How does drinking 2 litres of diet soda in a day affect a senior man?

• Can Disrupt Gut Health
Diet soda contains artificial sweeteners, which may adversely affect the gut microbiome, and your body’s bacterial community. The gut microbiome is essential for many health factors, including immunity, nutrient absorption, heart health, etc. In a small study of seven participants, saccharin affected blood sugar management and imbalanced gut bacteria among 57%. 
In other studies, acesulfame, another artificial sweetener in diet soda, has been shown to damage mice’s gut microbiome. Nevertheless, further studies on humans and animals have produced conflicting results.
Diet soda’s most common non-nutritive sweetener is aspartame. Many studies have examined its safety; however, few have examined how it might affect the human microbiome. The body breaks down aspartame before it reaches the large intestine, home to the gut microbiome. We need more large-scale, high-quality studies to understand how artificial sweeteners and diet soda may affect human gut health.
 
• Erodes Tooth Enamel 
Despite the lack of sugar in diet soda, acidity can seriously damage your teeth over time. In a test tube study, regular and diet soda significantly affected tooth enamel surface roughness, suggesting that both can contribute to tooth erosion. 
 
• May Cause Headaches
 Drinking diet soda multiple times a day may cause headaches for some people; this may be caused by artificial sweeteners in diet soda, such as aspartame. There are several side effects associated with consuming large amounts of aspartame. The side effects may include:
• Headaches 
• Migraines
• Irritability 
There is evidence that caffeine, found in some diet sodas, may cause headaches for some people, but the evidence is mixed.
 
• Can Decrease Bone Density 
Several compounds found in diet soda ( caffeine and phosphoric acid) may negatively affect bone health and bone loss. One study found that women who drank diet and regular cola had lower bone mineral density.
Plus, soft drink consumers were more likely to sustain bone fractures over five years. However, diet soda does not necessarily pose a greater risk of bone fractures or reduced bone mineral density than regular soda. When consumed in high quantities, you may be at risk for both.
 
• May Affect Heart Health
 Similarly, long-term heart problems are more common in people who regularly drink diet soda. In a study of 59,614 women, drinking at least two diet drinks per day increased the risk of heart disease and death from heart disease over nine years.
 In older studies, both diet and regular soda have been associated with an increased risk of stroke. Plus, diet soda has been linked to metabolic syndrome, leading to heart disease, stroke, and type 2 diabetes.
Still, researchers need to examine whether and how other factors may also contribute.