“You are what you eat” is an expression usually deployed as an insult — a joke made at the expense of others about their diets by those asserting their healthy diets and fitness.

But it’s no joke. What you eat has major effects on you, something the originator of the phrase, the renowned nineteenth-century French gastronome Jean-Anthelme Brillat-Savarin in Physiology of Taste, understood when he wrote: “Tell me what you eat, and I will tell you what you are.”

It has been two centuries since Brillat-Savarin made connections between the external (social and political) and internal (anatomy, histology). Yet many of us still struggle to make this connection ourselves and act in spite of all the scientific advances in understanding the biomes within and on us.

The 27 feet of bowel we all have helps us to digest food and absorb it. It degrades complex fats, carbohydrates, and proteins and gives you energy.

But it is also prevents harmful organisms from gaining a foothold. This microbe-host relationship is vital to our wellbeing and is thus deeply affected by diet.

Research has revealed unambiguous and startling differences in the health and effectiveness of gut microorganisms of thinner versus obese people. Studies for decades have shown that your diet can increase or decrease the risk of cardiovascular disease in both men and women, which is the most common cause of death in developing countries.

Yet our food chain remains as much about class, politics, and calories than what might be right for you — and be doing to you inside and out. While food and diet research and debate are constantly evolving, we need another gut check as the health of the communities of organisms essential to our biological souls hangs in the balance— something that antibiotics not only cannot fix but can also hurt.

Dr. Bob is the author of IMMUNITY STRONG: Boost Your Body's Natural Healing Power and Live to 100.