and regular physical exercise could be helpful. I myself have to trim down a bit. So I have to keep in mind the metabolic pathways via the citric acid cycle and the respiratory chain. Just enough food for body maintenance, and physical activities to burn down excessive stored fats work well.

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Dr. Elsie C. Jimenez, a biochemist and molecular biologist, is a professor emeritus at the University of the Philippines Baguio. She taught biochemistry for many years. Her research interests include protein chemistry, proteomics, and toxinology. She also has an interest in nutritional biochemistry and its applications to health and wellness. She can be reached at [email protected].