Don’t treat your skin like a digestive organ!
There are distinct anatomical and physiological differences between the skin and the digestive tract that make them react differently. These differences include:
You may have heard that a basic, or alkaline, diet rich in leafy greens and other vegetables is great for your health. And some acidic foods, like lemons and other citrus fruits, actually leave an alkaline ash in your digestive tract.
Your stomach has a low pH, of approximately 2, so it neutralizes alkaline foods and residues much more readily than acidic foods.
On the other hand, the exact opposite is true with your skin: Basic, or alkaline, products harm your skin. In fact, the skin’s delicate acid mantle is destroyed by agents with a pH higher than 7.0 (Skin Pharmacology and Physiology, 2006). Not only that, but your skin’s immunological protection is maintained at a neutral pH (Acta Dermato-venereologica, 1990).
So, obviously, if a food is acidic, it is best for the skin. If a food is alkaline or leaves an alkaline ash when digested, it is best to be applied to the skin.
The skin evolved to protect. On the other hand, the digestive tract evolved to absorb.
Obviously, there are key differences in protection and absorption. The skin requires large molecules to be microencapsulated, esterified, or micronized in order to be absorbed. On the other hand, the digestive tract readily breaks down and subsequently absorbs large molecules without much of an issue.
That’s why large proteins like collagen have been found to be beneficial when ingested, but not when topically applied. Collagen supplements can increase skin’s elasticity and suppleness with regular use over time (Bioscience, Biotechnology, and Biochemistry). However, topical application of collagen does very little for the skin except hydrate (Cosmetic Dermatology), as proteins need to be broken into much smaller amino acids to be absorbed by the skin.
Enzymes affect what the digestive tract can absorb. But there are no enzymes in the skin that break down skin care ingredients, so absorption is different.
Turmeric is a great example. A spice used in Indian food for centuries, turmeric has been proven to have potent anti-carcinogenic activity when it is used as a food additive.
But when applied to the skin, turmeric is still a double-edged sword, because there are no enzymes (like there in the digestive tract) to break down the tetrahydrocurcumin in turmeric. What happens? Turmeric will dye the skin orange, unless the tetrahydrocurcumin is removed from it, but tetrahydrocurcumin is potent and hard to remove (Molecular Nutrition and Food Research, 2011).
4. Sun Sensitivity
We all know that tomatoes and lemons are great to eat, right?
But applying citrus fruits to your skin is asking for trouble. Matrix metalloproteinases are enzymes that degrade collagen. At least one study shows applying lycopene or beta-carotene increases matrix metalloproteinase formation 1.5 to 2 times after UVA light exposure! (Free Radical Biology and Medicine, 2002).
When used on the skin, lemon is also sun-sensitizing, to the point that regular use of lemons on the skin can cause blistering (Photoimmunology, Photodermatology, and Photomedicine, 2005).
Again, another example that your skin is NOT a part of your digestive tract!
5. Blood Flow
When it comes to digestion, more blood is a good thing. That’s why the largest blood vessels away from the heart lead to the digestive organs — you need more blood flow to the region to digest effectively!
On the other hand, the microcirculation in the skin (particularly the face and neck regions) can benefit from careful regulation of tightening (or constricting) blood vessels where puffiness occurs, but loosening (or dilating) blood vessels slightly where dark circles, hyperpigmentation, and other instances of blood pooling occur.
For instance, when caffeine is consumed, it can cause chest pain, arrhythmias, trouble breathing, hallucinations, and convulsions (amongst other symptoms) when overdosed (Healthline, 2014).
On the other hand, topical application of caffeine has been shown by Lu et. al to have a mild sun-protective effect. The exact mechanism by which caffeine achieves these aims is not yet known, but it may be related to the fact that the caffeic acid in caffeine has been found to have some antioxidant activity. Topical application of caffeine additionally dehydrates skin cells, making the skin temporarily appear smoother, and may reduce the appearance of under-eye puffiness.
The skin is not a part of the digestive tract, due to differences in pH, absorption, enzymes, sensitivity, and blood flow.